Investigations on the structure and properties of novel mixed-metal borohydrides

Thesis

Reference

Investigations on the structure and properties of novel mixed-metal borohydrides

SCHOUWINK, Pascal

Abstract

This thesis deals with structural topologies of different dimensionalities in novel complex hydrides based on the tetrahydroborate anion. While classical applications of hydrides such as mobile hydrogen storage are discussed, the use of hydrogen-storage incompatible heavy metals, in especial lanthanides, yields new structural features and functionalities in borohydride chemistry. In this context, the photophysical properties as well as extensive structural dynamics provide means of venturing into new fields such as solid state phosphors and solid state electrolytes. Extensive characterizations are presented to correlate physical behaviour with the crystal structure. These include synchrotron X-ray diffraction, quasi-elastic neutron scattering and vibrational spectroscopies as well as optical spectroscopy and thermal analyses. In particular, di-hydrogen contacts are revealed to play a dominant role in phase transition mechanisms and it is shown that the tetrahydroborate anion can be utilized to engineer both lattice instabilities and physical properties in perovskite type complex hydrides.

SCHOUWINK, Pascal. Investigations on the structure and properties of novel mixed-metal borohydrides. Thèse de doctorat : Univ. Genève, 2014, no. Sc. 4723

URN : urn:nbn:ch:unige-456846

DOI : 10.13097/archive-ouverte/unige:45684

Available at:

http://archive-ouverte.unige.ch/unige:45684

Disclaimer: layout of this document may differ from the published version.

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Investigations on the Structure and Properties of Novel Mixed-Metal Borohydrides

TH` ESE

pr´ esent´ ee ` a la Facult´ e des sciences de l’Universit´ e de Gen` eve pour obtenir le grade de Docteur ` es Sciences, mention

cristallographie

par

Pascal Schouwink de Heidelberg, Allemagne

Th`ese N

4723

GEN` EVE

Atelier d’impression ReproMail

2014

The work presented in this thesis was carried out at the Department of Con- densed Matter of the University of Geneva in the Laboratory of Crystallogra- phy, under the supervision of Radovan ˘Cern´y. In first terms, I wish to thank Radovan for the support and means he has provided me with to develop my own ideas and projects. The time in our laboratory has allowed me to learn many things essential in all the beautiful areas of scientific research, such as the management and development of scientific research projects. Our work together has been mutually enriching and I would like to thank him for his reliability as colleague and friend. It is an honour to have Yaroslav Filinchuk, Magnus Sørby and Dirk van der Marel in the jury of my PhD defense and I am grateful for their willingness to accept the invitation.

Furthermore I would briefly like to thank Ronald Miletich and in especial Thomas Pippinger from my previous group in Heidelberg, who accompanied me throughout the beginnings of my scientific research. Next to my labmates Yolanda Sadhikin, Matteo Brighi, Laure Guen´ee and C´eline Besnard I have found a great colleague in Jean-Luc Lorenzoni. Many memorable moments have been shared together reparing machines and I will always remember that problems are often not as complicated as they seem. Thanks for your time and evergood spirit Jean-Luc!

Thanks to my principal collaborators Torben from Aarhus, and in especial L’ubomir Smr˘cok, who has had a watchful eye on me to keep my scientific imagination in limits. Many of the synchrotron beamtimes leading to the re- sults of this study have been shard with different people. In particular the common times shard with Dorthe Ravensbæk from Aarhus and Yaroslav Fil- inchuk and his group from Louvain-la-Neuve have been delightful.

A great deal of appreciation needs to go out to my girlfriend Mia Milos, who has more often than not come up with the necessary energy to motivate me through the downs every thesis entails. Our vivid discussions about science have surely been of great help more than once.

Of course nothing in life works without parents. Thank you, mom and dad, I hope I will continue to fascinate you with my love for science.

R´ esum´ e

Apr´es des dizaines d’ann´ees d’efforts globaux dans le domaine du stockage d’hydrog`ene dans les solides, les applications des compos´es intermetalliques impliquant l’hydrog´ene sont nombreuses. Parmi elles figurent le stockage d’´energie ´elechtrochimique sous forme de batteries ou d’´energie solaire ther- male dans des reacteurs d’hydrures des m´etaux, des senseurs et des vitres intelligentes. Le stockage d’hydrog`ene dans l’´etat solide inclut des applica- tions stationnaires, par exemple le c´el´ebre interm´etallique LaNi₅, et mobiles, comme c’est le cas des alliages de Fe et Ti. Ces derniers apportent des applica- tions mobiles `a la marine allemande pour des sous-marins propuls´es avec des piles `a combustibles. Toutefois, le probl`eme principal de notre soci´et´e moderne est le developpement d’un syst`eme d’´energie durable ”propre” et ´economique.

Ceci concerne principalement l’automobile. L’abondance et l’´enorme densit´e d’´energie (5.6 MJ/L pour 70 MPa) emmagasin´ee par l’hydrog`ene le pr´edispose comme candidat pour r´esoudre ce probl`eme. Pour envisager un tel syst`eme bas´e sur l’hydrog`ene il est primordial de trouver des moyens de stockage effi- caces et surs. Un tel materiel doit r´epondre aux exigences dures et imp´eratives concernant le poids du syst`eme en question et les propri´et´ees cin´etiques et thermodynamiques de son comportement de sorption et desorption. Il doit aussi ˆetre cyclable des milliers de fois et finalement, pour ˆetre applicable `a grande ´echelle, le coˆut ne doit pas d´epasser celui des ´energies fossiles.

Ainsi, pour un syst´eme de stockage mobile parmis les diff´erents hydrures ces crit`eres ´eliminent les hydrures interstitiaux interm´etalliques `a cause de leur ca- pacit´e gravim´etrique wt%(H₂) inf´erieure, mˆeme si ils apportent des avantages d’un point de vue thermodynamique, et les m´etalloporeux MOF (physisorp- tion) dont l’enthalpie de sorption est trop basse pour r´ealiser le stockage `a temp´eratures ambiantes. En revanche, les hydrures inorganiques complexes o`u l’hydrog`ene est stoqu´e dans des liaisons covalentes ont, `a premi`ere vue, des caract´eristiques gravim´etriques et volum´etriques optimales concernant les applications mobiles. C’est pour cette raison que le stockage on board dans le solide a ´et´e jug´e le plus viable dans les compos´es bas´es sur les anions complexes comprennant les AlH<sub>4</sub><sup>−</sup>, Si<sub>n</sub>H<sub>2n+2</sub> ou notamment le BH<sub>4</sub><sup>−</sup> . C’est la cristallo- chimie de ce dernier groupement, le tetrahydroborate, qui est le sujet de cette th`ese, dans laquelle les compos´es sont dor´enavant d´enomm´es borohydrures.

En cons´equence, une grande partie de cette th`ese est consacr´ee `a la cristallo- chimie des nouveaux syst`emes des borohydrures comportant deux ou trois m´etaux diff´erents. Avec un rapport m´etal:hydrog`ene 1:4 l’anion tetrahydrob- orate BH₄⁻ est l’ion le plus prometteur tant au niveau volum´etrique que gravim´etrique, la masse atomique de bore ´etant inf´erieure `a celle de l’aluminum, de l’azote ou du silicium. La lib´eration de la mol´ecule H<sub>2</sub> proc`ede via la

d´estabilisation par diff´erents mechanismes (pyrolyse, hydrolyse, etc.) du groupe BH₄ et la formation de produits de d´ecomposition, souvent des borides ou des hydrures de m´etaux. Malheureusement le r´ecyclage du syst`eme d´epend large- ment du m´echanisme de d´ecomposition. Par exemple, une re-hydrog´enation compl`ete n’est pas possible si les gaz ´evacu´es contiennent le diborane B₂H₆ ou des boranes sup´erieurs B_yH_x, vue que cela pose une perte de boron. Les m´echanismes de d´ecomposition ne sont pas le sujet principal de cette th´ese.

Appliqu´ee de fa¸con appropri´ee, la cristallo-chimie syst´ematique est un in- strument tr`es performant pour d´evelopper de nouveaux mat´eriaux functionel- ls se basant sur des analogies avec des syst`emes chimiques semblables. En particulier, le Laboratoire de Cristallographie de l’Universit´e de Gen`eve fait partie de la communaut´e travaillant sur les hydrures complexes et recherchant des applications dans le domaine du stockage d’´energie ´electro-chimique. Des

´

etudes montrent que la dynamique structurelle g´en´er´ee sp´ecialement par les re- orientations charact´eristiques du groupe BH₄ entraˆıne comme effet secondaire l’augmentation de la mobilit´e des ions mobiles, parmis eux surtout leLi⁺, grace

`

a l’effet paddle wheel. Les hydrures manifestent plusieurs propriet´ees physico- chimiques, dont la nature r´eductrice de ces derniers, repr´esentant des effets favorables pour la construction de batteries utilisant des ´electrolytes solides.

Afin d’exploiter enti`erement les relations entre la structure cristalline et les propri´et´ees physico-chimiques, il est essentiel d’acqu´erir une connaissance pro- fonde et d´etaill´ee sur le comportement du lithium et de ses coordinations dans un environment homoleptique de ligands BH<sub>4</sub>. Il est d´ecouvert que le cation Li<sup>+</sup> est une esp`ece mobile dans plusieurs compos´ees borohydrudres ainsi que capable d’adopter des coordination vari´ees dans des diff´erentes sub-topologies m´etal-BH₄ de dimensions diverses.

Effectivement, l’analyse topologique ne doit pas ˆetre consid´er´ee comme un moyen de classification mais est ´egalement utile pour la pr´ediction et l’ingenieurie des cristaux dans les sciences des mat´eriaux. Nous trouvons une forte tendance des borohydrures `a suivre les principes d’architecture des oxides iso-´electroniques. Cette certaine similarit´e, illustr´ee dans cette th`ese comme analogie borohydrures-oxides n’est pas surprenante, vue qu’il s’agit de com- pos´es iono-covalentes dans les deux cas.

La topologie des borohydrures comprends des membres plutˆot ioniques, r´epre- sent´es par des anions complexes de charact`ere g´en´erale [Mⁿ⁺(BH₄)_m]^(n−m)−, et des structures poreuses attestant d’un r´esaux de liaison plus directionnel et par cons´equent plus covalent, comme c’est le cas pour la modifciation ”γ” de Mg(BH₄)₂. Ce dernier adopte une structure construite sur les mˆemes principes que le groupe d’alumosilicates d´enom´e z´eolites. Entre ces deux extr´emes, il ex- iste une multitude de topologies diff´erentes construites sur des connectivit´es (4)- ou (3,4)-, qui enveloppe des r´esaux trimetalliques dans Li₃MZn₅(BH₄)₁₅

et des couches de topologie honeycomb (topologiquement identiques augra- phene) dans KLiMg(BH₄)₄ et des polymorphes monocliniques et hexagonaux de Li₂Cs(BH₄)₃.

La ressemblance aux oxides est refl´et´ee ´egalement dans les topologies de con- nectivit´e (6)-, les borohydrures type perovskites, qui constituent la partie finale de cette th`ese. Etant probablement la structure la plus ´etudi´ee et certaine- ment une des plus importantes dans la chimie et la physique du solide, mais aussi dans la technologie moderne grace `a ses fonctinalit´ees interminables et ajustables par des distortions structuralles, ici nous pr´esentons des caracteris- tiques enti`erement nouvelles apport´ees par le groupe BH₄. L’incorporation de ce dernier dans la structure perovskite a des cons´equences ´etonnantes, pr´ecisement pour les faibles distortions et transitions de phase au niveau struc- turel, fondamentalement importantes pour le groupe des oxides de m´etaux de transition ferroics. Ici nous incorporons de fa¸con controll´ee une dynamique structurelle sur le site anion dans la famille ABX₃. Ainsi, nous unifions les concepts ´etablis de cette famille de mat´eriaux ceramiqes avec les concepts provennant de la chimie moleculaire, ceux-ci consituant les interactions faibles, en particulier les contacts homopolaires di-hydrog´enes.

Dans le cadre de cette th´eses des propriet´es physiques ont ´et´e explor´ees au niveau ´electronique pour les compos´es du type perovskite, hors du context du stockage d’hydrog´ene.

1 Introduction to Borohydrides 1

1.1 Applications . . . 3

1.2 Crystal Chemistry of Borohydrides . . . 4

1.2.1 Different types of disorder . . . 6

1.3 Outline . . . 7

2 Experimental Methods 9 2.1 Sample Preparation . . . 9

2.1.1 Mechano-Chemical Synthesis . . . 9

2.1.2 Wet-Chemical Procedures . . . 11

2.1.3 Single Crystal Synthesis . . . 11

2.1.4 Sample Treatment under Gas Pressures . . . 12

2.2 Sample Characterization . . . 12

2.2.1 Diffraction . . . 12

X-Ray Powder Diffraction . . . 13

Line width and line shape . . . 14

Neutron Powder Diffraction . . . 14

Direct Space Methods . . . 14

Rietveld Refinement . . . 16

Sample environment . . . 17

2.2.2 Spectroscopy . . . 17

Vibrational Spectroscopy . . . 17

Raman line width and position . . . 18

Quasi-Elastic Neutron Scattering . . . 18

Optical Spectroscopy . . . 21

2.2.3 Thermal Analysis . . . 21

2.2.4 Computational Methods . . . 22

3 Results and Discussion 23 3.1 Complex anions . . . 23

3.2 Frameworks . . . 24

3.2.1 Metal-BH₄ sublattices in bimetallic A_xLi_y(BH₄)_x-y . . . . 25 ix

3.2.2 Metal-BH₄ sublattices in trimetallic borohydrides . . . . 25

3.3 Perovskite-type Borohydrides . . . 26

3.3.1 Chemistry of Borohydride Perovskites . . . 29

3.3.2 Phase Transitions in AB(BH₄)₃ . . . 29

Vibrational Spectroscopy of AB(BH₄)₃ . . . 31

KCa(BH₄)₃: A low symmetry structural transformation . 32 CsCa(BH₄)₃: A high-symmetry order-disorder transition 37 RbCa(BH₄)₃: A structural low symmetry transition fol- lowed by order-disorder . . . 38

Structural Dynamics in AB(BH₄)₃. . . 41

Displacively modulated superstructures in AB(BH₄)₃ . . 55

3.3.3 Hydrogen Storage in NH₄Ca(BH₄)₃ . . . 58

3.3.4 Photoluminescence in REE - perovskites . . . 62

CsEu(BH₄)₃ and CsCa(BH₄)₃:Eu²⁺ . . . 64

3.3.5 Single Crystal Synthesis of KCa(BH₄)₃ . . . 65

Single crystal structure at 180 K . . . 68

Single crystal structure at 250 K . . . 71

Discrepancies between ball-milled and single crystal sam- ples . . . 73

3.3.6 High Pressure studies on perovskite-type and ReO₃-type borohydrides . . . 74

REE(BH₄)₃ . . . 75

3.4 Transition Metal Compounds . . . 79

Scandium-based syntheses . . . 80

Chromium-based syntheses . . . 83

Iron-, Cobalt- and Nickel-based syntheses . . . 84

Copper-based syntheses . . . 84

Titanium-based syntheses . . . 85

3.4.1 Conclusions . . . 91

4 Conclusions and Perspectives 92 A Appendix 104 A.1 List of publications resulting from this thesis . . . 104

Introduction to Borohydrides

Also known as borohydrides, complex hydrides based on the tetrahydroborate anion BH₄^– are a rapidly evolving field in sustainable energy research. Their unique gravimetric hydrogen density results in light-weight materials, that can meet various requirements set by the Department of Energy for future hydrogen storage materials. A large fraction of today’s energy consumption (about 1/4 in Europe) is related to transportation. Hydrogen is an excellent energy carrier and has the highest mass energy density of any fuel, 120 MJ/kg.

However, the volumetric density of hydrogen gas at 1 atm is only 10.7 kJ/l.

Compressing or liquifying hydrogen gas is energetically a very expensive pro- cess, which is why the storage of hydrogen has been suggested to be most viable in the solid state.

Borohydrides belong to the group of complex hydrides. So far the ther- modynamic and kinetic properties of complex hydrides are not favourable as compared to the traditional interstitial metal hydrides. However, any on-board storage system must necessarily be of light weight, which is where no other materials can meet the excellent properties, i.e. high mass density and high volume density, of borohydrides. Chemically, borohydrides can be consid- ered somewhat of an outlier, since the covalently bound hydrogen encountered in molecular or supramolecular chemistry is usually positive charged, i.e. a proton, and hence called protic. The hydrogen in the BH₄-molecule on the other hand carries a partial negative charge, and is called hydridic. This is the situation encountered in metal hydrides. The discovery of the first metal borohydride Al(BH₄)₃ dates back to 1939 [105]. Alkali-borohydrides were first synthesized in 1940 by Hermann Irving Schlesinger and Herbert C. Brown [102, 103], and developed as reducing agents in organic chemistry, a discov- ery for which they were awarded the nobel prize in chemistry in 1972. While NaBH₄ was used to produce hydrogen gas for weather balloons in World War II, it also presented an intermediate in the synthesis of uranium borohydride.

Schlesinger was assigned the task of preparing this compounds by the National 1

Figure 1.1: Different mechanisms of hydrogen storage compounds of increasing hydrogen density. From bottom to top: Physisorption of H₂molecules on surfaces of e.g. metal-organic frameworks (MOFs) aimed at mobile applications, interstitial metal hydride used in stationary applications, complex hydride for mobile applications where hydrogen is released by pyrolysis or hydrolysis (reaction with H₂O, NH₃, H₂S, etc.), chemical hydride.

Research Defense Committe during the Manhattan Project, on the search for volatile uranium compounds. This research remained top secret and was pub- lished only 8 years after the end of WWII [104]. Since their discovery, borohy- drides are being used in every chemistry laboratory. During over half a century however, their potential application beyond organic synthesis were not at all considered. In 2001 the first report on LiBH₄ as a potential hydrogen storage material was published [101]. The number of related publications in the field has been sky-rocketing ever since, in spite of significant issues that need to be solved, concerning hydrogen yields during gas release, slow kinetics and un- favourable thermodynamics hampering reversibility of most systems. Strongly driven by today’s need of a sustainable energy society application-oriented re- search is often based on trial-and-error approaches, fundamental properties of the most important unit, the underling structural architecture of the crystal lattice, not being sufficiently considered.

This thesis deals with structural topologies of different dimensionalities in novel complex hydrides based on the tetrahydroborate anion (BH₄)^–, and their physical properties. The use of metals not commonly considered to be hydrogen-storage compatible due to their high weight yields new structural features and functionalities in borohydride crystal chemistry.

Figure 1.2: State-of-the-art applications of metal-hydrogen systems based on metal-hydrides, taken from ref [66].

1.1 Applications

As opposed to complex hydrides, metallic hydrides have a long tradition in various research fields and numerous applications have resulted from decades of efforts. They are usually based on intermetallic compounds or solid-solution alloys. While traditionally the target application used to be hydrogen storage or related fields such as fuel cells, modern applications fabricated with metal hydride thin films or nanoparticles include smart windows [125], where use is made of the metal-insulator transition caused by hydrogenation, or sensors that make use of plasmonic properties in nano-devices [117]. A summary of many metal hydride applications is shown in figure 1.2 and some recent re- views are available in references [66, 33]. Though the favourable hydrogen sorption-desorption kinetics make many famous intermetallic compounds, e.g.

LaNi₅, excellent stationary storage materials, the low mass density of hydrogen in such compounds makes their implementation impossible in mobile applica- tions. Complex hydrides on the other hand, amongst which borohydrides are the lightest in weight, initially attracted significant interest due to achieving record values for both the volumetric and the gravimetric hydrogen density (see figure 1.3).

Aside of hydrogen storage, various borohydrides have been found to favour the mechanism of ionic conduction [121] as has been reported for the mobile species Li, Na and even Mg, the latter in the ionic liquid phase. As will be seen later, BH₄ is an inherently mobile anion, and the positive effect of borohydrides on the activation energy of mobile species, has been found to have its origin in the so-called ”paddle-wheel” effect [78]. A number of borohydrides and complex hydrides in general has been reported in the past years, ranging from spinell-like Li-conducting compounds [65] over vacant-site Na-conducting

Figure 1.3: The energy density landscape of systems currently being evaluated for on-board hydrogen storage, taken from reference [59]. The yellow field represents the requirements for on-board application in fuel cell vehicles (FCV) imposed by the Department of Energy.

borohydrides [113, 79] or related higher borane materials [119] to mobile Mg- species in molten Mg(BH₄)₂ [82]. Future efforts in rational design will require increasingly detailed information on the structural behaviour of each of the building blocks. One of the chapters of this thesis deals with the study of the relevant Li-species in different coordination environments.

1.2 Crystal Chemistry of Borohydrides

Throughout this thesis it will be frequently referred to the borohydride-oxide analogy, which essentially refers to structural relationships between both groups of iono-covalent compounds. Borohydrides have just recently appeared on the stage of materials science, and analogies to other materials families are ex- pected to be an efficient and fruitful tool to design new compounds and tailor their physical properties. The relationship to metal-oxides was recognized early and manifests itself most readily in the simple binary M-BH₄-phases based on alkaline earth metals. For instance, most of the high-pressure high- temperature phase diagramm of silicon di-oxide (Quartz) SiO₂ can be found amongst the different polymorphs of Mg(BH₄)₂ [20, 38] and Cd(BH₄)₂ [91].

Likewise, the analogy proceeds between the modifications of Ca(BH₄)₂ and those of titanium di-oxide TiO₂ [2]. However, there exist two main differences between the oxide and borohydride anions O²⁻ and BH₄⁻. While the oxide can be considered to be approximately spherical, the tetrahedral configura-

tion of the BH₄ molecule leads to a tetrahedral charge distribution. In real crystal lattices, moreover, BH₄ tends to be inherently unstable with respect to rotations around different symmetry axes. Due to these differences very inter- esting deviations from intuitive structural behaviour can result, which will be of particular importance in the discussion on perovskites in chapter 3.3. The general borohydride-oxide analogy hence implies lower-symmetric distortions in the borohydride compound when compare to the parent oxide.

Figure 1.4: Schematic difference between approximately spherical oxide anion O²⁻and non-spherical, highly dynamic borohydride anion BH₄⁻.

Borohydrides, and complex hydrides in general, contain both ionic and co- valent bonding contributions. Boranes have been studied extremely well in solution, a nobel prize was awarded for this to William Lipscomb in 1976.

There is no doubt about the predominantly covalent character of the B-H bond. Many boranes are electron-deficient molecules, leading to 3 center - 2 electron bonding schemes. In the case of the tetrahydroborate anion (BH₄)^– the molecule is stabilized by the formally negative charge. Hence the BH₄ group may therefore be destabilized by removing this charge with electrostatic forces, e.g. by an electronegative counter-cation. In fact, an empirical rela- tionship between the stability of the respective borohydride compound and the electronegativity of the involved metal counter-action was proposed already in the 90’s of the 20th century [108] and more recenly has been confirmed theo- retically in 2006 [83], the upper limit of electronegativity that allows for stable compounds coming to lie at approximately χ <1.7.

In figure 1.5 it can be seen how this relationship is being actively exploited to tailor decomposition temperatures of novel metal borohydrides. In partic- ular, considerable effort has been invested by many groups in the synthesis of transition-metal compounds. While their upper thermal stability limit is expected to be below, or close to room temperature, the use of an additional electropositive metal can provide means of stabilizing and destabilizing new dual-cation compounds at the desired conditions. This has lead to a vast spectrum of bimetallic borohydrides [49, 24, 21, 55, 56, 88, 91, 71, 22, 107].

Comprehensive reviews are found in [90, 98, 37]. Concerning the pure mono- metallic compounds of the kind TM(BH₄)_n, where n is the oxidation state of the metal, there is a considerable amount of reports in the literature that

Figure 1.5: Thermal stability vs. electronegativity of selected borohydrides, taken from [98].

claim their existence, but no structure has been reported for any compound containing a genuine transition metal, where the electronic configuration is neither an empty, full, nor half-full d-shell.

In general, borohyrides containing more than two metals are unlikely to become realistic chemical hydrogen storage candidates. Such systems are hard to understand and control thermodynamically. The harsh physical and chem- ical operating conditions of hydrogen release and uptake processes imply that any potential system be kept as simple as possible.

However, the recent discovery of the nano-porousγ-polymorph of Mg(BH₄)₂ is possibly currently opening a new direction of research in borohydrides. The construction of charge-balanced networks based on the BH₄⁻anion is aimed at providing hydridic frameworks with selective gas-sorption properties [38]. The construction of such frameworks may well profit from a certain complexity in the system, as for instance in zeolithes or metal-organic frameworks (MOF).

The increase of chemical components in the structure is also driven by the already mentioned borohydride-oxide analogy. Currently the potential struc- tural roles of different metals in borohydride environment is being scanned in order to reconstruct technologically relevant oxides. This approach has shown first results in the design of spinells, garnets and perovskites.

1.2.1 Different types of disorder

A characteristic property of the borohydride anion is that it tends to perform different local dynamic motions with different characteristic times, leading to positional disorder of the hydrogen atoms in an average picture. For instance, the high-temperature phases of the alkali-borohydrides are stabilized by the entropic contribution owed to full disorder. This kind of disorder has been shown to promote superionic conduction as for instance in hexagonal HT- LiBH₄, where borohydride anions perform a polar tumbling motion around

the hexagonal axis. A second kind of disorder frequently encountered on the anion site is that of chemical disorder. In anion-mixed compounds the anions can be ordered or disordered, hence occupy the same Wyckoff position or different ones. In anion-ordered compounds it is expected that effects due to the non-spherical nature of one of the anions will have a greater impact on the structure than in anion-disordered variants. This results in a complicated interplay with disorder on the cation-site, which also occurs, for instance in a series of double-perovskites briefly mentioned in chapter 3. It hence needs to be summarized, that a huge spectrum of disorder-mechanisms occurs within these compounds, and it will be seen that it constitutes the critical component dominating high temperature phase transitions.

1.3 Outline

This thesis attempts to establish systematic relationships in the chemistry of borohydrides, by exploring not only the light-metal range of the periodic table, but extending this to heavier elements, that could provide physical properties going beyond hydrogen storage. Crystal chemical concepts are used to place structure-property relationships into context with the aim of further under- standing the individual functionalities attainable with each structural archi- tecture. Introducing heavy metals into systems where structural behaviour is dominated by a rich spectrum of properties specific to the structural building blocks made of light-elements implies severe complications at the level of data analysis, structural characterization and interpretation. Chapter 2 deals with the different approaches adopted to obtain a maximum of information on the structural level.

During the extensive and systematic synthetic investigation of this thesis over fifty new compounds and polymorphs have been discovered, which belong to three different structural building principles. The most significant results are discussed in detail in chapter 3, as well as in six full articles found in the appendix. The reader may get the impression that less attention has been payed to chapters 3.1 and 3.2. This however is due to the fact that the corresponding full results have been published in papers 1-5. Next to paper 6, chapter 3.3 deals largely with unpublished results.

The work presented in papers 1-3 is based on the approach where elements of different electronegativities are combined to tailor hydrogen release prop- erties, especially to stabilize transition metals in borohydride environment.

Papers 1 and 2 extend these concepts to dual-cation compounds based on Mg, Mn and Zn. The structures are built from complex anions made of the more electronegative element and the ligand BH₄, that are counterbalanced by large electropositive cations. The decomposition mechanisms of such structures are

investigated to evaluate their potential as hydrogen storage systems.

In paper 3, this same concept is extended to borohydride systems con- taining three metals. The first trimetallic compounds Li₃MZn₅(BH₄)₁₅ are discovered and the resulting structures become increasingly complex with the number of contained elements. Li₃MZn₅(BH₄)₁₅ is built from two interpen- etrated three-dimensional frameworks. Methodologically, paper 3 employs a combined approach to obtain a maximum information on Li₃MZn₅(BH₄)₁₅, where ordered variants of average structural models obtained by synchrotron powder diffraction are evaluated by means of density functional theory calcu- lations. Details such as potential Li-motion are investigated and conclusions are drawn on future crystal design of such phases. This approach is rigorously applied in paper 3 for the first time, and will be applied again in papers 4-6.

In papers 4 and 5 we use large electropositive heavy metal cations to ex- plore the versatile behaviour of light metals such as lithium and magnesium in charged metal-BH₄ substructures, that are balanced by the counter-cation. In paper 4 the lithium node is revealed to be surprisingly flexible and different co- ordination modes occurring in Li-BH₄ substructures of various dimensionalities are discussed in the rich phase diagrams of LiBH₄−RbBH₄and LiBH₄−CsBH₄. Importantly, the versatility of the Li-node in such architectures testifies of new coordination modes that can be rationally employed during the design of new superionic conductors.

In paper 5 the light-metal system LiBH₄−Mg(BH₄)₂, very relevant to hy- drogen storage, is stabilized for the first time in trimetallic K-Li-Mg-borohydri- des. In this study general aspects preventing the formability of bimetallic Li- Mg-phases are discussed on the basis of analogies to Al-Si-based oxide systems.

In chapter 3 and paper 6 the essentials of our investigations on a new fam- ily of perovskite materials, discovered in this thesis, are presented. Paper 6 serves as an introduction to the perovskite-type borohydrides, which summa- rizes their exotic structural behaviour, photophysical and electronic properties, as well as hydrogen-storage potential. It is found that the incorporation of BH₄ on the anion site has significant impact on the lattice instabilities dominating all structural behaviour and many physical functionalities in the famous transi- tion metal oxide perovskites. The specific hydrogen-related features controlling distortions in perovskite-type borohydrides and extensive invetigations on the structural dynamics are discussed in detail in chapter 3. It is found that weak interactions are responsible for high-temperature polymorphs of low symmetry.

This is remarkable, since all functional perovskite-type compounds evolve to higher symmetry with temperature, which eliminates many simple but impor- tant properties of polar symmetry. It is suggested that the implementation of molecular chemistry concepts, such as dihydrogen contacts, can be a new tool to control distortions, and in the future possibly symmetry, in perovskites.

Experimental Methods

2.1 Sample Preparation

Borohydrides are very reactive materials and their high reducing power makes them prone to oxidation. They easily hydrolyse and the sensitivity not only to oxygen, but in especial to moisture makes sample manipulation under inert gas atmosphere indispensible. All sample handling connected with the results presented in this thesis was performed under inert conditions making use of argon- and nitrogen supplied gloveboxes.

2.1.1 Mechano-Chemical Synthesis

Mechano-chemical synthetic methods, i.e. ball milling, are tools originating from metallurgy where they were developed to reduce particle size and for mechanical alloying. The ball-mills used in this study are used in so-called high-energy ball milling, where the grinding bowls are located off-center on a rotating plate (see scheme in figure 2.1). The sense of rotation of the base-plate and the bowls is opposite, leading to balls travelling on trajectories through the center of the respective grinding bowl. This trajectory results in high- energy impacts of the grinding balls. The conditions hence experienced by the sample are ”extremer” in terms of temperature and mechanical stress, than in conventional rotating or shaker mills. While this promotes the completion of chemical reactions it entails various undesired side-effects, that have severe consequences on subsequent sample characterization and data analysis. These issues include ”over-milled” samples, where the phase of interest has exceeded its stability field and decomposed to the more stable products, as well as effects on the crystallinity of the sample. The grain-sizes are commonly in the 1-5 µm range, but the coherence-length of individual grains can drop well below 50 nm. This reduction in size plus defects introduced in the crystal lattice can result in significant broadening of Bragg signals in X-ray or neutron diffraction

9

Figure 2.1: Scheme of the process of high-energy ball milling and listed variable parameters.

experiments. In polycrystalline samples inferior crystallinity is tantamount to peak overlap, which not only complicates peak-shape modelling, but more importantly, can render phase separation and phase indexing impossible. This aspects need to be considered with great care when interpreting the results [19, 106].

Different reactions can be performed to produce borohydrides by mecho- chemistry [45], depending on the reactants available and the product that the synthesis is designed for. Metal-chlorides are commonly used if the borohy- dride compound is not available. In simple generalized cases this results in metathesis reactions producing homoleptic borohydrides according to

x ABH_4(s)+MCl_x(s)−−→M(BH₄)_x(s)+xACl_(s) and anion-mixed variants resulting from addition reactions

x ABH_4(s)+yMCl_n(s)−−→A_xM_y(BH₄)_xCl_yn(s)

The reactions used in most cases in this study often contain contributions of both mechanisms and are discussed where necessary in the respective sections.

Due to competing reactions and often more than one unknown phase on the products size it is common to perform identical synthesis with different ratios of the starting materials, thus modifying the phase yields of the products. An introductory guide on how to deal with the characterization of borohydride systems is provided in [106].

Thanks to the simplicity of its setup ball-milling is an extremely versatile technique, not restricted to solid state chemistry. It has rapidly left its histori- cal application and is nowadays being employed in many different fields ranging from solid state sciences, e.g. ceramics or quantum dots, to soft matter and pharmaceutics, as well as metal-organic frameworks [17]. The nature of reac- tants may range from liquids to gases, ball-milling at cryogenic temperatures

as well as under high gas pressures is readily accessible by minor modifications to existing setups. An attractive property of mechano-chemical treatment is that solvant-free reactions can be developed, these efforts are relevant to the green-chemistry trends in solvant-free organic chemistry. The relative ease of implementation has recently lead to first reports on in-situ ball-milling at synchrotrons beamlines [50, 40].

2.1.2 Wet-Chemical Procedures

Recently wet chemistry procedures have been developed [38, 67] to prepare many borohydrides without chlorine impurities. Mainly this has been moti- vated by the need of chlorine-free reactants for the synthesis of multi-cation compounds. In collaboration with the group of Torben Jensen from the Univer- sity of Aarhus these procedures were used in this study to obtain the necessary pure lanthanide borohydrides used to produce perovskite-type borohydrides by high-energy ball milling. The protocoll makes use of the good solubility of borohydrides in di-methyl sulfide [67]. In a first step, mechano-chemistry is used to carry out an exchange reaction between LiBH₄ and LnCln, where n is the oxidation state of the lanthanide cation. The produced LiCl is not soluble in DMS. The as-milled mixture is stirred under inert atmoshphere in DMS for over 10 h at 400 K. The solution is the filtered and the residual evaporated under dynamic vacuum, resulting in a chlorine free lanthanide borohydride.

2.1.3 Single Crystal Synthesis

Quantities experimentally observed on powders are often projections of a higher-dimensional property onto two- or one-dimensional space. For instance, the three-dimensional reciprocal space is projected onto one dimension in a powder diffraction pattern. Similarly, an inelastic scattering curve measured on a powder sample provides the experimental phonon density of states, but does not contain any information about the dispersion of different phonon- branches in momentum space. Assignment of Raman or IR-active modes by means of vibrational spectroscopy suffers from the same complications. To obtain three-dimensional information single crystals are required. It is obvi- ous that single crystal samples are not accessible with mechano-chemistry. In a preparation for future phonon studies on perovskite-type borohydrides dif- ferent crystallization procedures were evaluated. In a first approach it was attempted to grow single crystal by slow evaporation from different solvants, including DMS, DMSO, THF, di-ethyl-ether, aceto-nitrile and various others.

Hereby the ball-milled powder is dissolved in the respective solvant and left to evaporate over days or weeks in a glass recipient. It is a very simple, but com- mon method to obtain very high quality crystals. However this approach was

soon abandoned due to the crystallisation of different solvates of the mono- metallic borohydrides, especially of the solvant THF and DMS. Solvates of the latter have been reported for Mg(BH₄)₂ [38] and Y(BH₄)₃ [66]. The removal of the solvant molecule in fine-grained powders requires quite severe conditions in these cases. This suggests that solvant-removal would be either incomplete or result in cracking in the case of a single crystal. To avoid these problems, in a second approach the synthesis was attempted with different methods in the solid state. The postitive results are presented in the corresponding section in chapter 3.

2.1.4 Sample Treatment under Gas Pressures

The crystallinity of a sample can be improved by thermal treatment. This requires the sample to be stable at the specific temperature conditions. Ap- plying pressure to a sample shifts its thermodynamic equilibrium. Physical pressure may be applied in high-pressure environments such as diamond anvil cells or multi-anvil apparatus. High gas pressure on the other hand can be generated in autoclaves connected to a high pressure line.

2.2 Sample Characterization

2.2.1 Diffraction

The condition for constructive interference in elastic, coherent scattering pro- cesses, i.e. the condition for Bragg intensities, is described by the Laue equa- tion, the diffraction condition reads in reciprocal space

G=k_s−k_i (2.1)

where s and i denote the wave vector of incoming and scattered waves and G is the reciprocal lattice vector. Taking into account momentum conservation of elastic scattering |k_i| = |k_s| this reduces to

2dsinθ=nλ (2.2)

In the Frauenhofer approximation (far-field) the constructive interference of plane waves giving rise to measured intensities can be described as

I =

"

X

j

A_jexp(iφ_j)

#2

(2.3) This far-field approximation is valid as the distance between scattering centers, atoms, is much smaller than the distance between scatterers and the source,

detector. In crystallography, the interference pattern generated by complying with equation 2.1 is interpreted by means of the structure factor Fh, which relates to the measured intensity as I ∝ |F_h|² and the following

F_h =

N

X

j

f_jexp(2πir_j·h) (2.4) Hence, the phase contains the position vector r of atom j and the reciprocal space vector h. The phases ofFh entirely describe the interference conditions, while the intensities scale with the scattering power f_j of atomj. F_h is thus a function of the scattering power density of the crystal ρ(r) of the crystal. The structure factor can thus be expressed as

F_h = Z

cell

ρ(r)exp(2πir·h)dr=F T(ρ(r)) (2.5) and presents the fourier transform of direct space, r, to reciprocal space, h.

The function ρ(r) can describe the electron density, as in the case of X-ray diffraction, or the density of atomic nuclei or magnetic moments, for neutron diffraction.The crystal being of periodic nature, it suffices to sum over all struc- ture factors in reciprocal space, the summation cut-off defining the resolution of ρ(r), in the inverse fourier transform, which allows to calculate the electron density. Correct scaling implies multiplication by the factor 1/V.

ρ(r) = 1 V

X

h

F_hexp(−2πir·h) (2.6) Since the measured intensity is a quantity proportional to the squared struc- ture factor (kinematic approximation), the phases of F_h can not be retrieved directly. Phase determination can be achieved both in reciprocal and in direct space. The nature of samples investigated in this studies however makes the former impossible, a brief outline of the direct space methods is given at the end of this chapter.

X-Ray Powder Diffraction

Electromagnetic radiation, such as X-rays, is scattered off charged particles (Thomson scattering). The most common terminology of diffraction refers to coherent elastic scattering of an array of points. In three dimension this leads to a reciprocal space defined by the structure factors F_h. In a single crystal experiment, these are measured throughout the volume accessible by the respective wavelength. Unlike single crystal diffraction, powder diffraction probes only the length but not the orientation of the reciprocal lattice vector, the information in reciprocal space is thus projected onto one dimension. In

particular, this means that all reciprocal space vectors of the same length are superposed. It is easily understood that this implies great complications when determining space group symmetry, which contains translations in three dimensions. The procedure of indexing, i.e. assignment of a unit cell and its respective space group, which is quite trivial in single-crystal diffraction hence becomes the bottle-neck of structure solution in powder-diffraction.

Line width and line shape

Decreasing the coherence length of diffracting crystallites in direct space leads to broadening of Bragg signals in reciprocal space. It was mentioned above that, in specific case of soft borohydrides, mechano-chemistry produces grain- aggregates in theµm range. The stress on the sample produces different strain- induced defects that result in coherence-length much below the grainsize. If an accurate structural model is available, size-strain analysis can be extracted directly from the peak profile parameters refined with Rietveld analysis. To account for instrumental broadening, e.g. due to non-ideal optics, detector res- olution and other parameters of the specific setup, an instrumental resolution function is obtained on a standard with a well defined particle size, commonly LaB₆ or Silicon.

Neutron Powder Diffraction

Neutrons are scattered off the nucleus, unlike electromagnetic waves. This means that the atomic form factor, which arises from the free-electron like shell, is no longer a coefficient in the calculation of structure factors. It is replaced by the neutron scattering length that becomes the dominant factor in summations, being part of the total scattering cross section. Hence, the amplitudes of waves Fhkl scattered of the same lattice planes, i.e. giving rise to identical Bragg peaks, are not the same in both techniques.

This is most important in structural studies that involve the determination of hydrogen atomic positions. When hydrogen, as in borohydrides, becomes an important quantity, significant details are often overlooked by X-ray pow- der diffraction. This results not only in wrong atomic positions, but more importantly in wrongly determined space groups, that are easily overlooked.

Different space groups belonging to a respective extinction symbol are often most readily evaluated by the goodness of fit at higher angles, where the con- tribution of hydrogen to the scattered intensities is close to zero.

Direct Space Methods

The approach to structure solution is mainly dictated by the quality of the in- vestigated sample. Modern laboratory machines have become powerful enough

Figure 2.2: Global optimization in crystal structure parameter space as implemented in direct space methods.

Trial structures are evaluated by means of cost functions, shown on thez-axis.

to measure and solve small molecule crystal structures entirely without human interfering , this however being solely restricted to cases where the chemistry of the sample and the quality of the crystal are appropriate. The opposite of this extreme is found in powder diffraction experiments. Reciprocal space methods such as direct methods, Patterson methods or charge flipping all rely on an initial set of accurately determined observed intensities. In powder diffraction, this is achieved by a profile fit to the observed data. This fit is model-independent but does contain information on space group symmetry.

Essentially, peak overlap poses an enormous problem, which however can be addressed either with different statistical methods or increasing resolution by improving the sample quality or changing the experimental setup. The prob- lem of peak overlap may additionally be dealt with experimentally by applying external stimuli such as stress, which creates texturized samples [123], or tem- perature, to separate peaks by non-linear thermal dilatation. Therefore, also in powder diffraction structure solution in reciprocal space is becoming more and more frequent, thought still far from presenting a standard approach. At the same time it is remarkable, that just one single such case has been reported for borohydrides [51] where the polymorphs of Mg(BH₄)₂ were solved with di- rect methods. The numerous sample-dependent issues mentioned above, such as multi-phase character, small coherence length, large peak overlap, light el- ements, etc. all preclude any attempts of extracting intensities for separate phases to attempt reciprocal space structure solution. Therefore, all crystal structures reported in this study were solved in direct space. These methods are also know as global optimization methods [29], as they deal with the task of finding the absolutely best set of parameters in order to optimize an ob- jective function, called the cost function. This is accomplished by scanning the crystal structure parameter space in the search of the global minimum, as

shown schematically in figure 2.2. Direct space methods rely on a maximum of user input. This means that prior knowledge of the structure is extremely useful, since it can actively be used in direct space, but not in reciprocal. For condensed solids such as borohydrides, this refers to information on expected coordination polyhedra and inter-atomic distances. The structural model is then parametrized in real space to minimize the degrees of freedom in the sys- tem during global optimization (see figure 2.2). The criteria to evaluate trial structures generated are defined as cost-functions.

Rietveld Refinement

Following structure solution the structural parameters are refined with a non- linear least squares algorithm that minimizes the function defining the differ- ence between observed and calculated intensities, called the Rietveld method.

Hereby the whole pattern is modelled, including the structural models ob- tained in direct space, and potential background terms. The classical intensity equation to model intensities of one phase is expressed as

I_i,calc=s

N(peaks)

X

k=1

L_km_k|F_k|²S(2Θ_i−2Θ_k)P_kA+bkg_i (2.7) where s is the scale factor, L_k is the Lorentz-polarisation correction, m_k the multipilicity of the k-th reflection, S the profile function. Θ_i and Θ_k the angle at the i-th point in the diffraction pattern and the Bragg angle , P_k accounts for preferred orientation andA refers to effects such as absorption or extinction due to multiple scattering which is approximated this way in the kinematic theory of diffraction. A backgroun term is added, usually modelled with a Chebyshev polynomial.

In a multiphase sample of number of phasesj the intensity if the summation over contribution from different phases, which is accounted for by

I_i,calc =

N(phases)

X

j=1

s_j

N(peaks)

X

k=1

L_km_k|F_k,l|²S_j(2Θ_i−2Θ_k,j)P_k,jA_j +bkg_i (2.8)

The weighted residual function which is minimized contains a statistical weight w_i for the i-th reflection and is expressed as

RS =X

i

w_i(I_i,obs−I_i,calc)² (2.9)

Sample environment

Most of the experimental in-situ X-ray diffraction performed in this study was conducted at a number of different synchrotron beam times with similar experimental setups, usually using two-dimensional detectors. Modern area de- tectors are well adapted toin-situ studies on reactive systems. The acquisition times are low, commonly between 10 seconds and one minute and the count- ing statistics are excellent. Area detectors are less sensitive to sample-texture, which is indispensable during reaction or recrystallization events where crys- tallinity tends to change. One disadvantage of area detectors when compared to focussing beam geometries, or to analyzer crystals or even curved posi- tion sensitive detectors is that they are lower in angular resolution. Arguably the latter may therefore be more applicaple to the decoding process of multi- phase patterns with large degrees of peak overlap. However, it is found that the lower resolution imposes no significant restrictions in the case of borohy- drides produced by mechano-chemistry, since the resolution for these systems is limited by the sample nature itself. Fine grained powders are measured in Debye-Scherrer (Transmission) geometry in rotating Quartz- or borosilicate- glass capillaries. Rotation of the capillary largely eliminates preferred ori- entation effects. A number of non-ambient studies next to in-situ T-ramping were employed to different extents. High-pressure gas cells provide pressures of many hundreds of barP(H₂), which are commonly used to study rehydrogena- tion and here were chosen to anneal different samples in-situ. High-pressure diamond anvil cells were used to study the structural high-pressure behaviour of selected borohydrides between room pressure and 15 GPa. These are briefly discussed in the respective section where necessary.

2.2.2 Spectroscopy

Vibrational Spectroscopy

Both Raman and Infrared spectroscopy where employed, in collaboration with Hans Hagemann from the Department of Physical Chemistry of thge Uni- versity of Geneva, to study the molecular B-H vibrations of the BH₄-anion.

According to results of [46] the B-H vibrations do not show significant disper- sion, meaning that measurements at the zone center such as those performed by Raman and IR spectroscopy are a good approximation. Presumably due to the sample nature of ball-milled mixtures it has been impossible to pre- cisely measure the lower energetic lattice vibrations. In this study the focus was hence on the stretching and bending vibrations of BH₄. Different reviews deal with the vibrational spectroscopy on complex hydrides and the charac- teristic signatures of different bonding schemes between the BH₄ molecule and the metal center [77, 86, 92, 28, 106]. The temperature-evolution of Raman-

active bands has been used to study phase transition in the alkali-borohydrides [48, 47]. In this study monitor the bandwidth across temperature-induced phase transition in perovskite-type borohydrides to obtain information on hys- teresis and activation energies of BH₄ reorientations. The high-energy region around 2200-2400 cm⁻¹, where the stretching modes are located, is dominated by fermi-resonances and is of limited suitability both for the determination of band-shapes as well as foot-printing known compounds and different B-H..M bonding schemes.

Raman line width and position

The band width Γ and shape of IR as well as Raman signals contain infor- mation on the dynamics of the vibrating group. In the case of Raman this information is hidden in the autocorrelation function of the transition moment and the Raman polarizability tensor. By following the band width of suffi- ciently isolated bands in the Raman spectrum as a function of temperature the approximate energy barriers of reorientational motion can be extracted by fitting different models to the data.

Γ = Γ0+A·e^-V/RT (2.10)

where the activation energy in this case corresponds to the energetic reori- entation barrier of the borohydride ligand. In more complicated cases empirical models can be fit involving two or more activation energies.

Γ = Γ₀ +A·e^-V1/RT +B·e^-V2/RT (2.11) Quasi-Elastic Neutron Scattering

Coherent scattering probes collective motions spatially correlated. The elas- tic part thereby provides information on the position of atoms, as in neutron diffraction, or the shape of objects in imaging techniques. The inelastic con- tribution relates to the excitation spectrum in crystalline materials, shown schemateically in figure 2.3 and is mostly used to investigate the phonons.

Quasi-elastic neutron scattering (QENS) makes use of the incoherent con- tribution of the scattered waves. It provides information on single particles and is used to study different processes of solid state diffusion. In figure 2.3 it can be seen that QENS investigates processes due to small amounts of energy transfer and around zero energy transfer. In contrast to the coherent methods that probe collective motions, with QENS the stochastic motions of particles that are behind the processes involving small energy transfers (µeV-range, see figure 2.4) can be investigated. The geometry of their diffusive motion man- ifests itself in the elastic incoherent structure factor. In the experiment, the

Figure 2.3: Schematic spetrum of inelastic scattering processes. Quasi-elastic neutrons scattering QENS involves processes at very low momentum transfer at the far left of the spectrum, centered on the elastic line. The inset schematically shows the general shape of the EISF as function of momentum transfer.

scattered neutrons are measured as a function of momentum transfer Q and energy transfer ω. The experimentally obtained incoherent scattering function Sinc(Q, ω) is also called the dynamic structure factor. It is not to be confused with the the quantity denoted structure factor by crystallographers, whose squared modulus is obtained in experiment. S_inc(Q, ω) is related to the inco- herent intermediate scattering function Iinc(Q, t) by Fourier transform. This is particularly useful, since I_inc(Q, t) is the self-correlation function (no pair cor- relations in QENS) that directly describes the time-dependence of the relative positions of particles, and it is calculable from atomic coordinates in molecular dynamics simulations, hence relates very well to theory.

I_inc(Q, t) = 1 N

X

k=1

hexp{iQ•R_i(t)}exp{−iQ•R_i(0)}i (2.12) If the motion of particles explores a volume that is small compared to the interatomic distances, hence restricted in space, the dynamic structure factor can be split into two terms.

S_inc(Q, ω) =S_inc^el (Q) +S_inc^inel(Q, ω) (2.13) The first term is a purely elastic in nature. The second term is the in- elastic contribution manifests itself as broadening of the elastic line (see figure 2.4). Experimentally, this is of great use when molecular dynamics calculation are not available. The motion of particles moving in a restricted space can be described by the elastic incoherent structure factor (EISF). To obtain the EISF both the elastic line, which is modelled with a delta function, and the quasi-elastic broadening described as one or several lorentzian functions, are convoluted with the instrumental resolution function. The EISF as a func- tion of Q is then obtained from the observed integral intensity which contains

both elasticI_el and quasi-elastic (inelastic with very small energy transfer)I_qel contributions according to.

EISF = I_el(Q)

Iel(Q) +Iqel(Q) (2.14) The general shape of the EISF is that of a Bessel function, shown in figure 2.4. The specific shape is dictated by the motion the particle under investiga- tion performs, and will be discussed in chapter 3. Bulk diffusion has charac- teristic length and time-scales that do not lead to the lorentzian broadening.

This has its origin exclusively in localized stochastic motions, such as those of the borohydride anion, which can be considered a solid rotor [63].

Figure 2.4: Schematic representation of the contributions at very small momentum transfer. The broadened quasi-elastic contribution is centered at the elastic line.

It is particularly purposeful to study hydrogen-related effects with QENS, due to the large incoherent scattering length of the hydrogen isotope, which is listed and compared to other elements below (units in fm).

Element H D ¹¹B C O N

b_coh -3.741 6.674 6.65 6.648 5.805 9.300 binc 25.217 4.022 -1.300 0.285 0.000 2.241

The quasi-elastic intensity arises from different jump motions performed by the BH₄ group with characteristic residence times in the ps-range. The ex- perimental, Q-dependent EISF provides information on the geometry of these jumps, which can be very characteristic of the crystal structure. QENS is therefore a well-suited method to obtain experimental information on struc- tural dynamics in borohydrides. The data presented in this thesis were col- lected at the cold neutron time-of-flight (TOF) spectrometer FOCUS of the Laboratory for Neutron Scattering of the Paul Scherrer Institute. The Instru- ment is a direct geometry spectrometer, where the neutrons are pulsed and

controlled with a chopper after the spallation source and a fermi chopper in front of the monochromator of the spectrometer. The upper limit of the ac- cessible time scale at a wavelength of 4 ˚A is approximately 15 ps, however slower motions can be modelled if good instrumental resolution functions are determined. The results are presented in chapter 3.

Optical Spectroscopy

This thesis contains the first reports on luminescence in borohydride com- pounds. Optical emission, results as a spontaneous emission of photons after an electron relaxes from its excited state to a lower level, not necessarily the ground state. The excitation spectrum in a luminescence measurement, where the emission monochromator is fixed at a certain value and the excitation en- ergy is scanned in give spectral range, in principal corresponds to the signal measured in absorption spectroscopy. The emission spectrum is collected at an excitation wavelength selected with the excitation monochromator. Lumines- cence spectra were collected on a series of Lanthanide-borohydrides crystalliz- ing with the perovskite-type and discussed in paper 6, using a Fluorolog where the sample is excited by means of a Xenon lamp and the emission recorded with a photo-multiplier tube. An excited state has a certain lifetime, depend- ing on the excited ion itself and its accessible energy levels, but also on the chemical environment and properties of the crystal lattice, such as the phonon distribution or the nature of lattice defetcs. Highly allowed transitions, such as the d→ f transitions in divalent Ln²⁺ activators have fast transition rates and contribute to characteristically short lifetimes and high efficiencies of e.g.

Eu²⁺. The life-times of excited states were determined in the temperature- range between 10 K and room temperature, using closed-cycle helium cryostat and a CCD camera to record intensities, with the help of Antoine Tissot from the Department of Physical Chemistry of the University of Geneva. The time- resolution is given by the duration of laser pulses (5 ns) generated by a pulsed Nd:YAG laser using the third harmonic (355 nm, 20 Hz).

2.2.3 Thermal Analysis

Both differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to quantify enthalpies of phase transitions and evolution of gaseous species, where necessary. In particular the series ACa(BH₄)₃, where A = K, Rb, Cs, was studied with DSC to complement temperature depen- dant in-situ diffraction studies. The ramping speed in such experiments is considerably high in order to save time and it is advisable to perform comple- mentary experiments to detect thermal events (structural phase transitions, melting, etc) in equlibrium. The transition enthalpy of a polymorphic trans-

formation as well as the hysteresis provide information on the character of a phase transition, which can be of second or first order, and involve different mechanisms such as atom displacements and the different kinds of disporder introduced above. The findings are discussed in chapter 3. All thermal analy- sis performed here were done with a Netzsch 404 F3 Pegasus apparatus under nitrogen flow. Calibration runs on empty BN crucibles were run before the actual experiment to determine the baseline. The heating rate was 1 K/min if not specified otherwise.

2.2.4 Computational Methods

The calculations performed throughout this work were done in collaboration with L’ubomir Smr˘cok from the Slovak Academy of Sciences in Bratislava and will not be discussed at this point.