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MOLECULAR AND MAGNETIC ORDER IN ALKALI
HYPEROXIDES : A SHORT REVIEW OP RECENT
WORK
W. Känzig, M. Labhart
To cite this version:
1. Introduction. — It is well known to the physi- Mixed crystals hyperoxide-halide have not been pro-cists working in the field of color centers that the duced so far. The alkali hyperoxides can be considered molecule-ion O J can substitute for a halide ion in the as ionic crystals with the paramagnetic anion 02 . alkali halides. The O^ center has been of considerable They are among the rare candidates for 2p-electron interest in the past for several reasons : magnetic order. In fact, neutron diffraction experiments — It is an example of a relatively simple molecule have indicated long range antiferromagnetic order in with the ground state 2/7g in a well defined crystal field K° 2 below 7 K [20].
of orthorhombic symmetry, and it is amenable to a It turns out that the magnetic behaviour of the alkali detailed study by means of paramagnetic resonance [1, hyperoxides is rather complicated. The possibility of 2, 3]. molecular reorientation gives rise to a number of phase — Its luminescence spectrum gives detailed infor- transitions that are not driven by magnetic interactions, mation on the electronic and vibrational structure of
the ground and excited state and on the elastic coupling TABLE I
between the molecule and the host lattice [4, 5, 6, 7, 8]. Molecular and magnetic order — Its reorientation under mechanical stress applied /„ condensed molecular oxygen to the host lattice can be conveniently studied. The O J Phase
center represents one of the simplest elastic dipoles in transition temp. alkali halides [9, 10, 11, 12, 13]. at atmospheric
— The OJ-center lends itself to a quantitative pressure Molecular order Magnetic order
experimental and theoretical study of the interaction ,. , - , , , • , , ,
. . , .. .• , r i„ 1C 1iC n , _ . e liquid translational and disordered
(pa-between elastic dipoles [14, 15, 16, 171. The energy of . . , ,. . . „ . . . ~ - onentational di- ramagnetic)
the elastic interaction of nearest neighbour 02 centers ,
in alkali halides is of the order of 100 K. CA . . ,
54.4 K.
— The magnetic interaction between two nearest and ,., , . . ... , . ,. , , , ° _ y-oxygen solid cubic lattice, hin- disordered (pa-next nearest neighbour 02 centers can be mvestigat- d e r e d r o t a t i o n r a m a g n e t i c ) ed directly by means of electron paramagnetic reso- of molecules
nance [18]. The energy corresponding to the isotropic 43 8 K
part of the exchange is of the order 1 K and can be p ^ soM r h o r n b o h e dr a l short-range
anti-positive or negative depending upon the relative posi- ,a t t i c e > h i n d e r e d f e r r o m a g n e t i c tion and orientation of the partners of a pair. precession of order
It is not so well known, however, that crystals can molecules be produced in which the concentration of O^ centers 23.9 K
is so to speak 100 per cent. These compounds are the a-oxygen solid monoclinic lat- long-range anti-alkali hyperoxides, N a 02, K 02, RbOz and C s 02. tice, molecules ferromagnetic Single crystals can be grown in liquid ammonia [19]. parallel order
MOLECULAR AND MAGNETIC ORDER IN ALKALI HYPEROXIDES :
A SHORT REVIEW OF RECENT WORK
W. KANZIG and M. LABHART
Laboratory of Solid State Physics, Swiss Federal Institute of Technology, 8093 Zurich, Switzerland
Abstract. — The properties and the structures of the alkali-hyperoxides N a 02 and KO2 deduced from magnetic and caloric measurements, X-ray and neutron scattering, paramagnetic resonance, optic and spectroscopic investigations are reviewed with special emphasis on the single crystal work done in Zurich.
JOURNAL DE PHYSIQUE Colloque Cl, supplément au n° 12, Tome 37, Décembre 1976, page Cl-39
Résumé. — On donne un aperçu des propriétés et des structures des hyperoxides alcalins NaC>2 et KO 2, basé sur des mesures magnétiques et caloriques, sur la diffraction et la diffusion des rayons X et des neutrons, sur des expériences de résonance paramagnétique et sur des recherches optiques et spectroscopiques, avec égard spécial aux travaux du groupe de Zurich sur des monocristaux.
but nevertheless affect the magnetic properties via spin-orbit coupling and crystal field effects. Changes in magnetic and molecular order are thus inter- dependent, at least at low temperatures. A corres- ponding phenomenon has been observed earlier in condensed molecular oxygen as is summarized in table 1 [21, 221. There is, however, a very significant difference between solid molecular oxygen and the hyperoxides : The ground state of the oxygen molecule 0, is a
3Z,
state. Spin-orbit coupling can be considered a small second-order effect. The order of magnitude is 2 cm-'. The crystal field is also weak since one has a van der Waals crystal. On the other hand, the ground state of the hyperoxide ion 0; is'IZ,
with a spin-orbit coupling of the order 200 cm-'. Since the hyperoxides are ionic crystals the crystal field is expected to be strong. For example the orthorhombic crystal field parameter for the 0; center in alkali halides is of the order l 000 cm-' [2].The electronic structure of the hyperoxide ion 0; is not profoundly influenced by the compound of which it is a constituent. The frequency of the stretch- ing vibration which is near 1155 cm-' in all alkali hyperoxides is little affected by the various phase transitions 1191. Also the tensor of the optical polariza- bility does not depend upon the anion [23].
2. Structures and phase transitions. - The struc- ture determination of the alkali hyperoxides is not a
routine affair for the following main reasons.
-
In the phases with incomplete molecular orderPhase transition temp.
-
I Transition entropy-
R log 2.35 (first order) R log 2.46 (first order) very smallthe intensity of the Bragg peaks decreases rapidly with increasing diffraction order.
- As a consequence of the phase transitions the crystals consist of domains. If one fails to resolve the Bragg peaks due to different domains a symmetrized (average) structure results which contains little infor- mation about the phase transitions. In some cases important information on the true structure has to be taken from the scattering between the Bragg peaks (diffuse scattering and satellite reflections) and from a detailed analysis of the paramagnetic resonance spectra of single crystals.
With regard to the structural phase transitions there is a significant difference between NaO, and the group KO,, RbO,, CsO,.
2.1 SODIUM HYPEROXIDE, NAO,. - Table 11 gives a survey over the phases of NaO, based on X-ray and neutron diffraction [24], magnetic and caloric measu- rements [25] and optical data [19]. In the high tempera- ture phase I the Na+-ions form together with the centers of the 0; molecule-ions a NaCI-type cubic lattice. The equilibrium orientations of the anions are presumably
<
11 1>
and they undergo hindered rotation. Diffuse X-ray scattering indicates that the distribution over the four<
11 1>
axes is not entirely random. Correlations exist that correspond to pyrite- type local order. The structure of phase I can be desi- gnated as disordered pyrite-type structure. The crystals are grown in this phase.In phase II the pyrite-type orientational order is long range. Since there are two equivalent possibilities to
Survey over the phases of NaO,
Structure space group (of a domain)
-
disordered pyrite, average structure NaC1-type, cubic 0: (no domains) ordered pyrite, cu-bic T:, 2 domains marcasite orthorhombic D:: 12 domains marcasite orthorhombic 12 domains Molecular order - orientational disor- der with correla- tions
long range orienta- tional order
long range orientatio- nal order, molecules parallel in alternat- ing planes
long range orienta- tional order, mole- cules parallel in al- ternating planes Magnetic order effective magnetic moment Curie-Weiss temp.
-
paramagnetic .97 h o h r 8 = - 3 1 K paramagnetic .79 h o h r O = + 3 3 K short range magneticorder of unknown kind
MOLECULAR AND MAGNETIC ORDER IN ALKALI HYPEROXIDES 0 - 4 1
establish this order the crystals consist of crystallo- graphic domains.
The structure of phase 111 is of the marcasite-type. It can be derived from the pyrite-type structure by reo- rientation of every second anion combined with an orthorhombic deformation of the unit cell. Figure 1 "" illustrates the relation between the pyrite-type and the marcasite-type structure. There are six equivalent ways oi to transform a given pyrite-type domain into the marcasite-type structure. Consequently, crystals that are cooled from phase I over phase I1 into phase I11
PT,* Morcwle
have 12 domain orientations. These can be distinguish-
ed in X-ray and neutron-diffraction experiments on FIG. 1 . - Relation between the ordered pyrite structure and
single crystals. the marcasite structure.
The transition 111 -t 1V at 43 K manifests itself by a
small discontinuous decrease in the magnetic suscepti- 2 . 2 POTASSIUM HYPEROXIDE, KO,. - Potassium bility. It escapes observation by calorimetric measure- hyperoxide has at least six phases as is summarized in ments and X-ray diffraction. table I11 [24,25]. The (average) structure of phase I is of
Survey over the phases of KO,
Phase Symmetrized Magnetic order
transition temp. (average) Symmetry of eff. magn. moment
transition entropy structure a domain Molecular order Curie-Weiss temp.
-
- - - -I NaCl chemically instable, no detailed information
cubic no domains hindered rotation paramagnetic
x 4400K
unknown CaC, orthorhombic stacking of planes paramagnetic
tetragonal differing in molecu- 2.01 h 0 h r
lar orientation and 8 = - 1 8 K displacement. Weak
correlation. Diffrac-
I1 tion satellites very
diffuse 231 K
A S
x
0.06 R CaC, orthorhombic strongly correlated paramagnetictetragonal stacking, modulat- 2.10 k 0 h r
ed structure, presu- 8 = - 4 4 K
I11 mably antiphase
domains, sharp in- commensurate dif- fraction satellites referred to CaC,- cell, 3.40 a 197 K A S x 0.15 R (first order) 12 K A S z 0.37 R (first order)
v
7 K A S z 0.4 R (second order) V1ordered, molecules in paramagnetic a domain are paral- 1 -76 k b h r lel
commensurate super- 0 =
+
2.3 K structure referred toCaC, cell, 2 a
ordered, molecules local antiferromagne in a domain are pa- tic order ? meta-
rallel magnetic
the NaC1-type. as for NaO,. However, the equilibrium orientations of the anions and the correlations are expected to differ from those in NaO, since phase I1 does not have pyrite-type order. In the older literature the structure of the phases IT and IT1 is given as tetra- gonal and of CaC,-type. It is derived from the face centered NaC1-type disordered structure by ali- gning all the 0; molecule-ions parallel to the same cube edge and by stretching the face centered cell along this edge. Then one chooses a new tetragonal unit cell of half the volume with the same c-axis but with the a-axes rotated by 45". Halverson [26] has recognized long ago that the tetragonal CaC,-type structure can be at best a syn~metrized or average structure because of the Jahn-Teller effect. In fact recent X-ray diffraction experiments with single crystals indicate a larger cell of orthorhombic symmetry as is illustrated schematically in figure 2. The drawing corresponds to a tripling of the CaC,-type cell along the y-axis. This is a simplifica- tion. In phase IIT the X-ray diffraction patterns show satellites corresponding to an incommensurate super- structure of (3.40 _+ 0.02) a. This indicates that the stacking of the planes can fall out of step, i. e. that the tripling is not rigorous. (Similar diffraction phenomena are observed e. g. in NaNO, and in certain alloys exhibiting so called antiphase domains). The modula- tion of the structure is transverse, the ionic shifts being
syrnrnetr~zed (overoged) structure
CO C, - type tetrogonol
what \S ovecoged over
7 Q Q
KO2 ~n phcses
I
and5
FIG. 2.
-
Relation between the symmetri~ed (average) structure and the structure of an orthorhombic domain in the phases I1and I11 of KO2 (schematic).
along the x-axis. Note that a faultless tripling in the sense of figure 2 would correspond to a net electric polarization, but that such a polarization is apparently prevented by the formation of stacking faults. Tn phase I1 the satellite reflections are very diffuse and they condense into rather sharp peaks at the transition into phase 111. The superperiod 3.40 a seems to be temperature independent in the narrow temperature range in which phase I11 is stable. An accurate deter- mination of the structure parameters of phases 11 and 111 is not possible. Extrapolating from phase IV, where the conditions for an X-ray analysis are more favorable, one expects transverse shifts of the order of a few thenth of an Angstrom unit, but it cannot be said whether there is also a tilt of the molecular axes with respect to the c-axis of the CaC,-type pseudocell. The crystals are grown in phase IT. The orthorhombic distortion is so small that domains form during growth since the modulation can propagate along either a-axis of the tetragonal pseudocell. The Bragg peaks due to different domains could not be resolved in the X-ray diffraction patterns, however, the orthorhombicity can be inferred from the fact that the volume ratio of the two possible domains varies from crystal to crystal as can be seen by comparing the intensities of the
satellite reflections.
The unit cell of phase 1V is clearly monoclinic and twice as large as the CaC,-type pseudocell as shown in figure 3 [27]. If one interprets this as a superstructure,
FIG. 3. - Structure of phase IV of KOz. (Reference [27].)
MOLECULAR AND MAGNETIC ORDER I N ALKALI HYPEROXIDES C7-43
Phase V emerges from phase IV by a deformation that lowers the symmetry to triclinic and thus further complicates the domain structure. The determination of the structure is in progress. Paramagnetic resonance indicates that the molecules in a domain are parallel and that the above defined tilt angle jumps from 210 to 300 at the first order transition IV -r V.
Neutron diffraction has indicated that the second order transition V -+ V1 leads to long range anti- ferromagnetic order [20]. It is not yet known wether it is connected with a change of the molecular arrangement.
RbO, and CsO, undergo similar phase transitions as KO, 125, 241. In particular, there is also a phase 111 showing satellite reflections corresponding to an incommensurate superstructure of (3.12 & 0.02) a for
RbO, and (3.45 +_ 0.02) a for CsO,.
3. Magnetic properties of alkali hyperoxides.
-
3 . 1 SODIUM HYPEROXLDE [25]. - Figure 4 shows the temperature dependence of the magnetic suscepti- bility of a sample consisting of a large number of small single crystals grown in liquid ammonia. In the disor- dered and the ordered pyrite phase the material is paramagnetic. At the transition into the marcasite-type phase the susceptibility drops and then decreases with decreasing temperature, suggesting some kind of magnetic order. Neutron diffraction indicates that this order is not long range. The changes of magnetic order that occur at the transition 111 -+ IV are stillobscure. The increase of the magnetic susceptibility at low temperatures corresponds to a dilute para- magnet with a concentration of about one mole per cent. Its origin is not known. It could be due to non- stoichiometric composition or to 0; molecule-ions in the walls of crystallographic domains. If one saturates the paramagnetic contribution in high magnetic fields a susceptibility remains that decreases with decreasing temperature. (In poor quality samples the para- magnetic contribution to the susceptibility a t low temperatures is much larger.) There is no evidence for metagnetic behaviour in fields up to 225 kOe.
FIG. 4.
-
Temperature dependence of the magnetic suscepti-bility of polycrystalline NaOz. (Reference [25].)
3.2 POTASSIUM HYPEROXIDE [25].
-
The tempe- rature dependence of the reciprocal magnetic sus- ceptibility of KO, is given in figure 5. The phases I, 11, 111 and IV are paramagnetic. There is no profound change of the magnetic behaviour at the phase tran- sitions II -r 111 and 111 -+ IV, which may be due tothe fact that the patterns of molecular order in these phases are similar. Nevertheless, in view of the beha- viour of NaO, in the marcasite-type phase it is surpris- ing that the parallel ordering of the molecules in phase IV of KO, does not manifest itself drastically in the magnetic susceptibility. It is conceivable that the model depicted in figure 3 represents an average struc- ture. It cannot be said what is averaged over. If it is a motion between two different equilibrium orientations of the anions its frequency has to be so high that it escapes observation by means of paramagnetic reso- nance.
FIG. 5. - Temperature dependence of the reciprocal magnetic
susceptibility of polycrystalline KOz. (Reference [25].)
Recent paramagnetic resonance experiments indicate a rather drastic change of the state of magnetic order (disorder) at the first order transition
TV
-+ V at 12 K.The paramagnetic resonance lines which are about 200 gauss wide in phase TV narrow to about one half of this width at the transition 1V -+ V indicating an increase of exchange interaction. There is no evidence for long-range magnetic order in phase V. Presumably there is local antiferromagnetic order.
The transition into the antiferromagnetic phase V1 is just barely discernible in the susceptibility of a poly- crystalline sample. However, there is an anomaly of the specific heat that clearly indicates a second-order phase transition.
The long range antiferromagnetic order in phase V1 as well as the short range (antiferromagnetic ?) order in phase V can be broken up in a magnetic field of about 70 kOe. The resulting magnetization corresponds at sufficiently large fields to almost full alignment of all anion magnetic moments.
center in the alkali halides is well understood with the exception of NaI. The orthorhombic crystal field requires the introduction of a single crystal field para- meter A , which is of the order of magnitude
1 000 cm-', and the admixture of a single excited state
2 ~ : to the ground state is sufficient to arrive at a
satisfactory quantitative interpretation of the g- tensor [2]. For a magnetic field along the molecular axis the g-factor is between 2.3 and 2.5 and perpen- dicular to it between 1.93 and 1.98, depending upon the matrix. The paramagnetic resonance spectrum of nearest neighbour pairs of 02 centers indicates that the g-tensors of the partners of a pair are rather close to the g-tensor of single 0; centers [l81 in the same matrix crystal.
Unfortunately the paramagnetic relaxation times are so short that an accurate investigation of the g-tensor (with a Q-band spectrometer) can only be made below about 50 K. A detailed investigation of the para- magnetic resonance of 0; in the phases IV and V of carefully oriented KO, single crystals permitted a determination of the orientation of the molecular axis
(see section 2.2). The principal components of the
g-tensor are listed in table IV. Since the crystal field in KO, has lower than orthorhombic symmetry the interpretation of these g-tensors cannot be taken over from the 0; center work [l, 21. At the transition IV -t V the axes of the g-tensor change their orienta-
tion, but their magnitude remains unaltered within a rather small experimental error, suggesting that there is very little change of the electronic structure at the onset of local magnetic order. The contrary observation is made at the approach of the transition V -+ VI. Here the apparent principal values change whereas the orientation remains.
Principal components of g-tensor of 0; in KO,
single crystals, corrected for demagnetization and dipole-dipole interaction. The z-axis corresponds to the internuclear axis of the molecule.
gxx g y y g z z Error
-
- --
Phase IV (1 3 K) 1.975 2.004 2.259
+
0.005 Phase V (9 K) 1.970 1.996 2.275+
0.005 4. Lattice dynamics of sodium hyperoxide. - The main results of an investigation of the lattice vibrations of NaO, single crystals by means of Raman spectro- scopy [28], far infrared reflection [l91 and inelastic neutron scattering [29] can be summarized as follows :- The stretching frequency of the 0; molecule-ion changes very little at the phase transitions.
-
The restrahlen bands are centered near 240 cm-' indicating that the binding is not very different from the binding in NaCI.-
There are reflection bands centered near 100 cm-' due to the fact that the long range molecular order in the marcasite-type phase and the ordered pyrite-type phase demand a unit cell that is larger than the primitive NaC1-type cell. Smeared out these bands survive the transition into the disordered pyrite-type phase thus confirming the local order detected by means of X-ray scattering.The far infrared reflection spectra are given in figure 6.
0 100 200 300 400
--D- W (cm-')
FIG. 6.
-
Far infrared reflection spectra of NaOz singlecrystals. (Reference (191).
5. Electronic spectra. - The 0; centers in the alkali halides give rise to an absorption band centered at 5.1 eV for KCl. It corresponds to the transition
211,-2h', of the 0; molecule-ion [2, 301. A weak
absorption band at 6.4 eV was interpreted as a transi- tion corresponding to charge transfer to the host lattice [30]. e
-
T-
'.-
.- > ....
-
U o a l .-
-
t! 9. 4-FIG. 7.
-
Ultraviolet reflection spectra of NaOz single crystalsMOLECULAR A N D MAGNETIC ORDER IN ALKALI HYPEROXIDES C7-45 The present information on the electronic spectra of
the alkali hyperoxides is still very limited. It has not been possible so far to produce thin films of these compounds suitable for ultraviolet transmission spec- troscopy. The spectra of powders are not trustworthy because of the highly reactive surface. (The magnetic susceptibility of commercial hyperoxide powders deviates significantly from the susceptibility of care- fully crystallized material.) The honey-yellow-orange color of single crystals is presumably due to the intra- molecular transition
'II,
+ ' A , .Reproducible reflection spectra of the surface of NaO, single crystals could be obtained for all phases (Fig. 7). The structured band at about 8 eV has been assigned to the r-exciton. The assignment of the other bands is still open to speculation. It is surprising that the intramolecular transition 217, '17, (which is dominant in the optical absorption of the 0; e n t e r s in the alkali halides) does not manifest itself clearly. The great puzzle, however, is the reflection band at 2.5 eV which appears at the transition into the mar- casite phase.
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DISCUSSION
F. LUTY. - YOU started out with the isolated 0; tern of the anions. I am not optimistic about predic- elastic dipole defect, and a pair of these defects for tions or extrapolations based on the interaction of which the elastic dipole interaction can be calculated elastic dipoles in the alkali-halide matrix. At best and understood. Is there any way that this elastic the order of magnitude of the ordering temperature dipole (+ interaction) picture can be extrapolated can be predicted.
and used in the crystal with 100
%
0; defects D,not to understand f i e details of the structure, but B-
R.
CHowDAR1. - you 'Omment about some gross featurer e. g. why the elastic dipoles go the potentiality of these systems to study radiationbasically parallel (fei-ro-elastic) in KO,, but crossed effects ? ((c antiferroelastic n) in NaO, ?
