Structure of D1.65MoO3 by neutron diffraction

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Structure of D1.65MoO3 by neutron diffraction

M. Anne, Daniel Fruchart, S. Derdour, D. Tinet

To cite this version:

M. Anne, Daniel Fruchart, S. Derdour, D. Tinet. Structure of D1.65MoO3 by neutron diffraction.

Journal de Physique, 1988, 49 (3), pp.505-509. �10.1051/jphys:01988004903050500�. �jpa-00210723�

Structure of D1.65MoO3 by ^neutron diffraction

M. Anne, ^D. Fruchart, ^S. Derdour and D. Tinet (*)

Laboratoire de Cristallographie du CNRS associé à l’USTMG, 166X, 38042 Grenoble Cedex, ^France (*) CRSOCI-CNRS, ^1B

^{de la} Férollerie, 45071 Orléans Cedex 2, ^France

(Requ le 17 decembre 1985, révisé le 30 novembre 1987, accepté ^le 3 decembre 1987)

Résumé.

La structure de D1.65MoO3

été affinée dans le groupe d’espace C2/m. L’étude par diffraction des neutrons de D1,65MoO3

^forme polycristalline,

permis de localiser la moitié du deutérium dans la structure, l’autre moitié étant dispersée

de nombreux sites cristallographiques. L’insertion de deutérium dans le réseau MoO3 n’introduit que de faibles modifications structurales.

Abstract.

The structure of D1.65MoO3 has been refined in the space group C2/m. Half of the deuterium in

D1.65MoO3 has been localized using powder ^neutron diffraction, the other half being dispersed

^numerous

sites throughout ^the structure. Insertion of deuterium in the MoO3 ^network produces only weak structural modifications.

Classification

Physics ^Abstracts

61.12

66.30F

1. Introduction.

The Hl,6sMo03 ^{bronze is a} phase ^{of the} hydrogen molybdenum ^bronze (HxMo03, 0 x 2 ) series, which has been characterized by ^its X-ray analysis [1], by its electronic properties [2] ^and by ^{’H and} 2D NMR [3-7].

The crystallographic ^{structure of} Do,36Mo03, ^de-

termined by ^neutron diffraction by Dickens et al. [8],

shows that the introduction of 0.36 deuterium atom per unit formula produces only slight modifications of the structure of the Mo03 ^network.

The insertion of hydrogen ⁱⁿ molybdenum trioxide, up ^{to a} H/Mo ratio of 1.65 defines a phase clearly identified by X-ray diffraction [1]. ^{The struc-}

tural modifications are still weak. There is however a

lowering ^{of the} symmetry ^{from an} orthorhombic to a

monoclinic lattice.

Furthermore, ^{from NMR} measurements, hydro-

gen in Hl,6sMo03 ^{is not static. Some authors} [3, ^4, 6]

have split ^the hydrogen motion into a diffusion process and ^a rotation of OH2 entities around the oxygen ^atom. These results are now still under discussion and it is important ^{to have a better} knowledge ^of hydrogen localization by ^neutron

diffraction.

Samples ^were prepared by ^the hydrogen spillover technique [9]. ^{The amount of absorbed} hydrogen

was determined by volumetric measurement. Dif- fraction studies were performed ^{on a deuterated}

sample ^since deuterium has a very low incoherent

scattering ^cross section and a higher ^{coherent cross}

section than hydrogen.

2. Experimental ^method.

The powder diffractograms ^{were obtained} using ^a position ^{sensitive detector at the} Institut Laue-

Langevin (D1B) ^{and at} the Nuclear Research Center

(Siloe) ⁱⁿ Grenoble, ^{at room} temperature ^{and 4.2 K.}

The wavelength ^{was A}

^{1.2818 A} (D1B) ^{with an} angular range 8° 2 0 88° in steps ^{of 0.1°. The} coherent scattering lengths ^{used were :}

The results were analysed by the least squares method of profile refinement given by ^Rietveld [10],

modified so that the optimized results could further be treated by ^Fourier synthesis [11].

The optimization process ^{is based on} the weighted

value of the count at each point ^{of the} profile, leading ^{to the} minimization of a weighted reliability

factor. In order to compare these results with those obtained by integrating ^the intensities, the observed and calculated profiles ^were separated into individual intensities, ^thus defining ^an agreement ^{factor :}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01988004903050500

506

where C is a scaling ^factor.

3. Results.

The diffraction diagrams ^{show no} significant ^differ-

ence between 300 K and 4.2 K but only ^{the results at}

low temperature will be considered here, ^{as smaller} thermal vibrations are favourable to a better locali- zation of atoms.

The precise indexation of the diffraction peaks

corroborates the monoclinic lattice given by X-ray

diffraction [1]. The model is first composed ^{of the}

Mo and 0 atoms of Mo03 ^{and its} parameters ^refined in the monoclinic unit cell (the ^atomic positions ^and

widths at half height ^{of the} peaks). ^The optimized

unit cell parameters ^{are :}

An examination of all indexed peaks ^{in the} diffractogram shows that all the reflexions with non- zero intensities are compatible ^{with a} monoclinic C- face centred lattice. This gives ^a choice between the space group C2/m and the ^two subgroups ^{C2 and Cm}

which have the same extinction rule h + k

2 n + 1, ^{as found} experimentally.

It must be emphasized ^that the transformation of the orthorhombic group Pnma of Mo03 ^{into the}

monoclinic C2/m (or ^its subgroups) ^{cancels a mirror} plane since 13 =A ^90°.

The space group finally chosen for the structure of

Dl.65M003 ^is C2/m, ^Z

^{4. The} description by ^either

of the two subgroups ^{does not} improve the agree- ment factor.

Furthermore, ^no indication of any superstructure

could be found since there were no extra peaks.

A Fourier difference synthesis, calculated after the refinement of the position parameters ^{of the}

molybdenum and oxygen atoms, shows that an excess of nuclear density ^{could be} attributed to a

deuterium atom. Introducing ^{this new atom in the} model, ^and refining ^its position parameters improves

the agreement ^factor by ^{6 %. The} refinement of all parameters including ^atomic positions, temperature factors and site occupancy of deuterium, _converges

to an agreement ^factor R,

14.8 %. The optimized parameters ^are reported ^{in table I and} figure ¹

shows the agreement between the observed and

Table I.

Structural parameters ^{at 4.2 K.} Space Group : ^C2/m.

Fig. 1. - a) Comparison between the observed (+) and calculated (full line) profiles ^for D1.65Mo03 ^{at 4.2 K.}

b) Difference spectrum.

calculated profiles. Interatomic distances and bond

angles ^{in the structure of} D1.65Mo03 ^{are given in}

table II.

Table II. - Interatomic distances and angles ⁱⁿ D1.6SMo03.

The 8 ( j ) site of deuterium is only ^{41 %} occupied corresponding ^{to 0.82 atom of} deuterium per unit formula as compared ^to the value 1.65 found vol-

umetrically.

The improved ^{model is not} entirely satisfactory

since the reliability factor is still relatively high.

Further attempts ^{to include} other sites for

deuterium, corresponding ^{to weaker} density ^maxima

revealed by ^a Fourier difference map, ^{were not} conclusive. Some sites could be in the vicinity ^{of the} M006 octahedra, ^but they ^were very close ^to the atoms of the mean structure and the strong ^corre-

lations between the position parameters ^{do not allow} them to be refined accurately. Finally ^{a refinement}

of a model with the deuterium located in the site chosen by ^{Dickens et al.} [8], ^{in the} orthorhombic

phase Do,36Mo03 ^{did not} converge.

4. Discussion.

The increase of the hydrogen concentration (from

x

0.36 to x

1.65) ^{leads to a} partially ^disordered

structure which could not be completely ^resolved.

The disorder is due to half the deuterium atoms

statistically distributed on numerous sites inducing

local distorsions with repercussions ^{on the other}

atoms. This static disorder implies high ^thermal parameters. Nevertheless, the average ^structure of

Dl-65M003 ^is determined by ^neutron diffraction and half the deuterium atoms are localized. The structure is shown in figure ² together ^{with the} corresponding

structure of Do.36Mo03. ^Both structures can be described in terms of edge ^and corner-shared

Mo06 ^{octahedra more or less} distorted but the D atom sites are different.

In the orthorhombic phase DO.36MOO3 ^the

deuterium atoms, symmetrically ^related by ^{a mirror} plane, ^{are located in} the y

0 and y = 1/2 planes

defined by layers of Mo and 0.

In the monoclinic phase Dl.6sMo03, ^the

deuterium atoms form chains perpendicular ^{to these} planes, midway between the zig-zag ^{chains of} molyb-

denum atoms. The presence of ^more deuterium

atoms near the oxygen ^atoms modifies the 0-0 distances and this is probably ^{the cause of the}

monoclinic distortion which cancels the mirror plane relating ^{the deuterium atoms} in the orthorhombic

phase. Figure 3 shows the periodic pattern ^{of the} deuterium chains. The 0---0 lines form a grid ^on

which the deuterium atoms are fixed occupying ^half

the space between the oxygen ^atoms.

This arrangement ^of the deuterium atoms in

D1.65MOO3 has been described very approximately, by Slade et al. [7], ^{but these authors} suggested ^that

all the sites between the oxygen ^{atoms were} avail-

able, the deuterium atoms being statistically ^distri-

buted among 42 % of these sites (required by stoichiometry). ^This symmetrical distribution of the deuterium could not explain ^the origin ^{of the mono-}

clinic distortion.

Figure ⁴ gives ^{the distances and} angles ^between

the atoms. The distance between the A and B sites is

so short that the two sites can not simultaneously ^be occupied by deuterium. The maximum occupancy ^of the crystallographic sites will then be only ⁵⁰ %, ⁱⁿ agreement with the 41 % value obtained by ^diffrac-

tion.

The amplitude ^{of the thermal vibration around the}

deuterium site is very high. ^{This is} probably ^{due to a} dynamic disorder. The projection ^{on the 0---0 line}

of all nuclear density ^observed around the A and B sites (Fig. 5) clearly shows that the hydrogen ^{atom in}

A or B vibrates anisotropically ^{towards the other}

site. The shape ^{of this curve of nuclear} density

suggests that the deuterium ^atom is weakly ^{bound to}

508

Fig. ^2. - Comparison between the structure of Do,36Mo03 given by Dickens et al. [8] ^{and of} D1.65Mo03 (this work).

Fig. ^3.

Location of deuterium sites in the interlayer plane ^of Mo03, projected ^{on the} (b, c) plane. ^Crossed

oxygens ^are at y

0, the others at y ^{= 1/2.}

its site and can move easily ^{from A to} B and vice-

versa.

The D1.65Mo03 compound ^{is a} good protonic

conductor [3, 5], ^{and its} study by ^neutron diffraction has provided important ^{facts for} understanding hy- drogen ^motion through the lattice :

-

the ^structure is partially disordered due to numerous sites for the deuterium ;

-

one half of the deuterium is localized on a well defined crystallographic ^{site, with an} occupancy of

41%;

-

the observed nuclear density ^favours jumps ^of hydrogen ^{atoms between two sites} separated by

0.9 A (localized motion) ;

-

the number of hydrogen ^{atoms is} less than the

Fig. ^4.

Distances and angles between deuterium and oxygen atoms, crossed oxygen is in the plane ^y

0, ^the

two others

in the planes y

^{± 1/2.}

Fig. ^5.

^Nuclear density of deuterium around the A and

B sites showing the weak localization

A or B.

number of available sites, ^{which is a} necessary condition for translational diffusion ;

- the unsymmetrical arrangement ^{of the} hydro-

gen ^atoms around the oxygen ^atoms in the monoclin- ic lattice does not allow 180° reorientation of pseudo OH2 ^entities.

5. Conclusion.

This work shows that there are two species ^of protons ⁱⁿ Hl.65M003. ^{A first} species ^is widely dispersed in the lattice and a second type ^of proton

has been localized by ^neutron diffraction. The latter is probably involved in a diffusional motion by hopping ^{from site to} site, through the lattice. This

dynamical aspect ^{has been studied} by quasielastic

neutron scattering ^{and will be discussed in a forth-} coming ^article [12].

Acknowledgments.

We are grateful ^{to Dr.} J. Pannetier for helpful

discussion and his cooperation ⁱⁿ carrying ^{out the} experiments with the multidetector D1B at the Institut Laue-Langevin.

Addendum.

Since the submission of this work for publication,

two new articles on the hydrogen molybdenum

bronze series have been published [13, 14]. Very recently, ^{a second structure} determination of

Dl.7MOO3 ^has been carried out [15] ^{with results}

similar to ours in some aspects. Nevertheless, ^{in this} study, the authors have introduced in the refined model twice as many deuterium atoms as us. Since such a model could not be fitted to our data with

enough accuracy, it had previously ^been rejected.

References

[1] ^{BIRTILL, J. J. and DICKENS, P. G.,} Mat. Res. Bull. 13

(1978) ^311.

[2] ^TINET, D., CANESSON, P., ESTRADE, ^{H. and} FRIPIAT, J. J., J. Phys. Chem. Solids 41 (1979)

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[4] CIRILLO, A., RYAN, L., GERSTEIN, ^{B. C. and} FRIPIAT, J. J., J. Chem. Phys. ⁷³ (1980) ^3060.

[5] SLADE, ^{R. C.} T., HALSTEAD, ^{T. K. and} DICKENS, P.

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