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Orientational disorder in perovskite like structures of Li2X(OD) (XCl, Br) and LiBr · D2O
Physica B 234-236 (1997) 48-50
ELSEVIER
Orientational disorder in perovskite like structures of Li2X(OD) (X = C1, Br) and LiBr. D20 C. Eilbracht a, W. Kockelmann b, D. Hohlwein ~, H.
J a c o b s a'*
aAnorganische Chemie 1 der Universitiit Dortmund, D-44221 Dortmund, Germany bISIS, Rutherford Appleton Laboratory, Chilton, UK cInst. J~r Krist., Univ. Tiibingen, Germany; c/o Hahn-Meitner-Insitut, Berlin, Germany
Abstract
The crystal structures of Li2X(OD) (X = El, Br) and LiBr. D 2 0 were found closely related to the perovskite structure type of Li3BrO by means of single crystal X-ray structure determinations, temperature-dependent X-ray and neutron powder diffraction measurements. For Li2CI(OD) and LiBr. D20 a first order transition from orthorhombic to cubic symmetry was detected. In the case of Li2Br(OD) a phase transition without symmetry change was observed by neutron powder diffraction. Solid state 2H-NMR spectroscopy indicates a dynamical orientational disordering of the molecules OD- and D20 in the high-temperature (HT) modifications. Temperature-dependent neutron powder diffraction experiments show a nonspherical deuteron distribution in these phases which can be described with split models. Jump processes of the deuterons in the HT phases are probable. Corresponding accumulation of deuterium density in the HT phases of Li2CI(OD) and LiBr. D20 was found but is rather different from the orientation of D in OD- for Li2Br(OD). These results can be explained according to the different acceptor/donor strength of hydrogen bonds in these compounds.
Keywords: Crystal structures; Inorganic compounds; Phase transitions; Proton dynamics; Powder diffraction; NMR
Molecular dynamical disorder, phase transitions and ionic conductivities in Li2X(OD) (X = C1, Br) and LiBr. D 2 0 are the points of interest for these compounds [1-3]. Their cubic high-temperature modifications (HT) are closely related to the perovskite structure type of Li3BrO but with partially occupied Li-sites (Table 1). These structures bear a formal similarity in some respects to that of the classic electrolyte phase ~-AgI. Deuterated samples were investigated by neutron powder diffraction to get information about the deuteron density in the cubic modifications Where deuterium and lithium are disordered and to determine the structures of the low temperature modifications. * Corresponding author.
For Li2CI(OD) and L i B r ' D 2 0 a mainly firstorder transition from orthorhombic (LT) to cubic (HT) symmetry at T = 57°C and 32°C, respectively, was detected by X-ray powder investigations, neutron diffraction and DSC measurements. In the case of Li2Br(OD), we observed a discontinuity of the thermal expansion of the lattice parameter and volume at about 60°C. The cubic symmetry was found preserved by X-ray and neutron powder diffraction [4]. Solid state 2H-NMR spectroscopy indicates a dynamical orientational disordering of the hydroxide ions in the high-temperature modification of Li2CI(OD) and Li2Br(OD). In the low temperature modification the quadrupole coupling constant (QCC _-_280 kHz) and the asymmetry parameter (t/,,~ 0) for both compounds are characteristic of
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PI1 S 0 9 2 1 - 4 5 2 6 ( 9 6 ) 0 0 8 7 4 - 5
C. Eilbracht et al. / Physica B 234-236 (1997) 48-50
49
Table 1 CaTiO3 related types of structure: space group Pm3m Compound
CaTiO3 Li3BrO Li2X(OD) LiBr- D20
Wyckoff position
Occupation of side
lb
la
3d
3d
Ca Br CI/Br Br
Ti O O O
O Li Li Li
1 1 2/3 1/3
static hydroxide ions. E. Weiss interpreted the narrowing of the proton resonance line at T¢ by reorientation of water molecules in the HT-modification of LiBr • D20 [3]. Neutron powder diffraction data of all compounds in the HT-phases as well as the results of X-ray single crystal structure determinations of HT-LizCI(OD) and Li2Br(OD) fit with an antiCaTiO3 structure type concerning Li, O, and halide positions. Neutron powder analysis of the three compounds leads to a nonspherical deuteron distribution in the orientational disordered phases. For the HT-modifications of LiBr-D20 and Li2CI(OD) the structure refinements of the neutron diffraction data and the nuclear densities calculated from the observed structure factors support split models where the deuterons are located towards the octahedral faces of the Li6 octahedron (Figs. 1 and 2). The low temperature modifications of Li2CI(OD) and LiBr-D20 show weak superstructure reflections. Only small orthorhombic splitting of reflections made the structure determinations rather difficult and are still in progress. Neutron powder diffraction measurements from 40 up to 420 K were carried out on Li2Br(OD). In the low-temperature phase, deuterium positions could be refined on a 12-fold site, where the deuterons of the hydroxide ions point to the edges of a surrounding Li6 octahedron (Fig. 2). The above mentioned NMR-investigations and this refinement point out that the hydroxide ions are statically disordered in the [xx0] directions. The isotropic thermal parameters of Li, Br, and O show no conspicuous increase in the dynamically
Fig. 1. Unit cell of the cubic modification of LiBr. D20 at T = 330K.
(1/
LiBr*D20, T= 318 K
\
Li2Br(OD), T= 40 K
O LiBT * ~ O
Fig. 2. Fourier cut parallel to the triangular octahedral face of the OLi6 polyhedra of LiBr- D 2 0 and Li2Br(OD) ([xxx] with x = 0.150).
disordered high-temperature phase. The position of deuterium can also be refined on the 12-fold site (12i) as in the LT-modification but with a markedly higher Debye-Waller factor. The 2H-NMR spectra indicate a reorientation process with at least cubic symmetry. Taking into account the results of the neutron powder diffraction a 12-fold-jump process of the deuterons between the octahedral edges is probable. Different nonspherical deuteron distributions were found in the orientational disordered phases of LiEX (OD) (X = Cl, Br) and LiBr. D20. In the
50
C. Eilbracht et al. / Physica B 234-236 (1997) 48-50
case of HT Li2CI(OD) and LiBr. D20 the accumulation of deuterium density towards the octahedral faces can be explained by the stronger hydrogen bond acceptor/donor strength compared with Li2Br(OD) which favours hydroxide orientations towards the halides. For Li2Br(OD) the repulsive forces of the 2 statistically filled Li-octahedra around O D - seem to dominate the hydroxide orientation.
References [1] P. Hartwig, A. Rabenau and W. Weppner, J. Less-Com. Met. 78 (1981) 227. [2] R. Wortmann, S. Sitta and H. Sabrowsky, Z. Naturforsch. 44b (1989) 1348-1350. [3] E. Weiss, H. Hensel and H. Kfir, Chem. Ber. 102 (1969) 632-642. [4] C. Eilbracht, PhD Thesis, University of Dortmund (1997)
ELSEVIER
Orientational disorder in perovskite like structures of Li2X(OD) (X = C1, Br) and LiBr. D20 C. Eilbracht a, W. Kockelmann b, D. Hohlwein ~, H.
J a c o b s a'*
aAnorganische Chemie 1 der Universitiit Dortmund, D-44221 Dortmund, Germany bISIS, Rutherford Appleton Laboratory, Chilton, UK cInst. J~r Krist., Univ. Tiibingen, Germany; c/o Hahn-Meitner-Insitut, Berlin, Germany
Abstract
The crystal structures of Li2X(OD) (X = El, Br) and LiBr. D 2 0 were found closely related to the perovskite structure type of Li3BrO by means of single crystal X-ray structure determinations, temperature-dependent X-ray and neutron powder diffraction measurements. For Li2CI(OD) and LiBr. D20 a first order transition from orthorhombic to cubic symmetry was detected. In the case of Li2Br(OD) a phase transition without symmetry change was observed by neutron powder diffraction. Solid state 2H-NMR spectroscopy indicates a dynamical orientational disordering of the molecules OD- and D20 in the high-temperature (HT) modifications. Temperature-dependent neutron powder diffraction experiments show a nonspherical deuteron distribution in these phases which can be described with split models. Jump processes of the deuterons in the HT phases are probable. Corresponding accumulation of deuterium density in the HT phases of Li2CI(OD) and LiBr. D20 was found but is rather different from the orientation of D in OD- for Li2Br(OD). These results can be explained according to the different acceptor/donor strength of hydrogen bonds in these compounds.
Keywords: Crystal structures; Inorganic compounds; Phase transitions; Proton dynamics; Powder diffraction; NMR
Molecular dynamical disorder, phase transitions and ionic conductivities in Li2X(OD) (X = C1, Br) and LiBr. D 2 0 are the points of interest for these compounds [1-3]. Their cubic high-temperature modifications (HT) are closely related to the perovskite structure type of Li3BrO but with partially occupied Li-sites (Table 1). These structures bear a formal similarity in some respects to that of the classic electrolyte phase ~-AgI. Deuterated samples were investigated by neutron powder diffraction to get information about the deuteron density in the cubic modifications Where deuterium and lithium are disordered and to determine the structures of the low temperature modifications. * Corresponding author.
For Li2CI(OD) and L i B r ' D 2 0 a mainly firstorder transition from orthorhombic (LT) to cubic (HT) symmetry at T = 57°C and 32°C, respectively, was detected by X-ray powder investigations, neutron diffraction and DSC measurements. In the case of Li2Br(OD), we observed a discontinuity of the thermal expansion of the lattice parameter and volume at about 60°C. The cubic symmetry was found preserved by X-ray and neutron powder diffraction [4]. Solid state 2H-NMR spectroscopy indicates a dynamical orientational disordering of the hydroxide ions in the high-temperature modification of Li2CI(OD) and Li2Br(OD). In the low temperature modification the quadrupole coupling constant (QCC _-_280 kHz) and the asymmetry parameter (t/,,~ 0) for both compounds are characteristic of
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PI1 S 0 9 2 1 - 4 5 2 6 ( 9 6 ) 0 0 8 7 4 - 5
C. Eilbracht et al. / Physica B 234-236 (1997) 48-50
49
Table 1 CaTiO3 related types of structure: space group Pm3m Compound
CaTiO3 Li3BrO Li2X(OD) LiBr- D20
Wyckoff position
Occupation of side
lb
la
3d
3d
Ca Br CI/Br Br
Ti O O O
O Li Li Li
1 1 2/3 1/3
static hydroxide ions. E. Weiss interpreted the narrowing of the proton resonance line at T¢ by reorientation of water molecules in the HT-modification of LiBr • D20 [3]. Neutron powder diffraction data of all compounds in the HT-phases as well as the results of X-ray single crystal structure determinations of HT-LizCI(OD) and Li2Br(OD) fit with an antiCaTiO3 structure type concerning Li, O, and halide positions. Neutron powder analysis of the three compounds leads to a nonspherical deuteron distribution in the orientational disordered phases. For the HT-modifications of LiBr-D20 and Li2CI(OD) the structure refinements of the neutron diffraction data and the nuclear densities calculated from the observed structure factors support split models where the deuterons are located towards the octahedral faces of the Li6 octahedron (Figs. 1 and 2). The low temperature modifications of Li2CI(OD) and LiBr-D20 show weak superstructure reflections. Only small orthorhombic splitting of reflections made the structure determinations rather difficult and are still in progress. Neutron powder diffraction measurements from 40 up to 420 K were carried out on Li2Br(OD). In the low-temperature phase, deuterium positions could be refined on a 12-fold site, where the deuterons of the hydroxide ions point to the edges of a surrounding Li6 octahedron (Fig. 2). The above mentioned NMR-investigations and this refinement point out that the hydroxide ions are statically disordered in the [xx0] directions. The isotropic thermal parameters of Li, Br, and O show no conspicuous increase in the dynamically
Fig. 1. Unit cell of the cubic modification of LiBr. D20 at T = 330K.
(1/
LiBr*D20, T= 318 K
\
Li2Br(OD), T= 40 K
O LiBT * ~ O
Fig. 2. Fourier cut parallel to the triangular octahedral face of the OLi6 polyhedra of LiBr- D 2 0 and Li2Br(OD) ([xxx] with x = 0.150).
disordered high-temperature phase. The position of deuterium can also be refined on the 12-fold site (12i) as in the LT-modification but with a markedly higher Debye-Waller factor. The 2H-NMR spectra indicate a reorientation process with at least cubic symmetry. Taking into account the results of the neutron powder diffraction a 12-fold-jump process of the deuterons between the octahedral edges is probable. Different nonspherical deuteron distributions were found in the orientational disordered phases of LiEX (OD) (X = Cl, Br) and LiBr. D20. In the
50
C. Eilbracht et al. / Physica B 234-236 (1997) 48-50
case of HT Li2CI(OD) and LiBr. D20 the accumulation of deuterium density towards the octahedral faces can be explained by the stronger hydrogen bond acceptor/donor strength compared with Li2Br(OD) which favours hydroxide orientations towards the halides. For Li2Br(OD) the repulsive forces of the 2 statistically filled Li-octahedra around O D - seem to dominate the hydroxide orientation.
References [1] P. Hartwig, A. Rabenau and W. Weppner, J. Less-Com. Met. 78 (1981) 227. [2] R. Wortmann, S. Sitta and H. Sabrowsky, Z. Naturforsch. 44b (1989) 1348-1350. [3] E. Weiss, H. Hensel and H. Kfir, Chem. Ber. 102 (1969) 632-642. [4] C. Eilbracht, PhD Thesis, University of Dortmund (1997)