Structure of the orthorhombic γ-phase and phase transitions of Ca(BD₄)₂

Buchter, Florian; Łodziana, Z.; Remhof, A.; Friedrichs, O.; Borgschulte, A.; Mauron, Philippe; Züttel, Andreas; Sheptyakov, D.; Palatinus, L.; Chłopek, K.; Fichtner, M.; Barkhordarian, G.; Bormann, R.; Hauback, B. C.

doi:10.1021/jp810521p

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Structure of the orthorhombic γ-phase and phase transitions of Ca(BD₄)₂

Buchter, Florian Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Hydrogen & Energy, Dübendorf, Switzerland
Łodziana, Z. Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Hydrogen & Energy, Dübendorf, Switzerland - Institute of Nuclear Physics, Polish Academy of Sciences, Kraków, Poland
Remhof, A. Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Hydrogen & Energy, Dübendorf, Switzerland
Friedrichs, O. Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Hydrogen & Energy, Dübendorf, Switzerland
Borgschulte, A. Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Hydrogen & Energy, Dübendorf, Switzerland
Mauron, Philippe Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Hydrogen & Energy, Dübendorf, Switzerland
Züttel, Andreas Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Hydrogen & Energy, Dübendorf, Switzerland - Physics Department, University of Fribourg, Switzerland
Sheptyakov, D. Laboratory for Neutron Scattering, ETH Zurich & Paul Scherrer Institute, Villigen, Switzerland
Palatinus, L. Laboratoire de Cristallographie, Le Cubotron, Ecole Polytechnique Fédérale de Lausanne, Switzerland
Chłopek, K. Forschungszentrum Karlsruhe, Institute of Nanotechnology, Germany
Fichtner, M. Forschungszentrum Karlsruhe, Institute of Nanotechnology, Germany
Barkhordarian, G. GKSS-Research Center Geesthacht GmbH, Geesthacht, Germany
Bormann, R. GKSS-Research Center Geesthacht GmbH, Geesthacht, Germany
Hauback, B. C. Institute for Energy Technology, Kjeller, Norway

09.09.2009

Published in:

The Journal of Physical Chemistry C. - 2009, vol. 113, no. 39, p. 17223–17230

English We have solved and refined the crystal structure of the orthorhombic γ-phase of Ca(BD₄)₂ by combined synchrotron X-ray powder diffraction, neutron powder diffraction, and ab initio calculations. Among five structural candidates giving the same quality of the fit of the diffraction data, the structural model with the highest symmetry and space group Pbca is the most appropriate. This is supported by the implicit presence of the Pbca symmetry operations in the low-symmetry space groups in both experimental and DFT calculated structures. The Ca atoms are surrounded by six BD₄ groups that have similar distortions as reported for the β-phase of Ca(BD₄)₂. On the basis of the experimental structures, free energies of the α-, β-, and γ-phases are calculated in the range 300 K < T < 620 K. The phase transitions are observed in the same temperature range by means of X-ray diffraction on Ca(BD₄)₂ + MgD₂ and pure Ca(BD₄)₂ samples. According to the ab initio calculations, the α-phase is the ground state at 0 K. At room temperature, the calculated free energies of the α-phase, β-phase, and γ-phase were found to be within ~0.13 eV/f.u., in agreement with the observed coexistence of these phases. Moreover, calculations provide insight into trends for relative stabilities of α-, β-, and γ-phases. That is, with increasing temperature, the β-phase becomes more stable, while the metastable γ-phase becomes more destabilized with respect to the α-phase.

Faculty

Faculté des sciences et de médecine

Department

Département de Physique

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