Crystal structures and phase transformations in Ca(BH4)2

Yaroslav Filinchuk

doi:10.1016/j.actamat.2008.10.034

Crystal structures and phase transformations in Ca(BH4)2

2009-02-01

121

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AI

DOI：10.1016/j.actamat.2008.10.034

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Crystal structures of three polymorphs of Ca(BH4)(2), and related phase transitions, have been identified. Removal of solvent from Ca(BH4)(2)center dot 2THF results in various mixtures of alpha- and beta-polymorphs of Ca(BH4)(2). These mixtures were studied by in situ synchrotron powder diffraction in argon atmosphere. The alpha-Ca(BH4)(2) structure crystallizes in the noncentrosymmetric space group F2dd, and contains an ordered BH4- anion. Upon increasing the temperature, the cell parameters a and c of the alpha-phase approach each other, and at similar to 495 K, a second order alpha -> alpha' transition takes place resulting in a tetragonal alpha'-phase of space group I-42d, a supergroup of F2dd. The structure of beta-Ca(BH4)(2), which is 3.7-5.6% denser depending on temperature, was described in space group P-4. Crystal structures, variation of the cell parameters, and weight fraction are reported for the three Ca(BH4)(2) phases as a function of temperature. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keyword:

In situ synchrotron diffraction

Hydrides

Crystal structure

Crystal structures and phase transformations in Ca(BH4)2

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