Crystal structures and phase transformations in Ca(BH4)2

Crystal structures of three polymorphs of Ca(BH4)2, and related phase transitions, have been identified. Removal of solvent from Ca(BH4)2·2THF results in various mixtures of α- and β-polymorphs of Ca(BH4)2. These mixtures were studied by in situ synchrotron powder diffraction in argon atmosphere. The α-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 α-phase approach each other, and at ∼495 K, a second order α → α′ transition takes place resulting in a tetragonal α′-phase of space group I-42d, a supergroup of F2dd. The structure of β-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. © 2008 Acta Materialia Inc.