Crystal structures and phase transformations in Ca(BH4)2
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- 作者：Yaroslav Filinchuk ; Ewa Rö ; nnebro ; Dhanesh Ch ; ra
- 关键词：In situ synchrotron diffraction ; Hydrides ; Crystal structure
- 刊名：Acta Materialia
- 出版年：2009
- 出版时间：February 2009
- 年：2009
- 卷：57
- 期：3
- 页码：732-738
- 全文大小：521 K
文摘
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 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.