The structure, thermal properties and phase transformations of the cubic polymorph of magnesium tetrahydroborate

Abstract

The structure of the cubic polymorph of magnesium tetrahydroborate (γ-Mg(BH₄)₂) has been determined in space group Iad from a structural database of the isoelectronic compound SiO₂; this has been corroborated by DFT calculations. The structure is found to concur with that recently determined by Filinchuk et al. (Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, D. Chernyshov and H. Hagemann, Angew. Chem. Int. Ed., 2011, DOI: 10.1002/anie.201100675). The phase transformations and subsequent decomposition of γ-Mg(BH₄)₂ on heating have been ascertained from variable-temperature synchrotron X-ray diffraction data combined with thermogravimetric and mass spectrometry measurements. At ∼160 °C, conversion to a disordered variant of the β-Mg(BH₄)₂ phase (denoted as β′) is observed along with a further unidentified polymorph. There is evidence of amorphous phases during decomposition but there is no direct crystallographic indication of the existence of Mg(B₁₂H₁₂) or other intermediate Mg–B–H compounds. MgH₂ and finally Mg are observed in the X-ray diffraction data after decomposition.