Structural Phase Transitions of Mg(BH₄)₂ under Pressure

The structural stability of Mg(BH 4 ) 2, a promising hydrogen storage material, under pressure has been investigated in a diamond anvil cell up to 22 GPa with combined synchrotron X-ray diffraction and Raman spectroscopy. The analyses show a structural phase transition around 2.5 GPa and again around 14.4 GPa. An ambient-pressure phase of Mg(BH 4 ) 2 has a hexagonal structure (space group P 6 1, a = 10.047(3) Å, c = 36.34(1) Å, and V = 3176(1) Å 3 at 0.2 GPa), which agrees well with early reports. The structure of high-pressure phase is found to be different from reported theoretical predictions; it also does not match the high-temperature phase. The high-pressure polymorph of Mg(BH 4 ) 2 is found to be stable on decompression, similar to the case of the high-temperature phase. Raman spectroscopic study shows a similarity in high-pressure behavior of as-prepared Mg(BH 4 ) 2 and its high-temperature phase.