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First-Principles Determination of the Structure of Magnesium Borohydride

The energy landscape of Mg(BH4)2 under pressure is explored by ab initio evolutionary calculations. Two new tetragonal structures, with space groups P-4 and I41/acd are predicted to be lower in enthalpy, by 15.4 kJ/mol and 21.2 kJ/mol, respectively, than the earlier proposed P42nm phase. We have simulated X-ray diffraction (XRD) spectra, lattice dynamics, and equations of state (EOS) of these phases. The density, volume contraction, bulk modulus, and the simulated XRD patterns of I41/acd and P-4 structures are in excellent agreement with the experimental results.