5 Conclusions

In summary, we performed a systematic structure search for the high pressure phases of magnesium borohydrides and identified two tetragonal structures with space groups $I4_1/acd$ and $P\bar{4}$ as candidates for the unknown $\delta $ and $\delta ’$ phases, respectively, which can be kinetically stable upon the decompression to the ambient conditions. In particular, the $I4_1/acd$ structure is much lower in enthalpy than the earlier reported structures, and the calculated density, bulk modulus, and the simulated XRD patterns are in excellent agreement with the experimental results 126. This example highlights the importance of theoretical simulations in establishing crystal structures, when only powder XRD data are available: purely experimental solution may be dangerous even for simple structures, such as the structure of the $\delta $ phase with only 6 non-hydrogen atoms in the unit cell.