High-pressure polymorphs of Li2BeH4 predicted by first-principles calculations

We report two orthorhombic high-pressure polymorphs of beta-Na2SO4- and La2NiO4-type structures for lithium beryllium hydrides (Li2BeH4) predicted by first-principles calculations. The beta-Na2SO4-type structure possesses BeH4 tetrahedra, similar to the zero-pressure alpha-Li2BeH4 structure, but in dramatic contrast to the peculiar BeH4 octahedral layer in the La2NiO4-type structure. The beta-Na2SO4-type structure energetically surpasses the alpha-Li2BeH4 structure for stability above 7.2 GPa, which is nicely correlated with the experimental transition pressure of 9.1 GPa. Further transformation to the La2NiO4-type structure is predicted at 28.8 GPa. The two transitions are identified as first-order in nature with volume contractions of 3.32% and 5.17%, respectively. Our current discovery has ruled out the previously proposed Cs2MgH4-type structure as the candidate for the high-pressure phase.