Electronic structure and optical properties of LiXH₃ and XLiH₃ (X=Be, B or C)

The equilibrium lattice constant, the cohesive energy and the electronic properties of light metal hydrides LiXH₃ and XLiH₃ (X=Be, B or C) with perovskite lattice structures have been investigated by using the pseudopotential plane-wave method. Large energy gap of LiBeH₃ indicates that it is insulating, but other investigated hydrides are metallic. The pressure-induced metallization of LiBeH₃ is found at about 120GPa, which is attributed to the increase of Be-p electrons with pressure. The electronegativity of the p electrons of X atom is responsible for the metallicity of the investigated LiXH₃ hydrides, but the electronegativity of the s electrons of X atom plays an important role in the metallicity of the investigated XLiH₃ hydrides. In order to deeply understand the investigated hydrides, their optical properties have also been investigated. The optical absorption of either LiBeH₃ or BeLiH₃ has a strong peak at about 5eV, showing that their optical responses are qualitatively similar. It is also found that the optical responses of other investigated hydrides are stronger than those of LiBeH₃ and BeLiH₃ in lower energy ranges, especially in the case of CLiH₃.