Structural phase transformations and investigation of physical properties in some triple compounds under high pressure

The structural and electronic properties of XAlH4 (X=Li,Na) under high pressure were calculated with the Siesta package program using the ab-initio method. Although LiAlH4 crystallizes in the monoclinic structure with the space group of P21/c under ambient conditions, NaAlH4 crystallizes in the tetragonal type structure with the space group of I41/a. When gradually increasing pressures were applied on these structures, both LiAlH4 and NaAlH4 turned into a trigonal type structure with the space group of P-1. The lattice constant values of all the structures obtained were calculated and compared with the literature. These phase transformations seen in LiAlH4 and NaAlH4 were also obtained using total energy and enthalpy calculations. According to these calculations, LiAlH4 transformed from monoclinic type to trigonal type structure at a pressure of 4.35 GPa. NaAlH4, on the other hand, undergoes a phase transition from tetragonal type to trigonal type structure at a pressure of 30.31 GPa. In addition, the electronic properties of all structures obtained under 0 GPa and high pressure for LiAlH4 and NaAlH4 were also investigated. The electronic band structures calculations were done along the high symmetry directions. Although, LiAlH4 shows the properties of insulating materials at 0 GPa, shows the physical properties of metallic materials due to phase transformation under high pressure. On the other hand, while NaAlH4 is an insulator at 0 GPa, it is a semiconductor material under high pressure. Keywords: Density Functional Theory, Structural Phase Transition, Ab initio Method, Electronic Properties