First-principles studies of chalcopyrite-type (Cu, Li)GaS 2 and (Cu, Li)InS 2 systems

The first-principles studies of structural, electronic and elastic properties have been performed for chalcopyrite-type (Cu1-xLix)GaS2 solid solutions with x = 1/16, 1/8, 1/4, and 1/2 using general gradient approximations (GGA) with ultrasoft and norm-conserving pseudopotentials based on density functional theory. The band-gap energies have been also calculated using nonlocal screened hybrid density functional HSE06 for the structures optimized by GGA approximations. The results are compared with reported experimental data and the results of (Cu1-xLix)InS2 system calculated by the similar method. The Li-content (x) dependence of experimental band-gap energies in (Cu(1-x)Lix)GaS2 system is explained qualitatively where the band-gap energy increases with increasing Li content, x. The pronounced experimental Li content dependence of the tetragonal lattice parameters has been reproduced semi-quantitatively in which the lattice constant c of tetragonal (Cu1-xLix)GaS2 decreases whereas the lattice constant a increases with increasing Li content. A possible origin of the peculiar Li content dependence of the lattice parameters is proposed from the calculated difference in Young's modulus along the a and c axes, which suggests that the bonding interaction within the c-plane is stronger than that along the c-axis in the (Cu1-xLix)GaS2 system. (C) 2019 The Japan Society of Applied Physics