The First-principles Study on the Optical Anisotropy of R3 Space Group Chalcogenides AX_2MQ_6
【摘要】：In order to explain the opposite phase matchability of two types of newly discovered chalcogenides, AX_2MQ_6 vs AX_4M_5Q_(12) belonging to the same R3 space group, the linear and nonlinear optical properties are calculated. The calculated Electron Localization Function(ELF) show the acentricity of the bonds on the Q~(2-) ions with sp~3 hybridization is the main origin of the optical anisotropy. To quantify such an acentricity, a geometric parameter, the dihedral angle between the tetrahedral undersides and the xy-planes is defined. And the calculated birefringence depends on the above defined geometric parameter and the ion radius. This correlation reasonably explains the opposite phase matchability of two closely related chalcogenide families of AX_2MQ_6 and AX_4M_5Q_(12) and is shedding useful light on further exploration on phase matchable IR-NLO crystals.