First-principles studies of the Optical anisotropy of R3 space group chalcogenides crystal AX2MQ6
Abstract
In order to explain the reason that all the sythesised AX2MQ6 chalcogenides compounds are phase-matching while isomorphic chalcogenides compounds AX4M5Q12 are not phase-matched materials. The linear optical property birefringence and non-linear optical property of R3 space group crystals AX2MQ6 have been calculated to compare with AX4M5Q12. Their Electron Localization Function(ELF) was calculated to show that the acentricity of the bonds on sp3 hybridization Q2- ions is the main origin of optical anisotropy. To quantificat the acentricity, a geometric parameter dihedral angle between tetrahedral undersides and xy-planes was defined. We find theoretically the birefringence depend on the defined geometric parameter and ion radius. This relation between birefringence and structure resonalbly explain the abnormal difference of birefringence of isostructural AX2MQ6 and AX4M5Q12 and useful for exploring new phase-matching IR-NLO crystals.