First-principles studies of the Optical anisotropy of R3 space group chalcogenides crystal AX2MQ6

TL;DR

This study uses first-principles calculations to analyze the optical anisotropy of R3 space group chalcogenides X2MQ6, revealing how bond acentricity influences birefringence and phase-matching properties for nonlinear optics.

Contribution

It provides a theoretical explanation linking bond acentricity and geometric parameters to optical anisotropy in X2MQ6 crystals, aiding the design of phase-matching IR-NLO materials.

Findings

Acentricity of bonds correlates with birefringence.

Geometric dihedral angle influences optical anisotropy.

Theoretical relation explains differences between X2MQ6 and X4M5Q12.

Abstract

In order to explain the reason that all the sythesised \ce{AX2MQ6} chalcogenides compounds are phase-matching while isomorphic chalcogenides compounds \ce{AX4M5Q12} are not phase-matched materials. The linear optical property birefringence and non-linear optical property of $R3$ space group crystals \ce{AX2MQ6} have been calculated to compare with \ce{AX4M5Q12}. Their Electron Localization Function(ELF) was calculated to show that the acentricity of the bonds on $sp^3$ hybridization \ce{Q^2-} 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 \ce{AX2MQ6} and \ce{AX4M5Q12} and useful for exploring new phase-matching IR-NLO crystals.