Infrared birefringent crystals that hold significant importance for optoelectronic application have been rarely reported. Traditional tetrahedral PS₄, ethane-like P₂S₆, and octahedral InS₆ units in thiophosphates typically manifest near isotropy, often resulting in extremely small birefringence. However, this study prepares α-Rb₂InP₂S₇ (1), β-Rb₂InP₂S₇ (2), and Cs₂InP₂S₇ (3), consisting of the aforementioned microstructures, notably exhibiting the highest refractive index difference or birefringence values (0.247, 0.298, and 0.250 at 546 nm, respectively) among thiophosphates, the middle one being larger than that of commercial birefringent materials. This unusual increase in birefringence can be primarily attributed to two key factors: (1) simultaneous stretching and compressing of the P–S and In–S covalent bond interactions, generating high polarizability anisotropy of InS₆, PS₄, and P₂S₆ polyhedral units; (2) the additional incorporation of alkali metals that further reduces the dimensionality of the crystal structure, creating one-dimensional [InP₂S₇]²⁻ structures with increasing polarizability anisotropy. This study presents an alternative approach to enhance birefringent materials by reconstructing covalent bond interactions and specific spatial arrangements.

中文翻译：

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重建近乎各向同性的微观结构以构建一维框架，导致硫代磷酸盐的创纪录双折射


对光电应用具有重要意义的红外双折射晶体很少被报道。硫代磷酸盐中的传统四面体 PS₄、类似乙烷的 P₂S₆ 和八面体 InS₆ 单元通常表现为接近各向同性，通常会导致极小的双折射。然而，本研究制备了 α-Rb₂InP₂S₇ （1）、β-Rb₂InP₂S₇ （2） 和 Cs₂InP₂S₇ （3），由上述微观结构组成，显着表现出最高的折射率差或双折射值（在 546 nm 处为 0.247、0.298 和 0.250，分别）在硫代磷酸盐中，中间的比市售双折射材料的要大。这种双折射的异常增加主要归因于两个关键因素：（1） P-S 和 In-S 共价键相互作用的同时拉伸和压缩，产生 InS₆、PS₄ 和 P₂S₆ 多面体单元的高极化各向异性;（2） 碱金属的额外掺入进一步降低了晶体结构的维度，从而产生具有增加极化各向异性的一维 [InP₂S₇] ²⁻ 结构。本研究提出了一种通过重建共价键相互作用和特定空间排列来增强双折射材料的替代方法。