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.　©　2024　The　Royal　Society　of　Chemistry.