Greatly enhanced optical anisotropy in thiophosphates inspired by rational coupling of tetrahedra and ethane-like [P2S6]4− groups

Abstract

Exploring new potential infrared (IR) birefringent crystals is important for modulating the polarization of light in optical communication, but the rational structural design for the enhancement of optical anisotropy is still a great challenge. For that reason, we have proposed an effective strategy for the design of promising IR birefringent crystals by incorporation of Cu⁺ or Zn²⁺ with d¹⁰-configuration, or stereochemically active lone-pair (SCALP) Sb³⁺ cations into structures with the typical ternary thiophosphates as templates (A₃PS₄ and A₄P₂S₆: A = Na, K), affording the successful synthesis of six new mixed-metal thiophosphates (Na₄MgP₂S₈, A₃CuP₂S₆, KGaP₂S₆, Na₂ZnP₂S₆, Na₃SbP₂S₈). Systematic research results show that the birefringences (Δn) of A₃CuP₂S₆ (Δn, Na: 0.151; K: 0.124), Na₂ZnP₂S₆ (Δn = 0.136) and Na₃SbP₂S₈ (Δn = 0.107) are much larger than those of Na₄MgP₂S₈ (Δn = 0.037) and the typical A₃PS₄ (0.010 vs. 0.019) and A₄P₂S₆ (0.038 vs. 0.042), which indicates that the d¹⁰- or SCALP-cation-centered tetrahedra (CuS₄, ZnS₄ or SbS₄) produce large positive contributions to the enhancement of optical anisotropy; the ethane-like [P₂S₆]⁴⁻ dimer appears to have greater contribution than the [PS₄]³⁻ unit. Moreover, we extended the research study to the known A(Na/K)–M(Mg/Ga/Al/Cu/Sb/Zn)–P–S systems, and this comprehensive survey illustrates that their Δn values show an increasing trend for thiophosphates based on the order of cation substitutions, from alkaline earth (Ae²⁺) or alkali (A⁺) to IIIA (Ga³⁺/Al³⁺) to d¹⁰ or SCALP cations (Cu⁺/Zn²⁺/Sb³⁺). Special 1D chain or 2D layer structures are more inclined to gain a larger optical anisotropy. Therefore, this study demonstrates a feasible design strategy to achieve greatly enhanced optical anisotropy through the rational coupling of d¹⁰- or SCALP-cation-centered tetrahedra and ethane-like [P₂S₆]⁴⁻ groups into the structures of thiophosphates.