K6Mo8PO29OH·H2O and K6Mo5P2O23·7H2O: strongly distorted [MoO6] octahedral groups effectively enhance birefringence

Abstract

Two examples of molybdate phosphates, K₆Mo₈PO₂₉OH·H₂O and K₆Mo₅P₂O₂₃·7H₂O, were designed and synthesized using a hydrothermal method, introducing strongly distorted [MoO₆] octahedral groups. K₆Mo₈PO₂₉OH·H₂O crystallizes in the centrosymmetric space group Cmcm, where each [PO₄] combines [Mo₄O₁₅] and [Mo₄O₁₄(OH)] groups to form a unique [Mo₈PO₂₉(OH)] cluster. K₆Mo₅P₂O₂₃·7H₂O crystallizes in the non-centrosymmetric space group P2₁2₁2₁, where two [PO₄] link [Mo₅O₂₁] groups to form a closed hollow ellipsoidal [Mo₅P₂O₂₃] cluster. They possess wide experimental band gaps of 3.57 and 3.34 eV, respectively. Compared to K₃PO₄, the introduction of strongly distorted [MoO₆] octahedral groups enhances their birefringence from 0.006 to 0.127 and 0.077@1064 nm (about 21 × and 11 × K₃PO₄), with the source of the birefringence being dominated by the contribution of strongly distorted [MoO₆] octahedral groups. The relationship between its structure and optical properties is analyzed based on first-principles calculations. This work effectively enhances the birefringence properties of phosphate crystals by introducing highly distorted [MoO₆] groups, providing insights for designing and synthesizing ultraviolet optical crystal materials with superior performance.