K2MgMoP2O10 and K3Mg2MoP3O14: two new molybdophosphates exhibiting different optical anisotropies induced by variable dimensionality of the anion framework

Abstract

By introducing the d⁰ metal cation Mo⁶⁺ into phosphates, two new molybdophosphates, K₂MgMoP₂O₁₀ and K₃Mg₂MoP₃O₁₄, were synthesized by spontaneous crystallization, and their structures were determined by single-crystal X-ray diffraction. K₂MgMoP₂O₁₀ shows a two-dimensional layer composed of the uncommon eight-membered ring [Mo₂P₂O₁₆] formed by [MoO₆] and [PO₄], while K₃Mg₂MoP₃O₁₄ shows isolated [MoP₃O₁₄] clusters composed of [MoO₅] and [PO₄]. K₂MgMoP₂O₁₀ and K₃Mg₂MoP₃O₁₄ have UV cut-off wavelengths of 277 and 271 nm, respectively, which are significantly shorter than those of most recently published molybdophosphates. To the best of our acknowledge, K₂MgMoP₂O₁₀ exhibits the largest birefringence (a calculated value of 0.187 at 546 nm) among reported alkali metal or alkaline earth metal molybdophosphates, which provides a way to explore new birefringent materials. First-principles analysis of the electronic structure shows that the large birefringence of K₂MgMoP₂O₁₀ mainly originates from the [MoO₆] units.