Synthesis, structure and characterization of three new Mg-containing phosphates with deep-UV cut-off edges

Abstract

Three new Mg-containing phosphates, AMg₂P₃O₁₀ (A = Li, Cs) and Rb₂MgP₂O₇, have been successfully synthesized using a high-temperature solution method. Single-crystal X-ray diffraction measurements show that they crystallize in different space groups, P21/m for LiMg₂P₃O₁₀, Pbcn for CsMg₂P₃O₁₀ and Pnma for Rb₂MgP₂O₇. Interestingly, their structures contain two different types of P–O connection modes, i.e., P₃O₁₀ trimers in AMg₂P₃O₁₀ (A = Li, Cs) and P₂O₇ dimers in Rb₂MgP₂O₇. Correspondingly, the Mg²⁺-centered polyhedra also exhibit three different polymerizations, including one-dimensional (1D) [MgO₆]_∞ infinite chains in LiMg₂P₃O₁₀, Mg₂O₈ dimers in CsMg₂P₃O₁₀ and isolated MgO₅ square pyramids in Rb₂MgP₂O₇. Detailed structural analysis indicates that the different sizes of the A-site cations and flexible coordination of Mg²⁺ cations are responsible for the structural differences. Furthermore, optical measurements of the three compounds show that they all possess deep-UV absorption cut-off edges (<190 nm). Their thermal properties and bond valence sum difference map (BVS-DM) calculations were also studied.