Syntheses, structures and characterization of non-centrosymmetric Rb2Zn3(P2O7)2 and centrosymmetric Cs2M3(P2O7)2 (M = Zn and Mg)

Abstract

Three phosphates Rb₂Zn₃(P₂O₇)₂ and Cs₂M₃(P₂O₇)₂ (M = Zn and Mg) were successfully synthesized by a high-temperature solution method. Their structures were determined by single-crystal X-ray diffraction, which shows that the three structures all display three-dimensional [M₃P₄O₂₀]¹⁴⁻ (M = Zn and Mg) frameworks composed of isolated P₂O₇ dimers and MO₄ (M = Zn and Mg) tetrahedra with Rb⁺/Cs⁺ cations located in the space of the frameworks to balance charges. But owing to the cationic size effect, they are not iso-structural. Rb₂Zn₃(P₂O₇)₂ crystallizes in the non-centrosymmetric P2₁ space group, whereas Cs₂M₃(P₂O₇)₂ (M = Zn and Mg) crystallize in the centrosymmetric P2₁/c space group. And Rb₂Zn₃(P₂O₇)₂ exhibits a second harmonic generation (SHG) response as large as that of KH₂PO₄ (KDP). Combining electronic structure and SHG density calculations, we studied the effects of ZnO₄ tetrahedra on the SHG response of Rb₂Zn₃(P₂O₇)₂. Furthermore, a structural comparison is also carried out in Rb₂Zn₃(P₂O₇)₂ and Cs₂M₃(P₂O₇)₂ (M = Zn and Mg).