Phosphates with Two Types of Isolated P–O Groups: Noncentrosymmetric Na6Sr2Bi3(PO4)(P2O7)4 and Centrosymmetric Cs2CaBi2(PO4)2(P2O7)

Inorganic phosphates are of great interest, because of their rich structural chemistry and multiple functional properties. Compared with the phosphates that only contain the solely condensed P–O groups, the phosphates with various condensed P–O groups are less reported, especially for the noncentrosymmetric (NCS) ones. Here, two new bismuth phosphates, Na6Sr2Bi3(PO4)(P2O7)4 and Cs2CaBi2(PO4)2(P2O7), were synthesized by the solid-state reaction and both structures contain two types of isolated P–O groups. Remarkably, Na6Sr2Bi3(PO4)(P2O7)4 crystallizes in the tetragonal space group P4̅21c, which represents the first NCS bismuth phosphate with PO4 and P2O7 groups. Detailed structural comparisons among Bi3+-containing alkali/alkaline-earth metal phosphates show that the ratios of cations/phosphorus profoundly influence the condensed degree of P–O groups. Ultraviolet–visible–near-infrared (UV-vis-NIR) diffusion spectra show that both compounds have the relatively short UV cutoff edges. And Na6Sr2Bi3(PO4)(P2O7)4 has a second-harmonic generation response of 1.1 × KDP. The first-principles calculations are carried out to understand the structure–performance relationship.