New complex phosphates (Cs3MBi)-Bi-II(P2O7)(2) (M-II - Ca, Sr and Pb): synthesis, characterization, crystal and electronic structure

摘要

Herein, the peculiarities of complex phosphate formation in self-fluxes of Cs-M-II-Bi-P-O (M-II = Ca, Sr, Ba and Pb) systems with Cs/P = 0.7-1.3 at fixed ratios of Bi/P = 0.2 and Bi/M-II = 1.0 were studied and discussed. Three novel isostructural diphosphates with the general composition (Cs3MBi)-Bi-II(P2O7)(2) (M-II = Ca, Sr and Pb) and the original framework topology were synthesized and characterized via single-crystal and powder X-ray diffraction, SEM, DTA, and FTIR-and UV-VIS-spectroscopy. In addition, electronic structure (DFT) and Voronoi-Dirichlet polyhedra (VDP) characteristics calculations and crystallochemical analysis were performed. In the structure of the new compounds, the (MO7)-O-II and BiO6 polyhedra are connected via common oxygen vertices forming infinite helical-like chains, which are linked by P2O7 groups into a 3D-framework with pentagonal tunnels, where the Cs+ cations are located. The structural peculiarities are discussed considering perspectives for the creation of new luminescent materials. The dielectric band-gaps, E-g, of the (Cs3MBi)-Bi-II(P2O7)(2) crystals reveal an similar to 0.2 eV low-energy shift in the Ca-Sr-Pb sequence of M-II cations, which reveals the possibility to tune the optical absorbance spectra of the crystals via the synthesis of solid solutions with various contents of M-II cations. The glass-ceramic synthetic approach is also proposed as a convenient method for the creation of new diphosphates, and the applicability of this method is verified for Cs3CaBi(P2O7)(2).