New complex phosphates Cs3MIIBi(P2O7)2 (MII – Ca, Sr and Pb): synthesis, characterization, crystal and electronic structure

Abstract

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 Cs₃M^IIBi(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 M^IIO₇ 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 bandgaps, E_g, of the Cs₃M^IIBi(P₂O₇)₂ crystals reveal an ∼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.