A Novel Sodium and Chromium Borophosphate Na{Cr[BP2O7(OH)3]}: Synthesis, Crystal Structure, Hydrogen Bonding, and Comparative Crystal Chemistry

Abstract

A novel sodium and chromium borophosphate, obtained by hydrothermal synthesis in the Na2O–P2O5–B2O3–Cr2O3–Н2О system, has been investigated using X-ray diffraction and IR spectroscopy. The monoclinic cell parameters are found to be a = 10.4220(3) A, b = 8.2468(2) A, c = 9.2053(3) A, β = 116.568(4)°, and V = 707.63(4) A3; sp. gr. С2/с. The structure is interpreted and refined to the final reliability factor R1 = 2.23% in the anisotropic approximation of atomic displacements using 1311 reflections, I > 2σ(I). The novel compound is relative to the family of alkaline borophosphates with the general formula A{M[BP2O7(OH)3]} (A is an alkaline or alkaline-earth element; M is a transition метал) and is described by the crystallochemical formula (Z = 4) Na{Cr[BP2O7(OH)3]}, where the compositions of borophosphate anion and microporous framework are in square brackets and braces, respectively. The presence of alternative systems of strong symmetric hydrogen bonds is established; the localization of all hydrogen atomic sites has made it possible to analyze them in detail. An additional electron density peak is found, which can be interpreted as a statistically distributed proton H+. The IR spectrum is characteristic of compounds with strong hydrogen bonding; it confirms the statistical proton distribution over subsites around the inversion center. It is suggested that representatives of this family may possess proton conductivity.