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

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) Å, b = 8.2468(2) Å, c = 9.2053(3) Å, β = 116.568(4)°, and V = 707.63(4) ų; 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.