K2CaV2O7: a pyrovanadate with a new layered type of structure in the A2BV2O7 family

Abstract

The crystal structure of K₂CaV₂O₇ prepared by a conventional solid-state reaction has been solved by a direct method and refined using Rietveld full profile fitting based on X-ray powder diffraction data. This compound crystallises in the triclinic space group (P, Z = 2) with unit cell constants a = 7.1577(1) Å, b = 10.5104(2) Å, c = 5.8187(1) Å, α = 106.3368(9)°, β = 106.235(1)°, γ = 71.1375(9)°. The structure can be described as infinite undulating CaV₂O₇²⁻ layers parallel to the ac plane, which consist of pairs of edge-sharing CaO₆ octahedra connected to each other through V₂O₇ pyrogroups. The potassium atoms are positioned in two sites between the layers, with a distorted IX-fold coordination of oxygen atoms. The chemical composition obtained from the structural solution was confirmed by energy-dispersive X-ray analysis. The stability of compounds in the family of alkali metal calcium pyrovanadates is discussed based on an analysis of the correlation between anion and cation sizes and theoretical first-principles calculations.