Thermally activated transformations in stable and metastable copper(II) pyrovanadate polymorphs

The structural transformations of α- and β′-Cu₂V₂O₇ phases over the entire temperature range of their existence and α → β′-Cu₂V₂O₇ and β′ → β-Cu₂V₂O₇ polymorphic transitions in α-Cu₂V₂O₇ are described from the crystal-chemical standpoint. Variations in the parameters of the polyhedral blocks of the α-Cu₂V₂O₇ structure implies that the greatest deformations occur with a negative and near-zero bulk thermal expansion in the range from room temperature to 400°C. The compression and rotation of vanadium-oxygen diortho groups is accompanied by unbending of zigzag copper-oxygen chains, with the distances between them unchanged, which is the reason for the anomalous volume expansion of the structure. Thermal distortion of β′-Cu₂V₂O₇ is insignificant. The thermal expansion coefficients (TECs) of unit cell parameters are as follows: α _a = ?1.36 × 10^?5 1/K, α _b = 1.95 × 10^?5 1/K, α _c = 1.37 × 10^?5 1/K, α_β = ?0.18 × 10^?5 1/K, and α _V = 1.93 × 10^?5 1/K. We demonstrate that the low-temperature Cu₂V₂O₇ phase can be formed without admixtures of metastable β-Cu₂V₂O₇ upon slow cooling (at about 1 K/min) of the high-temperature phase.