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

Abstract

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: αa = −1.36 × 10−5 1/K, αb = 1.95 × 10−5 1/K, αc = 1.37 × 10−5 1/K, αβ = −0.18 × 10−5 1/K, and αV = 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.