Thermal strain in zinc pyrovanadate

The α- andc-axis thermal expansion coefficients of α-Zn₂V₂O₇ (monoclinic structure) were found to be negative between 20 and 560‡C. The volume expansion coefficient is negative above 260‡C. As a result, the zinc atoms tend to be in sixfold rather than fivefold coordination. Similar behavior results from substitution of larger sized ions on Zn sites. These effects are associated with the shear strain suffered by the soft Zn-0 polyhedra as a result of significant changes in Β. On cooling from 20‡C to liquid-nitrogen temperature, as well as upon substitution of smaller sized ions on Zn sites, the major structural changes occur along theb axis, without changes in theac plane: the soft polyhedra experience uniform compression and the Zn ions remain in fivefold coordination. Only in the latter case is the extent of substitutional solid solutions governed by the relative difference in size between the host and substituent ion.