Temperature- and composition-dependence of the ferroelastic phase transition in (Cu_xZn_1-x)WO₄

The lattice parameters of(Cu_xZn_1-x)WO₄ have been determined as a function of composition and temperature by high-temperature Guinier X-ray diffraction. The ferroelastic phase transition from PT to P 2 c (with increasing X and T) appears to be continuous (second-order) as a function of temperature for the sample (Zn_0.75Cu_0.25)WO₄. As the Cu-content increases, however, the high-temperature transition behaviour becomes increasingly first-order in character with a tricritical transition occurring at about (Zn_0.32Cu_0.68)WO₄ and about 650°C. The triclinic-monoclinic phase boundary has been mapped out in temperature-composition space. The critical transition temperature varies non-linearly as a function of composition. A modified Landau potential successfully models the transition as a function of both temperature and composition.