Ferroelastic phase transition in the sanmartinite (ZnWO₄)- cuproscheelite (CuWO₄) solid solution

Tungstates in the composition range ZnWO₄-CuWO₄ (sanmartinite-cuproscheelite) have been synthesized. Complete solid solutions between the end members are found for samples annealed at 600 degrees C. The lattice parameters across the solid solution vary continuously and reveal a structural phase transition between the P2/c sanmartinite structure and P1 cuproscheelite structure at around Zn_0.78Cu_0.22WO ₄. The spontaneous strain arising in the triclinic phase is large, amounting to some 6.5% in the cuproscheelite end member, and has been used as a measure of the order parameter for the transition. The structural phase transition is potentially ferroelastic and appears continuous as a function of composition at room temperature and pressure. The transition is successfully modelled in terms of a second-order Landau-type phenomenological potential.