Order parameter saturation (plateau effect) as a function of composition in the sanmartinite (ZnWO₄)-cuproscheelite (CuWO₄) solid solution

New data suggest that deviations from Vegard's law exist in certain dilute solid solutions, and that the plateau effect may be revealed by constant-temperature measurements of an order parameter far below T_c across a solid solution. As an example, the saturation of the order parameter revealed by the spontaneous strain below the P1/c-P 1 phase transition in (Zn_xCu_1-x)WO₄ has been studied as a function of composition at room temperature and pressure. This saturation is interpreted as evidence for the existence of a plateau regime for O < x < 0.12, indicating that order parameter saturation in this system is more extensive than has been observed in other materials. The origin of this large plateau regime in this structure-type is linked to the short-range character of the strain field associated with Zn solute atoms in the (Cu,Zn)O₆ chains running through the structure. Despite this, the critical behaviour between x ≈ 0.12 and the transition point (x ≈ 0.78 at RTP) conforms to a classical Landau model.