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 di lute solid solutions, and that the plateau effect may be revealed by c onstant-temperature measurements of an order parameter far below T-c a cross a solid solution. As an example, the saturation of the order par ameter revealed by the spontaneous strain below the P2/c-<P(1)over bar > phase transition in (ZnxCu1-x)WO4 has been studied as a function of composition at room temperature and pressure. This saturation is inter preted as evidence for the existence of a plateau regime for 0 < x < 0 .12, indicating that order parameter saturation in this system is more extensive than has been observed in other materials. The origin of th is 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)O-6 chains running through the structure. Despite this, th e critical behaviour between x approximate to 0.12 and the transition point (x approximate to 0.78 at RTP) conforms to a classical Landau mo del.