Rietveld study of the structural phase transition in the sanmartinite (ZnWO (sub 4) )-cuproscheelite (CuWO (sub 4) ) solid solution

The P<$E 1 back 40 up 40 -> to P2/c phase transition in Cu (sub 1-) (sub x) Zn (sub x) WO (sub 4) has been investigated by observing changes across the solid solution. The crystal structures of samples with x = 0.85, 0.55 and 0.30 have been refined by the Rietveld method from high-resolution synchrotron XRD data. The Cu/Zn and W atoms are in disordered octahedral coordination. The microscopic order parameter (defined in terms of changes in Cu/Zn-O bond lengths) behaves in a similar manner to the previously-recorded macroscopic spontaneous strain. Rigid-unit-mode analysis shows that the transition does not involve distortions of a framework of rigid octahedra; rather, the cooperative Jahn-Teller distortion of the CuO (sub 6) octahedra is identified as the driving mechanism, with WO (sub 6) octahedral distortions occurring as a secondary process.