RIETVELD STUDY OF THE STRUCTURAL PHASE-TRANSITION IN THE SANMARTINITE(ZNWO4)-CUPROSCHEELITE (CUWO4) SOLID-SOLUTION

The P (1) over bar to P2/c phase transition in Cu1-xZnxWO4 has been in vestigated by observing structural changes across the solid solution. The crystal structures of samples with x = 0.85, 0.55 and 0.30 have be en refined by the Rietveld method from high-resolution synchrotron X-r ay diffraction data. The Cu/Zn and W atoms are in distorted octahedral coordination. The microscopic order parameter (defined in terms of ch anges in Cu/Zn-O bond lengths) behaves in a similar manner to the prev iously-recorded macroscopic spontaneous strain. Rigid-unit-mode analys is shows that the transition does not involve distortions of a framewo rk of rigid octahedra. Rather, the co-operative Jahn-Teller distortion of the CuO6 octahedra is identified as the driving mechanism, with WO 6 octahedral distortions occurring as a secondary process.