DISTORTION CHARACTERISTICS ACROSS THE STRUCTURAL PHASE-TRANSITION IN (CU1-XZNX)WO4

Rietveld analysis of neutron powder diffraction data on the sanmartini te (ZnWO4)-cuproscheelite (CuWO4) solid solution has enabled the compa rison of microscopic and macroscopic order parameters associated with the P2/c-<P(1)over bar> structural phase transition. The macroscopic s pontaneous strain, calculated from the lattice parameters, conforms we ll with a second-order Landau model. Furthermore, this is also true of the symmetry-related atomistic M-O order parameter and the quadratic elongation of the MO(6) octahedra. It is clear that the Jahn-Teller ef fect, associated with the divalent Cu cation, is the driving force for the phase transition and the excess elongation evident in the M-O(2) bond giving rise to the non-symmetry predicted strain element e(22). T he existence of a large region of order parameter saturation at the co pper-rich end of the solid solution is also associated with the MO(6) elongation and the breakdown of the homogeneous strain field of the zi nc solute atoms.