Distortion Characteristics Across the Structural Phase Transition in (Cu_1-xZn_x)WO₄

Rietveld analysis of neutron powder diffraction data on the sanmartinite (ZnWO₄)-cuproscheelite (CuWO4) solid solution has enabled the comparison of microscopic and macroscopic order parameters associated with the P2/C-P1̄ structural phase transition. The macroscopic spontaneous strain, calculated from the lattice parameters, conforms well 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 MO₆ octahedra. It is clear that the Jahn-Teller effect, 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 e₂₂. The existence of a large region of order parameter saturation at the copper-rich end of the solid solution is also associated with the MO₆ elongation and the breakdown of the homogeneous strain field of the zinc solute atoms.