AN EXAFS STUDY OF CATION SITE DISTORTIONS THROUGH THE P2 C-P(1)OVER-BAR TRANSITION IN THE SYNTHETIC CUPROSCHEELITE SANMARTINITE SOLID-SOLUTION/

EXAFS-spectroscopy has been used to monitor changes in divalent cation site geometries across P2/c-P1BAR phase transition in the sanmartinit e (ZnWO4)-cuproscheelite (CuWO4) solid solution at ambient and liquid nitrogen temperatures. In the ZnWO4 end member, Zn occupies axially-co mpressed ZnO6 octahedra with two axial Zn-O bonds at approximately 1.9 5 angstrom and four square planar Zn-O bonds at approximately 2.11 ang strom. The substitution of Zn by Cu generates a second Zn environment with four short square planar Zn-O bonds and two longer axial Zn-O bon ds. The proportion of the latter site increases progressively as the C u content increases. Cu EXAFS reveals that the CuO6 octahedra maintain their Jahn-Teller axially-elongate geometry throughout the majority o f the solid solution and only occur as axially-compressed octahedra we ll within the stability field of the Zn-rich phase with monoclinic lon g-range order.