EFFECTS OF PRESSURE ON THE LOCAL ATOMIC STRUCTURE OF CAWO4 AND YLIF4: MECHANISM OF THE SCHEELITE-TO-WOLFRAMITE AND SCHEELITE-TO-FERGUSONITE TRANSITIONS

The pressure response of the scheelite phase of CaWO4 (YLiF4) and the occurrence of the pressure-induced scheelite-to-wolframite (M-fergusonite) transition are reviewed and discussed. It is shown that the change of the axial parameters under compression is related to the different pressure dependences of the W–O (Li–F) and Ca–O (Y–F) interatomic bonds. Phase transition mechanisms for both compounds are proposed. Furthermore, a systematic study of the phase transition in 16 different scheelite ABX4 compounds indicates that the transition pressure increases as the packing ratio of the anionic BX4 units around the A cations increases.