Drastic lowering of the order-disorder phase transition temperatures in Zr1-xMxW2O8-y (M=Sc,Y,In) solid solutions

ZrW2O8 undergoes an order-disorder phase transition associated with the orientational disordering of the WO4 tetrahedra at 440 K. We have showed previously that the phase transition temperatures of Zr1-xMxW2O8-y (M=Sc,Y,In) solid solutions are decreased by a maximum of 80 K with only 4% trivalent Sc content. In order to understand the drastic composition dependence on the transition temperature, precise x-ray diffraction experiments on the solid solutions were performed by using a synchrotron radiation. The results provided the evidence of the strong correlation between the phase transition temperature and the number of WO4 units with the ordered orientation. A model was proposed to interpret the drastic composition dependence on the transition temperature, by considering the existence of a local region including the WO4 pairs with the orientational disorder in Zr1-xMxW2O8-y(M=Sc,Y,In). The local region has the crystal structure of the high-temperature phase in a nonequilibrium state, and spreads out around the substituted trivalent cations. The size of the local region was estimated to be a few unit cells. The local regions play just like nonmagnetic impurities in a site-diluted magnetic system, leading to drastic lowering of the phase transition temperatures in Zr1-xMxW2O8-y solid solutions