Tungstates and molybdates with scheelite-type structure are technologically important materials with applications as phosphors, detectors, lasers, etc. In recent years, it have been found they exhibit high oxide-ion conduction in intermediate temperature ranges and there has arisen renewed interest in their evolution under pressure, providing valuable insight on structural, electronic and transport properties. The alpha-phases of rare earth molybdates are distorted scheelites and we are interested on its anomalous transport properties (with semiconductor, polaronic and ionic character), which we have correlated to their thermal dependence on the crystal structure. We have structural information from X ray data, but the precise determination of thermal dependence of oxygen displacements would help to explain the correlation with different conductivity mechanisms. We propose to study Tb2(MoO4)3 and Nd2(MoO4)3 compounds in the D2B beamline, at selected temperatures between 10 to 1000K.