An analysis of the thermal motion in the negative thermal expansion material Sc-2(WO4)(3) using isotopes in neutron diffraction

A full analysis of thermal motion has been carried out for the negative thermal expansion material Sc-2(WO4)(3) over the temperature range 50-823 K. By obtaining neutron diffraction data from isotopically pure samples of the compositions Sc-2((WO4)-W-184)(3), Sc-2((WO4)-W-186)(3) and natural Sc-2(WO4)(3), coupled With multidata set refinement methods, it is possible to extract anisotropic thermal parameters for individual atoms throughout the temperature range. Results indicate that the thermal motion of two Sc-O-W bridging oxygen atoms with the largest Sc-O-W angles is better represented by thermal toroids consistent with strong local motion of these units. The thermal behavior of the other oxygen atoms in the structures as a function of temperature is normal. Sc-O and W-O bond lengths corrected for the effect of correlated thermal motion show the expected increase with temperature