An analysis of the thermal motion in the negative thermal expansion material Sc₂(WO₄)₃ using isotopes in neutron diffraction

A full analysis of thermal motion has been carried out for the negative thermal expansion material Sc2(WO4)3 over the temperature range 50−823 K. By obtaining neutron diffraction data from isotopically pure samples of the compositions Sc2(184WO4)3, Sc2(186WO4)3 and natural Sc2(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.