Structural investigation of the negative thermal expansion in yttrium and rare earth molybdates

The Sc(2)(WO(4))(3)-type phase (Pbcn) of Y(2)(MoO(4))(3), Er(2)(MoO(4))(3) and Lu(2)(MoO(4))(3) has been prepared by the conventional solid-state synthesis with preheated oxides and the negative thermal expansion (NTE) has been investigated along with an exhaustive structural study, after water loss. Their crystal structures have been refined using the neutron and x-ray powder diffraction data of dehydrated samples from 150 to 400 K. The multi-pattern Rietveld method, using atomic displacements with respect to a known structure as parameters to refine, has been applied to facilitate the interpretation of the NTE behavior. Polyhedral distortions, transverse vibrations of A· · ·O-Mo (A = Y and rare earths) binding oxygen atoms, non-bonded distances A· · ·Mo and atomic displacements from the high temperature structure, have been evaluated as a function of the temperature and the ionic radii.