Thermal Expansion of Ln6WO12 (Ln = Y, Ho, Er, Yb) and Ln2WO6 (Ln = Gd, Dy, Ho) - an In Situ Synchrotron X-ray Diffraction Study

The thermal expansion (TE) behavior of cubic and rhombohedral Ln(6)WO(12) (Ln = Y, Ho, Er, Yb) and monoclinic Ln(2)WO(6) (Ln = Gd, Dy, Ho) was investigated by synchrotron X-ray diffraction from room temperature to approximately 1500 degrees C. The volumetric and lattice parameter expansions were measured for all compositions in both systems, and the respective expansion coefficients were derived and fitted over the observed temperature range with a second-order polynomial. The relative TE evolution along the unit cell edges in each phase was described from a crystallographic perspective, with the reasons for TE dynamics explained in terms of the fundamental constituent units of the structure. Analysis of the TE ellipsoid of the monoclinic Ln(2)WO(6) revealed a continuous change in both its shape and orientation, with the latter effect strongly manifested in the (010) plane. A noticeable reversal of the relative expansion rates between a, and b and c lattice parameters was attributed to the rotation of the ellipsoid cross section in this plane, bringing the larger of the two eigenvectors closer to a, while the smaller one to c. Investigating the structural dynamics of the constituent layers in Ln2WO6 suggested that the bridging role of the Ln polyhedron centered on the only general position in the unit cell may be the reason for the rotating TE ellipsoid.