Thermal Expansion of Ln₆ WO₁₂ (Ln = Y,Ho,Er,Yb) and Ln₂ WO₆ (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°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 Ln 2 WO 6 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.