Structural Phase Transition of Gd₃RuO₇

Structural phase transition of trigadorinium ruthenium heptaoxide, Gd3RuO7, has been investigated by in situ high-temperature single-crystal X-ray diffraction. A small shrinkage in b-length and an expansion in c-length were observed between 363 and 383 K with increasing temperature. No significant change occurred in a-length within experimental errors. The changes were essentially reversible against temperature. Structures of the high-temperature modification have been determined at 423, 773, and 1223 K assuming the orthorhombic Cmcm symmetry. The structure of the low-temperature modification has been determined at 293 K, assuming the orthorhombic P21nb symmetry with doubled unit cell along the b-axis of the high-temperature modification. The transition from high- to low-temperature modification can be structurally characterized by tilts about axes close to the c-axis of the unit cell occurring on half of the RuO6 octahedra. These octahedral tilts couple with a reduction in coordination number of the Gd atom bridging the adjacent RuO6 single chains along the b-axis. The present study also revealed the presence of structural disorder in the high-temperature Cmcm modification that had not been reported for the archetypal Cmcm structures of lanthanide ruthenates (Ln3RuO7) and osmates (Ln3OsO7) in the literature. The disorder includes a dynamical or static distribution of one-third of Gd atoms in the unit cell, which is presumably linked to the libration of the octahedral tilts about the axes close to c.