Phase transition in weberite-type Gd₃NbO₇

Gd₃NbO₇ has an orthorhombic weberite-type crystal structure and undergoes a phase transition at about 340 K. The phase transition was confirmed by second harmonic generation, heat capacity measurement, and high-resolution X-ray diffraction. Above 340 K, Gd₃NbO₇ has a centrosymmetric weberite-type structure with space group Cmcm (No. 63) and transforms at about 340 K into a non-centrosymmetric structure with space group Cm2m (No. 38). The phase transition is mainly due to off-center shifts of Nb⁵⁺ ions within NbO₆ octahedra, as well as off-center shifts of one-third of the Gd³⁺ ions within GdO₈ polyhedra.