Synchrotron and neutron powder diffraction study of phase transition in weberite-type Nd3NbO7 and La3NbO7

La3NbO7 and Nd3NbO7 are insulating compounds that have an orthorhombic weberite-type crystal structure and undergo a phase transition at about 360 and 450 K, respectively. The nature of the phase transitions was investigated via heat capacity measurements, synchrotron X-ray and neutron diffraction experiments. It is here shown that above the phase transition temperature, the compounds possess a weberite-type structure described by space group Cmcm (No. 63). Below the phase transition, the high temperature phase transforms into a weberite-type structure with space group Pmcn (No. 62). The phase transformation primarily involves the off-center shifting of Nb5+ ions inside the NbO6 octahedra, combined with shifts of one third of the Ln3+ (Ln3+=La3+ and Nd3+) ions at the center of the LnO8 polyhedra towards off-center positions. The phase transition was also proven to have great impacts on the dielectric properties.

Graphical abstractThe phase transition of La3NbO7 and Nd3NbO7 upon cooling primarily involves the off-center shifting of Nb5+ ions inside the NbO6 octahedra, combined with shifts of one third of the Ln3+ (Ln3+=La3+ and Nd3+) ions at the center of the LnO8 polyhedra towards off-center positions. Above the phase transition temperature, Ln3+ and Nb5+ sit at a position with mirror symmetry and 2-fold axis. Below the phase transition temperature, Ln3+ and Nb5+ shift away from the 2-fold axis but still in the mirror plane.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► La3NbO7 and Nd3NbO7 exhibit a phase transition at 360 and 450 K, respectively. ► The phase transformation primarily involves the off-center shifting of Nb5+ and Ln3+. ► The off-center ionic displacement decreases with increasing temperature. ► Dielectric properties are greatly related to the off-center displacement.