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.