Synchrotron and neutron powder diffraction study of phase transition in weberite-type Nd 3NbO 7 and La 3NbO 7

Abstract

La 3NbO 7 and Nd 3NbO 7 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 Nb 5+ ions inside the NbO 6 octahedra, combined with shifts of one third of the Ln 3+ ( Ln 3+=La 3+ and Nd 3+) ions at the center of the LnO 8 polyhedra towards off-center positions. The phase transition was also proven to have great impacts on the dielectric properties.