Structural characterisation of the Ce1−xLaxNbO4+δ solid solution series: In-situ high-temperature powder diffraction studies

High-resolution neutron powder diffraction has been utilised to investigate the effect of lanthanum substitution on the structure of cerium niobate, CeNbO4+δ, as a function of temperature. Two members of the Ce1−xLaxNbO4+δ solid solution series, Ce0.8La0.2NbO4+δ and Ce0.2La0.8NbO4+δ, were examined over a temperature range of 293–923 K under a positive pressure of O2 (500 mbar). From this data it was found that on increasing lanthanum substitution there was an associated reduction in the temperature of the monoclinic-to-tetragonal phase transition. The data also suggested that increasing lanthanum substitution caused an associated decrease in the excess oxygen content. In addition, high-temperature X-ray powder diffraction data recorded in static air were also examined for four compositions of the Ce1−xLaxNbO4+δ series (x=0.2, 0.4, 0.6 and 0.8). These data corroborated the results of the neutron diffraction experiments and also suggested that there was formation of an intermediate phase, analogous to the CeNbO4.08 phase of the parent material, during the phase transitions of the x=0.8 and 0.6 compositions.