Investigation of the hydrothermal crystallisation of the perovskite solid solution NaCe1−xLaxTi2O6 and its defect chemistry

• Direct hydrothermal synthesis allows crystallisation of a perovskite solid-solution.
• XANES spectroscopy shows some oxidation of Ce3+ to Ce4+.
• The paramagnetism of Ce3+ shifts and broadens the 23Na solid-state NMR.
• The perovskite materials incorporate water as an A-site defect.

Perovskites of nominal composition NaCe1−xLaxTi2O6 (0≤x≤1) crystallise directly under hydrothermal conditions at 240 °C. Raman spectroscopy shows distortion from the ideal cubic structure and Rietveld analysis of powder X-ray and neutron diffraction reveals that the materials represent a continuous series in rhombohedral space group R3̄c. Ce LIII-edge X-ray absorption near edge structure spectroscopy shows that while the majority of cerium is present as Ce3+ there is evidence for Ce4+. The paramagnetic Ce3+ affects the chemical shift and line width of 23Na MAS NMR spectra, which also show with no evidence for A-site ordering. 2H MAS NMR of samples prepared in D2O shows the inclusion of deuterium, which IR spectroscopy shows is most likely to be as D2O. The deuterium content is highest for the cerium-rich materials, consistent with oxidation of some cerium to Ce4+ to provide charge balance of A-site water.

A multi-element A-site perovskite crystallises directly from aqueous, basic solutions at 240 °C; while the paramagnetic effect of Ce3+ on the 23Na NMR shows a homogeneous solid-solution, the incorporation of A-site water is also found from 2H NMR and IR, with oxidation of some cerium to charge balance proved by XANES spectroscopy.Figure optionsDownload as PowerPoint slide