Crystal structures and phase stability of antiferroelectric R phases in undoped and Ca-modified sodium niobates

We investigate the crystal structures, phase transitions, and phase stability of undoped and Ca-modified $\mathrm{NaNb}{\mathrm{O}}₃$ through a combined analysis of high-resolution synchrotron radiation x-ray and neutron diffraction, convergent-beam electron diffraction, and density functional theory calculations. It is demonstrated that the antiferroelectric (AFE)-R phase is stabilized over a wide temperature range of 200--800 K by Ca modification, which is associated with the formation of Na vacancies at the A site. Our results reveal that the high-temperature AFE-R phase of undoped $\mathrm{NaNb}{\mathrm{O}}₃$ exhibits Pmmn symmetry whereas the room-temperature AFE-R phase of the Ca-modified $\mathrm{NaNb}{\mathrm{O}}₃$ adopts Pnma symmetry. We found that the Pmmn phase of $\mathrm{NaNb}{\mathrm{O}}₃$ is stabilized by temperature-driven isostatic pressure accompanied by lattice expansion, whereas the Pnma one of the Ca-modified $\mathrm{NaNb}{\mathrm{O}}₃$ is induced by composition-induced chemical pressure along with lattice shrinkage.