Isolating the Two Room-Temperature Polymorphs of NaNbO₃: Structural Features, Optical Band Gap, and Reactivity

The two room-temperature NaNbO3 polymorphs crystallizing with orthorhombic symmetry have been successfully isolated at using a new preparative method. The pure polar phase, annealed at 600 °C under air after hydrothermal treatment at 200 °C, adopts the P21ma space group, whereas the well-known and thermodynamically stable form with the Pbma structure is obtained at higher temperatures under air (T = 950 °C). Thanks to the combination of powder XRD Rietveld analysis, 23Na solid-state NMR spectroscopy, and second harmonic generation studies, structural features of both these polymorphs reveal clear structural differences. The stability of each atom site is investigated by means of bond distances, Madelung potentials, and DFT calculations. The key role of Na atomic positions is highlighted, especially how they influence the [NbO6] octahedron distortion. In the polar P21ma phase, the higher distortion along both the apical axis and the equatorial plane is consecutive to the relaxation of the overall network with the stabilization of Na atoms in two sites sharing the same symmetry. Focusing on oxygen mobility, both polymorphs show distinct reactivities toward reductive heat treatments: as characterized by thermogravimetric analysis and ESR measurements, the Pbma framework is relatively insensitive, while the P21ma one yields a nonstoichiometric oxide with a Nb4+ content of 18% corresponding to NaNbO2.91 chemical composition. The fine control in phasic purity together with advances on the structural features of room-temperature phases should benefit both nonlinear optical applications and photocatalytic performances of sodium niobates