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

Abstract

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, 23 Na 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 mean 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 both along 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 non-stoichiometric oxide with a Nb 4+ 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 non-linear optical applications and photocatalytic performances of sodium niobates.

Associating and Tuning Sodium and Oxygen Mixed‐Ion Conduction in Niobium‐Based Perovskites

“…The activation energy of 0.83 eV related to NaNbO 3 is rather close to values found in perovskite‐derived frameworks such as Na 3 x La 1.33− x □ 0.67−2 x Ti 2 O 6 (0.92 eV) than in Na/Zr/Mg‐based silicate‐phosphates (0.25 eV) . The slightly smaller value compared to the titanate can be attributed to facilitated Na diffusion pathways, limited by smaller bottleneck formed by four adjacent [TiO 6 ] octahedra in Na 3 x La 1.33− x □ 0.67−2 x Ti 2 O 6 (average Ti–O distance of 1.95 Å) than four adjacent [NbO 6 ] in NaNbO 3 (Nb‐O distances from 1.90 to 2.09 Å, average 1.97 Å) . For both oxygen and sodium, the absence of site vacancy accounts for poor overall conductivity.…”

“…NaNbO 3 ceramics with relative density of at least 94% are reached by SPS. After this treatment, the network remains identical with the Pbma space group ( Figure a, see also Table S1, Supporting Information), as confirmed by a typical signature in the region 35.5° < 2 Θ < 44.5° (Cu K α source) . Despite the similarities of X‐Ray diffraction (XRD) patterns of both RT polymorphs, the NbO 6 octahedron distortion and complex octahedral tilt schemes vary in connection with Na local environments (Figure b).…”

“…The closest NbO bond lengths involving the two apical oxygens (O1 and O2 in the vicinity of Na1 and Na2 respectively, Figure b) and the alternation of longer and shorter NbO bond distances in the equatorial plane (O3 and O4), underline the octahedral tilts schemes in three different directions and the key role of Na atomic positions which influence indirectly the second order Jahn–Teller distortion. As opposed to the acentric polymorph, the centrosymmetric structure in the Pbma space group displays distinct Na atomic sites by means of environment (symmetry, Na–O lengths) and stability, contributing to a less distorted but more rigid framework …”

“…Na sites contribute to constrain [NbO 6 ] octahedron to be more distorted but less rigid in P 2 1 ma framework by competitive bonds. Such distortions favor oxygen mobility . NaNb 1− x Ti x O 3−0.5 x (0 < x ≤ 0.20) complex oxides were very recently shown to preferentially crystallize in the low‐temperature form of sodium niobate, in the acentric P 2 1 ma space group .…”

“…[48][49][50][51][52][53] This structural complexity extends at room temperature for NaNb 1−x Ti x O 3−0.5x (0 ≤ x ≤ 0.20), with the occurrence of two structures, only one of them being polar and crystallizing in the acentric P2 1 ma space group. [54][55][56][57] Both the inequivalent Na sites are close in energy and stability in P2 1 ma network, whereas they are radically different in the centrosymmetric structure (Pbma space group). Nb 5+ (4d 0 ) and Ti 4+ (3d 0 ) stabilized in such perovskite networks exhibit second-order Jahn-Teller distortion (SOJT), which consists in the octahedral deformation due to high ionic charge, small ionic size (high polarizing power) and relatively low bandgap (3.5 eV) involving the mixing p(O) and d(Nb, Ti) orbitals.…”