High Ionic Conductivity in Oxygen-Deficient Ti-Substituted Sodium Niobates and the Key Role of Structural Features

NaNb_1–xTi_xO_3–0.5x (0 < x ≤ 0.15) compounds were prepared using a two-step synthesis process involving a hydrothermal route at T = 200 °C in an autoclave followed by heat treatments under air or reductive conditions. Rietveld structural refinements from X-ray diffraction data combined with ²³Na and ⁹³Nb nuclear magnetic resonance evidenced the formation of new complex oxides crystallizing in the P2₁ma space group. Ti substitution for Nb atoms contributes to stabilize the acentric polymorph rather than the well-known thermodynamically stabilized network (Pbma space group) of sodium niobate. Taking into account the competitive bond sequence Na1(2)–O1(2)–Nb–O3(4), the large variation of Na1–O1 and Na2–O2 bond lengths after Ti substitution leads to reduction of the Nb/Ti–O1 and Nb/Ti–O2 apical distortion (elongation in one direction) and consequently exaltation of the distortion in the equatorial plane. Then, transition metal crystal field splitting, as well as the second-order Jahn–Teller effect, increases, and the optical band gap red-shifts to visible range starting with low Ti content. Two phase transition sequences at moderated temperature are characterized by the relaxation of the perovskite framework with various [Nb­(Ti)­O₆] octahedral distortion and tilt modes: from the polar orthorhombic P2₁ma phase to the centrosymmetric orthorhombic Cmcm network above T = 300 °C and then to the ideal cubic perovskite structure above T = 600 °C. The pronounced decrease in phase transition temperature with Ti substitution, especially from P2₁ma to Cmcm, was correlated to the almost identical stabilities of the two Na sites and four oxygen positions in P2₁ma symmetry but also to larger distortion of the transition metal polyhedron enhancing the oxygen mobility. Moreover, the high ionic conductivity of oxygen-deficient NaNb_1–xTi_xO_3–0.5x was evidenced for the first time between 300 and 700 °C (σ_ion (T = 300 °C) = 3 × 10^–5 S·cm^–1 and σ_ion (T = 700 °C) = 2 × 10^–3 S·cm^–1, for x = 0.15).