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

NaNb1–xTixO3–0.5x (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 NaNb1–xTixO3–0.5x was evidenced for the first time between 300 and 700 °C (σion (T = 300 °C) = 3 × 10–5 S·cm–1 and σion (T = 700 °C) = 2 × 10–3 S·cm–1, for x = 0.15).NaNb₁₋ₓTiₓO₃₋₀.₅ₓ (0 < x ≤ 0.15) 化合物采用两步合成工艺制备：先在高压釜（autoclave）中于200 ℃下水热法（hydrothermal route）反应，随后在空气或还原气氛下进行热处理（heat treatments）。通过X射线衍射（X-ray diffraction）数据的里特维尔德结构精修（Rietveld structural refinements）结合²³Na与⁹³Nb核磁共振（nuclear magnetic resonance）谱，证实了新型复合氧化物的生成，该氧化物结晶于P2₁ma空间群（space group）。钛取代铌原子有助于稳定非中心对称晶型，而非铌酸钠已知的热力学稳定相（Pbma空间群）。结合Na1(2)–O1(2)–Nb–O3(4)的竞争性键序，钛取代后Na1–O1与Na2–O2键长的大幅变化，使Nb/Ti–O1和Nb/Ti–O2的轴向畸变（单方向伸长）得以降低，进而增强了赤道平面的畸变。随后，过渡金属晶体场分裂（crystal field splitting）以及二阶姜-泰勒效应（second-order Jahn–Teller effect）增强，光学带隙（optical band gap）发生红移（red-shift）至可见光范围，且仅需较低的钛掺杂量即可实现该效应。在中等温度区间内存在两条相变（phase transition）序列，其特征为钙钛矿骨架（perovskite framework）随[Nb(Ti)O₆]八面体（octahedral）畸变与倾斜模式发生弛豫：当温度高于300 ℃时，从极性正交晶系（orthorhombic）P2₁ma相转变为中心对称正交晶系Cmcm相；当温度高于600 ℃时，进一步转变为理想立方钙钛矿结构。钛取代导致相变温度显著降低，尤其是P2₁ma相向Cmcm相的转变，这与P2₁ma对称性下两个钠位点与四个氧位置的稳定性近乎一致有关，同时也与过渡金属多面体的更大畸变增强了氧迁移率相关。此外，缺氧型NaNb₁₋ₓTiₓO₃₋₀.₅ₓ的高离子电导率（ionic conductivity）首次在300~700 ℃区间得到证实：当x=0.15时，T=300 ℃下的离子电导率σ_ion=3×10⁻⁵ S·cm⁻¹，T=700 ℃下为2×10⁻³ S·cm⁻¹。