In this work, the ionic conductivity and charge carriers of acceptor-doped sodium tantalate (NaTaO 3 ) with perovskite structure were investigated at intermediate temperatures. The Ta-site of NaTaO 3 was doped with up to 20% titanium (Ti) with the conventional solid-state reaction method. After calcination at 900°C, samples nominally doped with 5, 10% Ti show X-ray diffraction (XRD) pattern of orthorhombic NaTaO 3 only, while peaks of Na 2 Ti 3 O 7 can be observed in those doped with 15, 20% Ti. The conductivity of undoped, 5% Ti and 10% Ti-doped NaTaO 3 at 300°C-700°C was measured with electrochemical impedance spectroscopy (EIS) under dry and wet O 2 atmospheres. Ti-doped NaTaO 3 samples have higher conductivity in the wet atmosphere than in the dry atmosphere, reaching 3 × 10 −4 S/cm at 700°C (10% Ti-doped NaTaO 3 ), which confirms the hydration and proton conduction in Ti-doped NaTaO 3 . Through the investigation on the dependence of conductivity on oxygen partial pressure, hole conduction in an oxidizing atmosphere, and electron conduction in reducing atmosphere can be verified. Na + conduction was proven to be negligible with direct current polarization.

NaTaO 3 shows significant proton conductivity and a high transport number of ionic conduction after being doped titanium (Ti). In this work, we aimed to determine the influence of sodium (Na) deficiency and Ti concentration in NaTaO 3 , for which NaTaO 3 samples with various Na deficiencies and Ti concentrations were prepared and characterized. The investigation was performed on microstructure, conductivity, transport number of ionic conduction, etc. Na deficiency was confirmed to decrease the conductivity of 1% Ti-doped NaTaO 3 , although it raises the conductivity of undoped NaTaO 3 . Summarizing the measured results of 1%-4% Ti-doped NaTaO 3 , a higher Ti concentration was found to significantly increase the proton conductivity of NaTaO 3 . However, when Ti is more than ∼3%, the higher concentration of Ti leads to a decrease in conductivity.Overall, ∼2% Ti was confirmed to be most beneficial to proton conductivity. The influence of phase transformation on proton conductivity was also discussed. In conclusion, precise control of Na amount is of great importance for the conduction properties of NaTaO 3 and ∼2% Ti is determined to be most beneficial to the conduction properties. In this case, through the adjustment of Ti concentration and control of Na deficiency, a bulk conductivity as high as 1.3 × 10 -3 S/cm at 600 • C and 7.8 × 10 -5 S/cm at 300 • C was realized with ∼2% Ti-doped NaTaO 3 in wet O 2 .

With the aim to elucidate the proton conduction mechanism in sodium tantalite with an orthorhombic perovskite structure, o‐NaTaO3, proton sites and elementary processes of proton jumps in the crystal were explored using first‐principles calculations. In this crystal, interoctahedral hopping plays a key role in the proton conduction, contrast to cubic perovskites where protons migrate over a long range only by rotation and intraoctahedral hopping. The interoctahedral hopping results from the tilting structure of TaO6 octahedra units, which accelerates the relatively fast proton migration along b‐ and c‐axes compared to that along a‐axis. This suggests that the low symmetry of the crystal structure can be beneficial to proton conduction in some cases, although the low symmetry is generally considered to make the potential barrier of long‐range proton migration higher. Considering the trapping effect of Ti dopants as the change in the concentration of mobile protons, the calculated proton conductivity is in excellent agreement with the experimental one.