Conduction properties of Ti‐doped NaTaO₃ at intermediate temperature

Abstract

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.

First‐principles analysis of proton conduction mechanism in perovskite‐structured sodium tantalate

“…For example, high oxide‐ion conductivities were observed in acceptor‐doped sodium and potassium niobates, NaNbO 3 and KNbO 3 , 10–12 and potassium tantalate, KTaO 3 , was reported to exhibit proton conductivity by Cu or Fe doping 7–9 . In our recent study, 14 we have confirmed the proton conductivity in titanium‐doped sodium tantalite (Ti‐doped NaTaO 3 ) at intermediate temperatures. By Ti‐doping, oxygen vacancies, $$V_{\rm{O}}^{ \bullet \bullet }$$, are introduced into the crystal, and Ti‐doped NaTaO 3 hydrates in humidity, generating proton charge carriers.…”

“…Conventionally, the tilting of TaO 6 octahedra and the anisotropy of the crystal structure are supposed to result in a deteriorated conductivity 17 . However, the measured conductivity of Ti‐doped NaTaO 3 is reported to be the highest class among all the reported I – V perovskite ceramic, 14 which means there might be some ignored mechanism promoting proton conduction in Ti‐doped NaTaO 3 . Some previous researches have theoretically investigated the proton conduction mechanism in orthorhombic perovskites CaZrO 3 and SrZrO 3 by first‐principles calculations 18–21 .…”

The influence of sodium deficiency and titanium doping concentration on conduction properties in NaTaO3−δ

“…Different from undoped samples, both 1% Ti‐doped samples have a greatly enhanced conductivity in a wet atmosphere, confirming the proton conduction generated by Ti‐doping. Same as demonstrated in our previous study, 20 there is supposed to be oxygen vacancy ($$V_{\rm{O}}^{ \bullet \bullet }$$) generated by Ti doping (Equation ). In this work, samples were sintered at 1200°C in dry O 2 , in which TiO 2 is quite stable and low oxygen partial pressure is necessary for the existence of Ti 2+ or Ti 3+ .…”

“…Therefore, the higher conductivity of Na‐deficient sample can be attributed to the increased charge carrier $$V_{{\rm{Na}}}^{\rm{^{\prime}}}$$, $$V_{\rm{O}}^{ \bullet \bullet }$$, and/or electronic carriers. In our previous work, 20 the undoped sample Na0.974 shows increased conductivity with hole conduction in high p (O 2 ) (Equation ), but the increase is limited and the ionic conduction is still dominant: $$\begin{equation}2{\rm{Na}}_{{\rm{Na}}}^ \times + {\rm{\;O}}_{\rm{O}}^ \times = 2V_{{\rm{Na}}}^{\rm{^{\prime}}}{\rm{\;}} + V_{\rm{O}}^{ \bullet \bullet } + {\rm{N}}{{\rm{a}}_2}{\rm{O}} \uparrow \end{equation}$$ $$\begin{equation}2V_{\rm{O}}^{ \cdot \cdot } + {\rm{\;}}{{\rm{O}}_2} = {\rm{\;}}2{\rm{O}}_{\rm{O}}^ \times + 4{{\rm{h}}^ \cdot }\end{equation}$$…”

“…The conductivities of the reported I–V materials are too low for further development or application, so it is of importance to improve the conductivities of I–V materials. Previously, we have confirmed the hydration and proton‐conduction in 5% and 10% titanium (Ti)‐doped NaTaO 3 and discussed the influence of Na conduction and oxygen partial pressure 20 . However, the conductivity of the reported 5% or 10% Ti‐doped NaTaO 3 is only limited to about 3 × 10 −4 S/cm even at 700°C.…”