Ca_1.46Ti_1.38Nb_1.11O₇ was synthesized using a solid-state reaction method. High-purity CaCO₃, TiO₂ and Nb₂O₅ powders (≥ 99.9%, Aladdin Biochemical Technology Co., Ltd., China) were used as starting materials, which were dried in an oven (DHG-9075A, Yiheng Scientific Instrument Co., Ltd., China) to remove adsorbed water prior to weighing. The starting materials were ground in anhydrous ethanol (AR, ≥ 99.7%, Sinopharm Chemical Reagent Co., Ltd., China) for approximately 30 min. The mixtures were pressed into pellets and pre-sintered at 1,000 °C for 12 h, and then sintered at 1,350 °C for 8 h. The sintered sample was ground, pressed into pellets, and finally sintered at 1,350 °C for 8 h for densification. All sintering processes were carried out in Ar gas (≥ 99.999%, Dalian Special Gases Co., Ltd., China) using a horizontal tube furnace (GYS-1700C, Hefei Weiyuans Laboratory Equipment Co., Ltd., China).

The X-ray diffraction (XRD) pattern was obtained using a powder diffractometer (X'Pert3 Powder, PANalytical, The Netherlands) with Cu Kα radiation. Rietveld refinement and Maximum Entropy Method (MEM) analysis were carried out using the Z-Rietveld software. BVSE calculations were performed using the softBV program to calculate the oxide-ion migration barriers. Visualization of the results was achieved using VESTA software. The elemental composition of the sample was determined by X-ray fluorescence spectrometry (XRF) using a wavelength-dispersive spectrometer (S8 Tiger, Bruker, Germany). Field emission scanning electron microscopy (SEM) (JSM-7900F, JEOL Ltd., Japan) equipped with energy-dispersive X-ray spectroscopy (EDS) was employed to investigate the morphology of the pellet samples and the elemental distribution of the powder samples. The particle size distribution of the sample was determined using a laser particle size analyzer (BT-9300H, Dandong Bettersize Instruments Co., Ltd., China). For electrochemical impedance spectroscopy measurements, pellets (approximately 10 mm in diameter and 2 mm in thickness) were used. Platinum paste electrodes (55H-1800, Shenzhen Saiya Electronic Paste Co., Ltd., China) were coated on its opposite faces. The sample was then heated at 1,000 °C for 2 h in a horizontal tube furnace (GYS-1700C, Hefei Weiyuans Laboratory Equipment Co., Ltd., China) to form dense platinum electrodes. Electrochemical measurements were performed on an electrochemical workstation (SP-300, Bio-Logic, France; or DH7002A, Donghua Analytical Instrument Co., Ltd., China) using the two-electrode method during the heating process. The sample was heated to temperatures between 500 and 900 °C in either a tubular furnace (JS-G5012-S, Tianjin Jiusuo Technology Development Co., Ltd., China) or a solid oxide fuel cell test station (HBSOC-5B, Zhejiang H₂-Bank Energy Technology Co., Ltd., China), followed by electrochemical measurements at each temperature. The process consisted of purging the system with various gases, including pure Ar, pure air, an Ar-H₂ mixture, an H₂-N₂ mixture, and an N₂-O₂ mixture. All gases (including the component gases in the mixtures) were of high purity (≥ 99.999%) and supplied by Dalian Special Gases Co., Ltd., China. The impedance was measured over a frequency range of 7 MHz to 0.1 Hz with an applied voltage of 100 mV. The obtained impedance data were fitted using Zview 3.1 software. The thermogravimetry (TG) curve was recorded on a thermogravimetric analyzer (STA 449 F3, NETZSCH-Gerätebau GmbH, Germany) over a temperature range of 80 to 1,000 °C. The gas flow rate was set at 20 mL/min, and the heating rate was set at 10 °C/min. High-purity Ar or air gas (≥ 99.999%, Dalian Special Gases Co., Ltd., China) was used during the measurement. X-Ray Photoelectron Spectroscopy (XPS) characterization was performed using an X-ray photoelectron spectrometer (Axis Supra+, Shimadzu, China) to analyze the chemical state of elements in the samples. The Al/Ag double anode X-ray source was used for excitation. The main peak of C1s at 284.8 eV was used as the reference for charge correction, and the spectra were analyzed using Avantage software. Electron paramagnetic resonance (EPR) spectra were recorded at room temperature using a spectrometer (E500, Bruker Corporation, Germany) operating in the X-band at a frequency of 9.42 GHz.

After the first sintering at 1,350 °C, the room-temperature XRD pattern of Ca_1.46Ti_1.38Nb_1.11O₇ confirmed the formation of the target phase, along with a small amount of TiO₂ as an impurity. This suggests that sintering temperatures below 1,350 °C may result in higher impurity levels due to incomplete reaction. To eliminate the TiO₂ impurity and achieve densification, the material was subjected to a second sintering at 1,350 °C for 8 h. Figure 1 shows the refined XRD pattern obtained at 700 °C of Ca_1.46Ti_1.38Nb_1.11O₇ after the second sintering at 1,350 °C. All diffraction peaks can be indexed to the pyrochlore structure, indicating the formation of a pure phase. Table 1 lists the refined structural parameters obtained from in situ XRD data. Because the data were collected at 700 °C, they provide a more realistic representation of the crystal structure at high temperatures, which is essential for accurately predicting the ionic conduction mechanism under operating conditions. Ca_1.46Ti_1.38Nb_1.11O₇ is a typical pyrochlore structure with a space group of Fd-3m (No. 227, origin choice 2). There are two different kinds of oxygen sites, O1 (48f) and O2 (32e). The O2 site contains a high concentration of oxygen vacancies, while the O1 site exhibits a relatively low vacancy concentration. These parameters were then used to predict the influence of structural changes on the material's properties. The occupancies of Ca, Ti, and Nb were determined based on the stoichiometry of starting materials. The R_F is below 7%, and R_wp and R_p are below 4%, indicating that the refined crystal structure is in excellent agreement with the experimental data^[16]. XRF analysis was conducted to verify the elemental composition of the synthesized material. Quantification was performed using a standardless fundamental parameters (FP) method. The measured oxide contents are 24.5 wt% CaO, 32.3 wt% TiO₂, and 43.2 wt% Nb₂O₅. Based on these values, the calculated molar ratios of Ca, Ti, Nb and O are 1.46:1.35:1.09:6.88, which are very close to the nominal stoichiometry. As shown in Figure 2, the EDS mapping diagrams of Ca, Ti, Nb, O confirm that the elements distribute homogeneously without any apparent clustering or segregation^[31]. Combined with the XRD results, this confirms the formation of a single-phase Ca_1.46Ti_1.38Nb_1.11O₇. Figure 3A shows the cross-section SEM image of the Ca_1.46Ti_1.38Nb_1.11O₇ pellet. A relatively dense microstructure can be observed, with particle sizes visually estimated to be in the range of several hundred nm. The particles are needle-like in shape. The relative density of the sample reached 92.6% as measured by the Archimedes method, which is close to the upper limit typically achieved by conventional solid-state sintering (manual grinding, uniaxial pressing). This indicates that a sintering temperature of 1,350 °C is sufficient for Ca_1.46Ti_1.38Nb_1.11O₇ and that higher temperatures are unnecessary. However, the particle size distribution appears inhomogeneous in the SEM image. This observation is consistent with the particle size distribution measured by the laser diffraction particle size analyzer, which gives an average particle size of approximately 412.8 nm with a PDI of 0.371 [Figure 3B], confirming a broad size distribution. The inhomogeneity in particle size can be attributed to non-uniform temperature distribution within the pellet during sintering.

Conductivity measurements were first performed in Ar to both prevent oxidation of the material and to assess its electrical response following sintering in Ar. Electrochemical impedance spectroscopy was used to characterize the electrical properties of Ca_1.46Ti_1.38Nb_1.11O₇. The resistance in the high frequency range represents the grain resistance, the resistance in the mid-frequency range corresponds to the grain-boundary resistance, and the resistance in the low-frequency range can be due to the polarization effect of the electrodes. The impedance plots [Figure 4A-C] exhibit a single, nearly symmetric semicircular arc, which arises from overlapping grain and grain-boundary responses and can be modeled using an equivalent circuit with a single RC (resistance-capacitance) element. The total conductivity (σ_b+gb, representing the sum of bulk and grain-boundary contributions) of Ca_1.46Ti_1.38Nb_1.11O₇ is presented in the Arrhenius plot in Figure 4D. These values were derived from the impedance spectra measured in Ar, 5% H₂/Ar, and air atmospheres over the temperature range of 500-900 °C. The conductivity (σ) was calculated from the resistance using Equation (1)^[32],

(1) $$ \begin{equation} \begin{aligned} \sigma=\frac{1}{R} \times \frac{l}{A} \end{aligned} \end{equation} $$

where R, l, and A represent the resistance, the spacing between the probes, and the cross-sectional area of the sample, respectively.

σ_b+gb of Ca_1.46Ti_1.38Nb_1.11O₇ increases gradually with increasing temperature from 500 to 900 °C, indicating a thermally activated ionic diffusion process. The conductivity of the sample varies approximately linearly with temperature in air. The temperature dependence of the conductivity in air can be described by Equation (2)^[33],

(2) $$ \begin{equation} \begin{aligned} \sigma T=\sigma_{0} e^{-\frac{E}{k_{B}T}}\end{aligned} \end{equation} $$

where σ, T, σ₀, E, and k_B are the conductivity, absolute temperature, pre-exponential factor, activation energy, and Boltzmann constant, respectively.

Across the entire temperature range, the conductivity follows the order: 5% H₂/Ar > Ar > air, indicating that the material exhibits n-type conduction. The conductivity of Ca_1.46Ti_1.38Nb_1.11O₇ was 4.20 × 10^-2 S cm^-1 in 5% H₂/Ar, 9.22 × 10^-3 S cm^-1 in Ar and 2.92 × 10^-5 S cm^-1 in air at 900 °C. Figure 5 demonstrates that the change in conductivity upon gas switching is reversible. This sensitive and reversible conductivity response to different atmospheres suggests that the material is a promising candidate for gas sensor applications. The DC conductivity σ_DC of Ca_1.46Ti_1.38Nb_1.11O₇ as a function of oxygen partial pressure (pO₂) is shown in Figure 6A. At 900 °C, σ_DC decreases with increasing pO₂, which is consistent with n-type conduction. σ_DC changes little with the oxygen partial pressure in the range of 1.0 × 10^-5 to 1.0 atm at 500 °C, indicating that the conductivity is primarily ionic at this temperature. Negligible proton conduction is observed because there is little difference in σ_DC of Ca_1.46Ti_1.38Nb_1.11O₇ under wet and dry air flows [Figure 6B]. Electromotive force (EMF) measurements were conducted to determine the ion transport number (t_ion) of the material under different atmospheres [Figure 7]. Using an air|N₂ gas concentration cell, the t_ion value is 0.995 at 500 °C, indicating that the material is an almost pure oxide-ion conductor at this temperature. The t_ion values in the entire temperature range are above 0.8, suggesting that oxide-ion conduction is dominant in oxidizing (air, O₂) and inert (Ar, N₂) atmospheres. In contrast, EMF measurements using an air|5% H₂/N₂ gas concentration cell revealed a drastically different behavior. The t_ion values are below 0.25 in the whole temperature range, indicating that the material exhibits predominantly electronic conduction under a reducing atmosphere (e.g., 5% H₂/N_2, 5% H₂/Ar). Figure 8 presents a comparison of the conductivity of Ca_1.46Ti_1.38Nb_1.11O₇ with that of recently reported pyrochlores. Compared with these pyrochlores, the conductivity of Ca_1.46Ti_1.38Nb_1.11O₇ is lower in air, comparable in Ar, and significantly higher in 5% H₂/Ar. σ_DC remains stable over 27 h [Figure 9A], demonstrating the excellent long-term stability of the material. No significant degradation in σ_b+gb is observed over 7 thermal cycles between 100 and 900 °C [Figure 9B], demonstrating the high structural stability of the material under repeated thermal cycling. These results indicate that Ca_1.46Ti_1.38Nb_1.11O₇ is suitable for high-temperature applications.