I. INTRODUCTION
Perovskite-type oxides, such as BaCeO3 and SrCeO3, are state-of-the-art high-temperature proton conductors as electrolytes in solid oxide fuel cells (SOFC) because of its high protonic conductivity when exposed to a humidified hydrogen-containing atmosphere at temperatures higher than 300 °C (Uchida et al., Reference Uchida, Maeda and Iwahara1983; Scherban et al., Reference Scherban, Lee and Nowick1988; Iwahara et al., Reference Iwahara, Asakura, Katahira and Tanaka2004; Tolchard and Grande, Reference Tolchard and Grande2007; Fu and Weng, Reference Fu and Weng2014; Knight et al., Reference Knight, Haynes, Bonanos and Azough2015). In order to improve proton conductivity, several kinds of rare-earth-doped BaCeO3 and SrCeO3 ceramics have been developed: (1) BaCe1−xRExO3−δ, via the B-site replacement of Ce4+ by acceptor-type trivalent rare-earth ions (RE3+) such as RE3+ = Y3+, Pr3+, Nd3+, Sm3+, Gd3+, Eu3+, Tb3+, and Yb3+ (Matsumoto et al., Reference Matsumoto, Suzuki and Iwahara1999; Wang et al., Reference Wang, Li, Campbell, Lv, Ji, Xue and Sua2004; Wu et al., Reference Wu, Li, Espinosa and Haile2004; Sharova et al., Reference Sharova, Gorelov and Balakireva2005; Malavasi et al., Reference Malavasi, Ritter and Chiodelli2008); (2) Ba(Ce0.8−yPryGd0.2)O2.9, via the B-site co-doping with double rare-earth ions Pr3+ and Gd3+ (Mukundan et al., Reference Mukundan, Davies and Worrell2001); and (3) BaCe0.2Zr0.7RE0.1O3−δ, via the B-site co-doping with Zr4+ and RE3+ (=Y3+, Sm3+) (Barison et al., Reference Barison, Battagliarin, Cavallin, Doubova, Fabrizio, Mortalò, Boldrini, Malavasic and Gerbasid2008; Ricote et al., Reference Ricote, Bonanos, Lenrick and Wallenberg2012; Kannan et al., Reference Kannan, Gill, Maffei and Thangadurai2013; Choi et al., Reference Choi, Lee, An, Hong, Kim, Yoon, Son, Kim, Lee and Lee2014). A more complex system Ba1−xSrxCe0.5Zr0.35Y0.1Sm0.05O3−δ with a cubic perovskite structure (the space group Pm-3m), in which Sr2+ and Zr4+/Y3+/Sm3+ are incorporated into the A- and B-sites in BaCeO3, respectively, shows high conductivity and high-density proton conductivity (Radenahmad et al., Reference Radenahmad, Afif, Petra, Rahman, Eriksson and Azad2016). From another perspective, the commercial application of materials has been hindered by the technical difficulty of creating fully dense ceramics with good mechanical properties (Dahl et al., Reference Dahl, Haugsrud, Lea Lein, Grande, Norby and Einarsrud2007). Thus, dense SrCeO3- or BaCeO3-based ceramics with simultaneous occupations of both the A- and B-sites may be promising candidates for high-temperature proton conductors in SOFC.
In this work, two ceramics with nominal compositions Ba1−x/3Srx/3Tb1−x/3Cex/3O3 (x = 1 and 1.5) (BSTC) were prepared using the mixed-oxides method. The refined crystal structures of BSTC are identified using Rietveld refinements. They exhibit higher densification and have perovskite structures with the general formula Ba2SrTb2CeO9 and Ba1.5Sr1.5Tb1.5Ce1.5O9, similar to Sr2CaFe2WO9 and Sr2PbFe2TeO9 perovskites, respectively (El Hachmi et al., Reference El Hachmi, Tamraoui, Manoun, Haloui, Elaamrani, Saadoune, Bih and Lazor2018). Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and electrical measurements were employed to discuss the crystalline structure and valence states of rare-earth ions Ce and Tb.
II. EXPERIMENTAL
A. Synthesis
Ceramic raw materials were reagent-grade BaCO3 (99.5%), SrCO3 (99.5%), Tb4O7 (99.9%), and CeO2 (99.9%) powders. Two ceramics were prepared according to the nominal formulas of Ba1−x/3Srx/3Tb1−x/3Cex/3O3 (x = 1 and 1.5) (abbreviated as BSTC1 and BSTC3/2, respectively) using a conventional mixed-oxides method. The stoichiometric mixtures in accordance with the above metal ratios Ba:Sr:Tb:Ce were carefully ground in an agate mortar, and then were calcined at 1100 °C for 5 h in air. After furnace cooling, the calcined mixtures were reground. Aqueous PVA (polyvinyl alcohol) solution was added into the calcined mixtures. The resulting powders were pressed uniaxially at 200 MPa into disk-like pellets of 12 mm in diameter. The discs were placed in a pile on the bottom of the Al2O3 crucible in an electric furnace. The discs were heated at a rate of 100 °C h−1 and sintered at 1400 °C for 12 h in air, cooled at a rate of −200 °C h−1 to 700 °C, and then furnace cooled to room temperature (RT) to form ceramics. In addition, BaCeO3, BaTbO3, SrCeO3, and SrTbO3 ceramics were prepared under the same conditions as BSTC.
B. X-ray powder diffraction
Diffraction data were collected at RT on a DX-2700 X-ray diffractometer (Dandong Haoyuan), with the Bragg–Brentano geometry, using CuKα radiation (λ = 1.5418 Å) with 35 kV and 30 mA, divergence slit of 1°; anti-scatter slit of 1°; receiving slit of 0.3 mm. The diffraction pattern was scanned between 5° ≤ 2θ ≤ 120° in 0.02° 2θ intervals with a fixed-time counting of 3 s step−1.
The Rietveld refinements were used to refine the crystal structure. The peak shape was described by a pseudo-Voigt function. The starting data needed for Rietveld refinements for BSTC1 and BSTC3/2 are the atomic positions and unit-cell parameters from BaCeO3 (88592-ICSD) and SrCeO3 (PDF#01-074-8250), respectively.
C. Scanning electron microscopy, backscattered electron, and energy-dispersive X-ray spectroscopy
Microstructures of polished and thermally etched ceramic surfaces were examined using an EVOMA 10 scanning electric microscope (SEM) (Zeiss) operated at 15 kV. To confirm the nature of single phase and the absence of secondary phases in BSTC, SEM investigations in the backscattered electron (BSE) mode were performed. Aztec 2.3 energy-dispersive X-ray (EDX) spectrometer (Oxford, UK) was attached to the SEM for compositional analyses.
D. Electrical measurements
Both surfaces of polished ceramic disks (10 mm in diameter and 0.8 mm in thickness) were sputtered with Au and silver paste to form electrodes for electric measurements. Temperature dependences of the electrical conductivity (σ), the dielectric permittivity (ε′), and the dielectric loss (tan δ) at a frequency of 1 kHz were measured from −75 to 200 °C at a heating rate of 2 °C min−1 using a Concept 41 Dielectric/Impedance spectrometer (Novocontrol) with an applied voltage of 1 V. Frequency dependences of σ, ε′, and tan δ were measured at RT. The accuracy in measurements of ε′, tan δ, and temperature control is less than 5%, 3 × 10−5, and ±0.3 °C, respectively.
E. Electron paramagnetic resonance
EPR spectra were measured using an A300 electron-spin resonance spectrometer system (Bruker BioSpin GMBH) at an X-band frequency of 9.148 GHz. The EPR of six samples, (1) 1 mg of BaTbO3 + 99 mg of BaCeO3, (2) 1 mg of SrTbO3 + 99 mg of SrCeO3, (3) 5 mg of BSTC1 + 95 mg of BaCeO3, (4) 5 mg of BSTC3/2 + 95 mg of BaCeO3, (5) 100 mg of BaCeO3, and (6) 100 mg of SrCeO3, were measured at RT. The g-factor is calculated by the formula hv 0 = gβH, where the Planck's constant is h = 6.0626 × 10−34 J s, v 0 is the frequency, β = 9.262 × 10−24 J T−1, and H is the magnetic field strength.
F. X-ray photoelectron spectroscopy
XPS measurements were performed at RT using an Escalab 250 Xi X-ray photoelectron spectrometer (Thermo Electron). XPS raw data were processed by smoothing multiple times. The core-level binding energy was calibrated using the C 1s peak located at 285 eV.
III. RESULTS AND DISCUSSION
A. Structure determination
For X-ray powder diffraction (XRPD) measurements, two factors are considered: (1) in general cases, the high-angle upper-limit 2θ m is taken to be 120° or larger, which is apt to yield satisfactory refined results in the crystal structure refinement (Chen et al., Reference Chen, Liang and Wang1995) and (2) the possible ordering of Ba2+/Sr2+ at the A-sites or Ce/Tb at the B-sites in the lattice will result in extra peaks, forming a so-called superstructure. The superstructure peaks with higher intensity are easier to be observed in the lower 2θ range of 10–20°, such as for A-site-ordered perovskites (Li1/2Nd1/2)TiO3 (Takahashi et al., Reference Takahashi, Baba, Ezaki, Okamoto, Shibata, Kuroki and Nakano1991) and (La1/2Na1/2)TiO3 (Ioshiyuki et al., Reference Ioshiyuki, Sohn, Kim, Itoh and Nakamura1992), as well as B-site-ordered perovskites Pb2CaTeO6 (Artner and Weil, Reference Artner and Weil2019), Ba2CaMoO6 (Nguyen et al., Reference Nguyen, Cava and Fry-Petit2019), and Ba2−xLaxFeMoO6 (Hussain et al., Reference Hussain, Khan, Rao, Kumar and Koo2019). For these two factors, the 2θ ranges in XRPD spectra of BSTC1 and BSTC3/2 are expanded from 5° to 120° and no diffraction peaks between 5° and 20° were observed. Their XRPD patterns (black line) show perovskite-like diffraction feature, as shown in Figure 1. On the basis of the trigonal BaCeO3 (88592-ICSD) and the orthorhombic SrCeO3 (PDF# 01-074-8250), the XRPD patterns of BSTC1 and BSTC3/2 can be refined by the Rietveld method, and their crystalline structures are indexed as a trigonal perovskite structure with the space group R-3c and an orthorhombic perovskite structure with the space group Pmcn, respectively. Table I gives the details of Rietveld refinements including lattice parameters, cell volumes, crystal system, space group, reliability factors (R p and R wp), and Uiso parameters. Table II gives positional parameters and occupancy at RT. The general formulas of BSTC1 and BSTC3/2 are expressed by Ba2SrTb2CeO9 and Ba1.5Sr1.5Tb1.5Ce1.5O9, respectively. Pure phases were obtained for these two compositions, as no secondary crystalline phase was detected in the measured XRD patterns.
Figure 1. (Colour online) Final Rietveld plots for (a) BSTC1 and (b) BSTC3/2 ceramics. The upper patterns illustrate the observed data (black line) and the calculated pattern (red line). The vertical green markers show the calculated positions of Bragg reflections. The lower blue curve is the difference diagram.
TABLE I. Details of Rietveld refinements for Ba2SrTb2CeO9 and Ba1.5Sr1.5Ce1.5Tb1.5O9 perovskites at RT.
TABLE II. Positional parameters and occupancy for Ba2SrTb2CeO9 (BSTC1) and Ba1.5Sr1.5Ce1.5Tb1.5O9 (BSTC3/2) perovskites at RT.
Under the current measuring conditions, no superstructure peaks were detected for BSTC. However, the superstructure may be imperceptible based on the XRPD, and the potential ordering in the structure is possibly undetectable by XRD. A long-time record of diffraction data benefits observations of superstructure peaks (Chen et al., Reference Chen, Bauernfeind and Braun1997). In order to investigate the possible ordering of Ba2+/Sr2+ at the A-sites or Ce/Tb at the B-sites in BSTC, the XRPD spectra of both samples were measured between 5° ≤ 2θ ≤ 20° in smaller 2θ intervals (0.01°) with a slow counting of 6 s step−1 (not presented here). Extra peaks were not observed. Thus, Ba2+/Sr2+ at the A-sites or Ce/Tb at the B-sites in the perovskite lattice are considered to be randomly disordered (Chen et al., Reference Chen, Chan and Harmer1989, Reference Chen, Liang and Wang1995; Ioshiyuki et al., Reference Ioshiyuki, Sohn, Kim, Itoh and Nakamura1992; Sahoo et al., Reference Sahoo, Zhang and Wang2016); their ordering is very weak if existed (Chen et al., Reference Chen, Bauernfeind and Braun1997). BSTC1 and BSTC3/2 can be thought to possess disordered perovskite structures.
The four ceramic phases prepared according to the same conditions as BSTC for structural and valence state analyses. The observed and simulated XRPD patterns of BaCeO3, BaTbO3, SrCeO3, and SrTbO3 are shown in Figure 2. BaCeO3, SrCeO3, and SrTbO3 exhibit orthorhombic perovskite structures, corresponding to PDF# 01-070-6741, PDF# 01-074-8250, and PDF# 01-089-5513, respectively, whereas BaTbO3 has a tetragonal structure (PDF# 01-074-4289).
Figure 2. (Colour online) Observed and simulated XRPD patterns of (a) BaCeO3, (b) BaTbO3, (c) SrCeO3, and (d) SrTbO3.
B. Microstructure and evidence of single phase
The BSE and SEM images, as well as EDX spectra for BSTC1 and BSTC3/2, are shown in Figure 3. Both samples show nonuniform microstructures with a grain size distribution from 0.2 to 3 µm, but they are denser. Their relative density (ρ r), which is referred to as the ratio of the volumetric mass density to the theoretical density (Lu et al., Reference Lu, Gao and Wang2019), was determined to be 93%. This reveals that dual doping with Sr and Tb in BaCeO3 is apt to the densification of ceramics.
Figure 3. (Colour online) BSE and SEM images of the polished and thermally etched surfaces as well as corresponding EDX investigations at a fine grain, a triple-grain boundary junction, and a coarse grain for (a) BSTC1 and (b) BSTC3/2 ceramics.
On the basis of BSE investigations, no difference in brightness was observed for all of the grains, suggesting the absence of secondary phases in BSTC. To further clarify the single-phase nature of BSTC, EDX investigations were made to provide evidence for compositional distributions in different grains and grain boundaries. For BSTC1, the ratios of Ba to Sr and Tb to Ce at a fine grain, a triple-grain boundary, and a coarse grain are 2.0–2.1 and 1.8–2.0, respectively, which are close to the theoretical values of Ba/Sr = Tb/Ce = 2. Similarly, the ratios of Ba/Sr = 0.9–1.0 and Tb/Ce = 0.8–1.0 for BSTC3/2 are also close to the theoretical value of Ba/Sr = Tb/Ce = 1. These EDXS results reveal the homogeneous concentration distributions of Ba, Sr, Tb, and Ce in BSTC. That is to say, Ba/Sr and Tb/Ce are completely incorporated into the A-sites and the B-sites in the BSTC perovskite lattice, respectively, and no secondary phase is present; both BSTC1 and BSTC3/2 are single-phase solutions. Figure 4 gives the schematic diagram of the crystal structure for BSTC1 as a representative.
Figure 4. (Colour online) A schematic diagram of the trigonal crystal structure for BSTC1. Red balls stand for O2− ions. Ba2+ and Sr2+ ions are represented by green/blue balls, whose surface consists of 2/3 green and 1/3 blue, representing the molar ratio of Ba2+/Sr2+ = 2:1. Tb and Ce ions by purple/yellow balls, and the corresponding colors reflect the molar ratio of Tb/Ce = 2:1.
C. Electrical properties
Figure 5 shows a plot of the electrical conductivity (σ) as a function of temperature (T) at 1 kHz for BSTC1 and BSTC3/2. The σ increases rapidly with increasing T. When T = 197 °C, the σ value of BSTC1 (σ = 5.8 × 10−6 S cm−1) is slightly greater than that of BSTC3/2 (σ = 4.5 × 10−6 S cm−1). This reveals that the difference in x has only a minor effect on the electrical conductivity of both samples. The σ at 25 °C increases slowly with increasing frequency (f), as shown in Figure 5 inset, which suggests that hole conduction is predominant, rather than electron conduction.
Figure 5. (Colour online) Plot of the electrical conductivity (σ) as a function of temperature for (a) BSTC1 and (b) BSTC3/2 ceramics, measured at 1 kHz. The inset depicts σ versus f at 25 °C.
Figure 6 shows the temperature dependences of the dielectric permittivity (ε′) at 1 kHz and the dielectric loss (tan δ) at RT for BSTC1 and BSTC3/2. Both ε′ and tan δ at 1 kHz increase rapidly with increasing T because of high electrical conducting behavior. This further confirms that BSTC1 and BSTC3/2 are semiconductors.
Figure 6. (Colour online) Temperature dependence of ε′ at 1 kHz for BSTC1 and BSTC3/2. The inset in (a) depicts tan δ versus T.
D. EPR investigations
The EPR technique can gather an insight into the valence state of rare-earth ions and vacancies in ceramics (Lu, Reference Lu2015; Lu et al., Reference Lu, Cui, Liu and Sun2016a). The EPR spectra of BaCeO3, SrCeO3, BaTbO3, SrTbO3, BSTC1, and BSTC3/2 at RT are shown Figure 7. Ce3+ (4f 1) Kramers ion in compounds is EPR silent at RT because of its short spin-lattice relaxation time. Ce4+ (4f 0) non-Kramers ion is also EPR silent in theory. Hence, no EPR signal was observed for BaCeO3 and SrCeO3.
Figure 7. (Colour online) EPR spectra of BaCeO3, SrCeO3, BaTbO3, SrTbO3, BSTC1, and BSTC3/2.
Our experiments clarified that the EPR cannot be observed for 100 mg of samples containing Tb because of the strong EPR from Tb4+. For this reason, 1 mg of BaTbO3 or SrTbO3 and 5 mg of BSTC1 or BSTC3/2 are dispersed into 99 and 95 mg of BaCeO3 or SrCeO3 for EPR observations. A very broad singlet signal at g = 2.020–2.029 appears in the four ceramics BaTbO3, SrTbO3, BSTC1, and BSTC3/2. This signal originates from Tb4+ (4f 7) Kramers ions because there is no EPR response for Ba2+, Sr2+, Ce4+, and O2−. For 5% Tb-doped BaTiO3 ceramics, however, Tb4+-related signal appears at g = ~6.5 (Lu, Reference Lu2015; Lu et al., Reference Lu, Peng, Yu and Sun2016b). This reveals that when TbO6 octahedrons act as the perovskite skeleton, the Tb4+-related EPR signal exhibits different g values. Thus, the EPR investigations provide evidence of the existence of a large number of Tb4+ ions in BSTC1 and BSTC3/2.
For doped BaTiO3 ceramics, oxygen vacancies (VO) can be detected in the low-temperature rhombohedral phase of T ≤ −100 °C (Lu et al., Reference Lu, Yuan, Liang and Zhu2016c). It is reported that SrCeO3 may remain orthorhombic, the space group Pbnm from 1.2 K up to the 1 atm melting point of 2266 K. (Knight et al., Reference Knight, Haynes, Bonanos and Azough2015). Our experiments confirm that no additional VO-related EPR signal was observed for BSTC1 and BSTC3/2 (not presented here) because no phase transition was observed in these two ceramics.
E. XPS investigations
XPS spectra of Ce 3d and Tb 3d core levels of BSTC1, BSTC3/2, SrCeO3, BaCeO3, SrTbO3, and Tb4O7 are shown in Figure 8. The Ce3+ and Ce4+ species in compounds can be differentiated by XPS, with distinct line shapes corresponding to various final states: Ce(III) = v 0 + v′ + u 0 + u′ and Ce(IV) = v + v″ + v′″ + u + u″ + u′″. The u′″ component, as a satellite peak, is a fingerprint of Ce4+ state, which arises from the so-called shake-down effect (Schneider et al., Reference Schneider, Laubschat, Nowik and Kaindl1981; Bêche et al., Reference Bêche, Charvin, Perarnau, Abanades and Flamant2008; Jaiswal et al., Reference Jaiswal, Hong, Yoon, Son, Lee and Lee2016; Xiong et al., Reference Xiong, Hua, Liu, Wu, Zhou, Wang, Jin and Lu2016). It can be seen from Figure 8(a) that the Ce 3d 3/2 and Ce 3d 5/2 photoelectron lines of all Ce-containing samples exhibit the characteristic features of tetravalent cerium, in good agreement with the reports from other authors (Braaten et al., Reference Braaten, Grepstadt and Raaen1989; Douillard et al., Reference Douillard, Gautier, Thromat, Henriot and Guittet1994; Bêche et al., Reference Bêche, Charvin, Perarnau, Abanades and Flamant2008). The final states corresponding to core-level binding energy are as follows: Ce 3d 94f 0 O 2p 6 final state: u′″ = 916.3 ± 0.1 eV, v′″ = 897.9 ± 0.1 eV; Ce 3d 94f 2 O 2p 4 final state: u = 900.4 ± 0.2 eV, v = 882.4 ± 0.2 eV; Ce 3d 94f 1 O 2p 5 final state: u″ = 907.0 ± 0.3 eV, v″ = 888.1 ± 0.1 eV; The Ce3+ spectrum (Blanco et al., Reference Blanco, Pintado, Bernal, Cauqui, Corchado, Galtayries, Ghijsen, Sporken, Eickhoff and Drube2002; Bêche et al., Reference Bêche, Charvin, Perarnau, Abanades and Flamant2008; Jaiswal et al., Reference Jaiswal, Hong, Yoon, Son, Lee and Lee2016; Xiong et al., Reference Xiong, Hua, Liu, Wu, Zhou, Wang, Jin and Lu2016; Liang, Reference Liang2019) is absent for BSTC. Thus, Ce is considered to be present in the form of Ce4+ and Ce3+ is negligible in BSTC1 and BSTC3/2.
Figure 8. (Colour online) XPS spectra corresponding to (a) Ce 3d core levels of BSTC1, BSTC3/2, SrCeO3, BaCeO3 and (b) Tb 3d core levels of BSTC1, BSTC3/2, SrTbO3, Tb4O7.
For terbium ions in compounds, the most intense photoemission peak corresponds to the 3d core level, peaking at ~1277 eV for Tb 3d 3/2 and ~1242 eV for Tb 3d 5/2, respectively (Van Den Bossche et al., Reference Van Den Bossche, Neyts, De Visschere, Corlatan, Pauwels, Vercaemst, Fierman, Poelma, Van Meirhaegh, Laflere and Cardon1994; Cao et al., Reference Cao, Shi, Xiu, Sun, Guo, Liu and Xue2010; Blanco et al., Reference Blanco, Pintado, Bernal, Cauqui, Corchado, Galtayries, Ghijsen, Sporken, Eickhoff and Drube2002). Tb3+ and Tb4+ due to their 3d-spectrum overlapping cannot be clearly identified. However, the core-level spectrum of Tb4+ was reported to show relatively intense 3d satellites with binding energy ca. 10 eV higher than those of the main peaks (Van Den Bossche et al., Reference Van Den Bossche, Neyts, De Visschere, Corlatan, Pauwels, Vercaemst, Fierman, Poelma, Van Meirhaegh, Laflere and Cardon1994; Martínez-Arias et al., Reference Martínez-Arias, Hungría, Fernandez-García, Iglesias-Juez, Conesa, Mather and Munuera2005; Blanco et al., Reference Blanco, Pintado, Bernal, Cauqui, Corchado, Galtayries, Ghijsen, Sporken, Eickhoff and Drube2002), but no 3d satellite of Tb3+ is present, as observed for TbF3 with Tb3+ (Van Den Bossche et al., Reference Van Den Bossche, Neyts, De Visschere, Corlatan, Pauwels, Vercaemst, Fierman, Poelma, Van Meirhaegh, Laflere and Cardon1994).
All XPS spectra of BSTC1, BSTC3/2, SrTbO3, and Tb4O7 show two Tb 3d binding energy peaks at 1276.7 ± 0.1 and 1242.1 ± 0.1 eV [Figure 8(b)], as mentioned above. A clear satellite at 1253.3 ± 0.1 eV confirms the existence of Tb4+ in these ceramics. The intensity of this satellite for SrTbO3 is obviously higher than the other three samples, which arises from the following two evidences: (1) the molar concentration of Tb in SrTbO3 is highest and (2) the high electrical conductivity (Figure 5) should be accompanied by oxygen vacancies, which are caused by some Tb3+ ions in BSTC.
F. Discussion on the structure, valence states of Tb and Ce ions, and defect chemistry
For BSTC1 and BSTC3/2, the EPR and XPS results provide evidence of the existence of Tb4+ (Figures 7 and 8), whereas the XPS results indirectly confirm the existence of some Tb3+ ions at the B-sites (Figure 8). Table III gives ionic radii versus coordinate number (CN) (Shannon, Reference Shannon1976; Lu, Reference Lu2015). For perovskite, the reduplicative orientations by BO6 octahedron form the skeleton of the perovskite lattice and the A-site ions locate on the interstitial space of BO6 skeleton. More stable Ce4+ at the B-sites in BSTC cannot be reduced to Ce3+ because of the existence of Tb4+ ions. The unit-cell volume (V 0) of the orthorhombic BSTC3/2 (V 0 = 317.02 Å3) is nearly the same as that of the orthorhombic SrCeO3 (V 0 = 317.47 Å3) reported (Ranlov et al., Reference Ranlov, Lebech and Nielsen1995). On the basis of the orthorhombic SrCeO3 lattice, dual doping with Ba and Tb results in only a little change in V 0. This is because the expansion in V 0 caused by the A-site Ba2+ ions is close to the contraction in V 0 caused by the B-site Tb4+ ions on the basis of ionic size comparisons between Ba2+ and Sr2+ as well as between Tb4+ and Ce4+. The V 0 of the orthorhombic BSTC3/2 (V 0 = 317.02 Å3) is evidently greater than the volume of two unit cells of the trigonal BSTC1 (2V 0 = 315.72 Å3) (see Table I), which arises from the higher Ce concentration at the B-sites in BSTC3/2.
TABLE III. Ionic radii versus coordinate number (CN).
The cation and vacancy defects in BSTC based on the SrCeO3 lattice can be expressed by 
${\rm Ba}_{{\rm Sr}}^ \times, {\rm Sr}_{{\rm Sr}}^ \times, {\rm Ce}_{{\rm Ce}}^ \times$, 
$ {\rm Tb}_{{\rm Ce}}^ \times, {\rm T}{\rm {b}^{\prime}}_{{\rm Ce}}$, and O vacancies 
${\rm (V}_{\rm O}^{ \bullet \bullet} )$. Tb ions at the B-sites in perovskites are confirmed to coexist in the form of the mixed-valence states of Tb3+/Tb4+ (Lu, Reference Lu2015; Lu and Peng, Reference Lu and Peng2016; Lu et al., Reference Lu, Cui, Liu and Sun2016a). The higher values in the electrical conductivity (Figure 5) and the dielectric loss (Figure 6) arise from the creation of oxygen vacancies, which are compensated by some Tb3+ ions at the B-sites based on a general lattice electroneutrality condition. The sixfold coordinated Tb3+ can coexist with Tb4+ and Ce4+ because of a complementary ionic radii relation 2R Ce(IV) ≈ R Tb(IV) + R Tb(III) (Table III).
$$\eqalign{{\rm 4BaO} + {\rm T}{\rm b}_{\rm 4}{\rm O}_{\rm 7} \to & 4{\rm Ba}_{{\rm Ba}}^ \times + 2{\rm Tb}_{{\rm Ce}}^ \times + 2{\rm T}{\rm {b}^{\prime}}_{{\rm Ce}} + {\rm 11}{\rm O}_{\rm O} \cr & + {\rm V}_{\rm O}^{ \bullet \bullet}}$$The tolerance factor (t) of ABO3-type perovskite is expressed by the following equation:
$$t = \displaystyle{{r_{\rm A} + r_{\rm O}} \over {\sqrt 2 (r_{\rm B} + r_{\rm O})}}$$where r A, r B, and r O are the ionic radii of the cation A, B, and the anion O2−, respectively. The ideal perovskite without distortion should show t = 1.0, and the t value of most perovskite-type oxides is empirically in the range of 0.75–1.0 (Fu et al., Reference Fu and Weng2014). For Ba1−x/3Srx/3Tb1−x/3Cex/3O3 (BSTC), the tolerance factor may be approximately expressed by the following equation:
$$t = \displaystyle{{[(1-x/3)r_{{\rm Ba}} + x/3\cdot r_{{\rm Sr}}] + r_{\rm O}} \over {\sqrt 2 \{ [(1-x/3)r_{{\rm Tb}} + x/3\cdot r_{{\rm Ce}}] + r_{\rm O}\}}} $$The tolerance factors of BSTC1 and BSTC3/2 are determined to be t = 0.956 and 0.939, respectively. These high t values show that the two BSTC phases are both nearly free of distortion of the perovskite structure.
IV. CONCLUSIONS
Two ceramics Ba1−x/3Srx/3Tb1−x/3Cex/3O3 (x = 1 and 1.5) (BSTC) with dense microstructures (ρ r = 93%, GS = 0.2–3 µm) were prepared at 1400 °C in air using the mixed-oxides method. Dual doping with Sr and Tb in the BaCeO3 perovskite results in a trigonal structure for BSTC1 and an orthorhombic perovskite structure for BSTC3/2. Ba/Sr ions at the A-sites and Ce/Tb ions at the B-sites in the perovskite lattice are randomly disordered and exhibit homogeneous concentration distributions. The EPR, XPS, and electrical results confirm that Ce and Tb ions in BSTC exist as Ce4+ and the mixed-valence state of Tb4+/Tb3+, respectively. At RT, two BSTC ceramics exhibit a similar semiconducting behavior, and the difference in x had little effect on their σ–T relations. The electrical conductivity (σ) of BSTC with x = 1.5 increases rapidly from 5.7 × 10−9 to 4.5 × 10−6 S cm−1 with increasing T from −50 to 200 °C, while the σ at RT increases slowly with increasing frequency, from 3.0 × 10−8 at 1 Hz to 2.9 × 10−7 S cm−1 at 106 Hz. Direct evidence of Tb3+, oxygen vacancies, and possible local ordering still need to be further explored.
ACKNOWLEDGEMENTS
This work was supported by the projects of the National Natural Science Foundations of China (21271084) and Jilin Province Development and Reform Commission (2019C044-1).
