Effect of Sn doping on sinterability and electrical conductivity of strontium hafnate published by Ural Federal University eISSN 2411-1414 chimicatechnoacta.ru ARTICLE 2023, vol. 10(1), No. 202310113 DOI: 10.15826/chimtech.2023.10.1.13 1 of 8 Effect of Sn doping on sinterability and electrical conductivity of strontium hafnate Adelya S. Khaliullina * , Anastasia N. Meshcherskikh , Liliya A. Dunyushkina Laboratory of Electrochemical Material Science, Institute of High Temperature Electrochemistry, Ekaterinburg 620066, Russia * Corresponding author: Adelia01@mail.ru This paper belongs to a Regular Issue. Abstract The effect of isovalent substitution of hafnium by tin in strontium hafnate on sinterability and electrical conductivity was studied for the first time. The ceramic samples SrHfxSn1–xO3–δ (x = 0–0.16) were synthesized by solid-state reaction and sintered at 1600 °C for 5 h. The samples were examined using the methods of X-ray diffraction, scanning electron microscopy, impedance spectroscopy, and four-probe direct current technique. It was shown that all samples were phase pure and had the orthorhombic structure of SrHfO3 with the Pnma space group. Sn doping resulted in an increase in grain size, rela- tive density and conductivity; the sample with x = 0.08 demonstrated the highest conductivity, which was ~830 times greater than that of undoped strontium hafnate at 600 °C. The conductivity of SrHf0.92Sn0.08O3–δ was 4.1∙10–6 S cm–1 at 800 °C in dry air. The possible reasons for the effect of Sn on the electrical properties of strontium hafnate were discussed. Keywords strontium hafnate perovskite electrical conductivity electrolyte Received: 16.02.23 Revised: 13.03.23 Accepted: 14.03.23 Available online: 17.03.23 Key findings ● Ceramic samples SrHfxSn1–xO3–δ (x = 0–0.16) were obtained by the solid-phase method. ● Sn doping enhances the sintering ability of ceramics. ● Sn doping results in an increase in conductivity by more than 2 orders of magnitude . © 2023, the Authors. This article is published in open access under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction Complex oxides with the ABO3 perovskite structure are of interest as functional elements of electronics, thermal bar- rier coatings, electrolytes and electrodes for high- temperature electrochemical applications [1–7]. In the 1980s, compounds capable of proton transfer were discov- ered among perovskites [8–11], and since then, active re- search has been carried out in the field of developing perov- skites with high proton conductivity. Among perovskite-like oxides of the A2+B4+O3 type, the materials based on BaCeO3 and SrCeO3 exhibit the highest proton conductivity [12–16], but their use in electrochemical cells is hindered by their low chemical resistance to carbon dioxide. Zirconates and hafnates of alkaline earth metals are characterized by high chemical stability, which makes them promising for practi- cal applications, and possess oxide-ion and proton conduc- tivity; however, the ionic conductivity of these materials is low [17–21]. It is known that the substitutions in A and B positions affect many characteristics of ABO3 perovskites; therefore, this approach is widely used to modify the ionic conductivity. As a rule, the method of acceptor doping is used, which makes it possible to increase the concentration of oxygen vacancies and, accordingly, the ionic conductivity [20–25]. In addition, doping is a common approach to im- proving the sinterability of solid oxides. Solid oxide electro- lytes, including those with a perovskite structure, are re- fractory materials, which are sintered at very high tempera- tures [28–34]. Yamanaka et al. [26] reported on the fabrication of ce- ramic samples of SrHfO3 with the relative density of 94% by the solid state method, followed by the prolonged sinter- ing at 1600 °C for 10 h. Qian et al. [29] used alumina as sintering aid to increase the density of Ce-doped SrHfO3 ceramics. The powders of Sr0.995Ce0.005HfO3 and Sr0.995Ce0.005Hf0.995Al0.005O3 were synthesized by the solid state method, pressed into pellets and sintered at 1800 °C for 20 h in vacuum; it was found that the addition of Al re- http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2023.10.1.13 mailto:Adelia01@mail.ruuz http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0002-9616-029X https://orcid.org/0000-0002-9541-6847 https://orcid.org/0000-0003-3369-5454 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2023.10.1.13&domain=pdf&date_stamp=2023-03-17 Chimica Techno Acta 2023, vol. 10(1), No. 202310113 ARTICLE 2 of 8 DOI: 10.15826/chimtech.2023.10.1.13 sulted in the increase in density up to 98.9%. The use of spark plasma sintering (SPS) method was reported to be effective for fabrication of the high-density ceramics of strontium hafnate [30, 31]. The powder synthesized by the solid state method was poured into a graphite die, placed in a SPS chamber, stepwise heated to a high temperature (1600–1900 °C), and kept at this temperature for 45 min; the relative density of the obtained samples was more than 99%. It is obvious that the use of sintering technologies, based on long-term high-temperature annealing, sometimes under vacuum conditions, or on the SPS method, leads to a significant increase in the cost of ceramics. Consequently, the search for additives that promote better sintering of alkaline earth hafnates is relevant. A positive effect of the partial substitution of hafnium by indium and tin on the sintering of BaHf0.9–xInxSn0.1O3–δ (x = 0.05–0.25) ceramics was reported in [22]. It was shown that the introduction of indium alone made it pos- sible to achieve a relative density of ~94%, while the co- doping led to an increase in density up to 96.25%. Isova- lent substitution should also affect the ionic conductivity of oxides, in addition to their sinterability, due to the dif- ference in ionic radii and the electronegativity of the host and dopant cations. It was shown in [35] that the intro- duction of tin into BaZr0.8Sc0.2O3–δ led to an increase in conductivity, although it was accompanied by a decrease in the average grain size in BaZr0.8–xSnxSc0.2O3–δ ceramics from 0.97 μm for x=0 to 0.39 μm for x=0.2. However, it is difficult to assess the effect of partial substitution of haf- nium by tin on the sintering ability of BaZr0.8Sc0.2O3–δ, since the authors additionally introduced a sintering aid, 0.5 wt.% CuO, and did not analyze the change in density depending on the tin content. To the best of the authors, knowledge, there are no data on the effect of Sn-doping on the sintering behavior and electrical properties of strontium hafnate. This research aims to study the effect of isovalent substitution of hafnium by tin in strontium hafnate on sinterability and electrical conductivity. The ceramic samples SrHfxSn1–xO3–δ (x = 0–0.16) were obtained by solid state synthesis, their phase composition and microstructure were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), electrical conductivity was measured by the impedance method and the four-probe direct current (DC) method. 2. Experimental part SrHfxSn1–xO3–δ (x = 0.04–0.16) compositions were synthe- sized by the solid state reaction method using HfO2, SrCO3 and SnO2 (all with 99.9% purity) as precursors. The start- ing materials were preliminary dried at a temperature of 150 °C for 5 hours to remove adsorbed water, and then thoroughly mixed in a planetary mill (Retsch PM 100) in a calculated ratio. The synthesis was carried out at 1200 °C for 6 hours with intermediate regrinding. Then the pow- der was pressed into pellets under a pressure of 300 MPa and sintered at a temperature of 1600 °C for 5 hours in air. A schematic diagram of the fabrication of SrHfxSn1–xO3–δ samples is presented in Figure 1. The apparent density of the samples was determined as the mass to volume ratio, which were determined by weighing and measuring the dimensions of the sample. The relative density was calculated as the ratio of meas- ured to theoretical density. The phase composition and crystal structure of the sintered samples were studied using an X-ray diffractometer D-Max 2200 (Rigaku, Tokyo, Japan) in Cu Kα radiation within 2θ range from 20 to 80° at a scanning speed of 0.02 rpm with a step of 0.1°. The X-ray diffraction (XRD) measurements were performed at room temperature. To study the microstructure of the samples by SEM, the surface of the samples was polished using diamond pastes and thermally etched at a temperature of 1200 °C for 4 hours in order to make the granular structure of ceram- ics visible. The microstructure of the samples was studied using a MIRA 3 LMU electron microscope (Tescan, Brno, Czech Republic). Impedance measurements were performed on a Bio- Logic SP-200 impedance meter in the frequency range from 0.1 Hz to 1 MHz with an amplitude of 30 mV in dry air (pH2O = 40 Pa). For these measurements, platinum paste was symmetrically applied to the opposite faces of the SrHfxSn1–xO3–δ pellets and sintered at 1000 °C (1 hour); the area of the Pt electrodes was 0.6 cm2. Dry air was ob- tained by blowing atmospheric air through a column filled with zeolite beads. The measurements were carried out in the temperature range of 550–800 °C. For the spectra analysis, the methods of equivalent circuits (ZView soft- ware) and distribution of relaxation times (DRT) (DRT- tools software) were applied. To measure the electrical conductivity by the four-probe DC method, a sample in the form of a bar was cut out from the pellet. Four Pt wires were wound on to the sample to serve as current electrodes and potential probes. The dis- tance between the potential probes was 0.75 cm, and the cross-sectional area of the sample was 0.05 cm2. Figure 1 Schematic diagram of SrHfxSn1–xO3–δ ceramics fabrication. https://doi.org/10.15826/chimtech.2023.10.1.13 Chimica Techno Acta 2023, vol. 10(1), No. 202310113 ARTICLE 3 of 8 DOI: 10.15826/chimtech.2023.10.1.13 Electrical conductivity σ was calculated using the fol- lowing formula: 𝜎 = 𝑈𝑆 𝐼𝐿 , (1) where S and L are the cross-sectional area of the sample and the distance between the potential probes, I is the current, and U is the voltage. The DC measurements were performed at 900 °С in the pO2-range of 10–18–0.21 atm using a ZIRCONIA-318 regula- tor which ensured the automatical change and control of the oxygen partial pressure. The schematic view of the DC measurement setup is shown in Figure 2. Figure 2 Schematic view of DC measurement setup: 1 – sample, 2 – platinum wires, 3 – YSZ tube, 4 – electrochemical pump, 5 – electrochemical sensor, 6 – gas inlet and outlet tubes, 7 – ther- mocouple, 8 – Zirconia-318 regulator, 9 – valve, 10 – fluoroplastic plug, 11 – bubbler, 12 – gas cylinder or compressor, 13 – sealant. 3. Results and Discussion 3.1. Characterization of the structure and morphology of the SrHfxSn1–xO3–δ samples Analysis of the XRD patterns of SrHfxSn1–xO3–δ sintered samples, which are presented in Figure 3a, showed that all the samples are single-phase and have the orthorhombic structure of SrHfO3 with the Pnma space group (ICDD 89- 5605). No impurity phases were found in the samples. Figure 3b shows that Sn doping results in the peak shifting towards higher 2θ angles. A linear decrease in the unit cell volume with an increase in the tin content in strontium hafnate (Figure 3c) is consistent with the assumption that tin incorporates into the SrHfO3 crystal lattice, replacing hafnium, since the ionic radius of tin is 0.069 nm, and the ionic radius of hafnium is 0.071 nm. A schematic structure of SrHfxSn1–xO3–δ is presented in Figure 3d. The morphology of SrHfxSn1–xO3–δ ceramic samples was evaluated from the SEM images displayed in Figure 4. As can be seen, the undoped strontium hafnate has a nanograin structure with an average grain size of ~100 nm, while Sn doping results in the grain growth: in the doped samples, the grain size varies from 1 to 5 μm, and the morphology of the ceramics practically does not change with the tin content. Figure 3 XRD patterns of SrHfxSn1–xO3–δ (a), the magnified fragment of diffractograms showing (002) peak (b), unit cell volume of SrHfxSn1–xO3–δ as a function of Sn content (c), and schematic structure of SrHfxSn1–xO3–δ (d). https://doi.org/10.15826/chimtech.2023.10.1.13 Chimica Techno Acta 2023, vol. 10(1), No. 202310113 ARTICLE 4 of 8 DOI: 10.15826/chimtech.2023.10.1.13 Figure 4 SEM images of the surface of SrHfxSn1–xO3–δ samples: x = 0 (a, b), x = 0.04 (c), x = 0.08 (d), x = 0.12 (e), х = 0.16 (f). The relative densities of the sintered SrHfxSn1–xO3–δ samples are presented in Table 1. As can be seen, the in- troduction of 4 at.% Sn improves the sintering of ceram- ics, however, an increase in the tin concentration has practically no effect on their density, which is close to 80% for all Sn-doped samples. The relatively low density of the ceramics indicates that further efforts are needed to sinter the high-density strontium hafnate-based ceramics. 3.2. Electrical conductivity of SrHfxSn1–xO3–δ The impedance measurements of SrHfxSn1–xO3–δ (x = 0.04– 0.16) samples were performed in the temperature range of 550–800 °C in dry (pH2O = 40 Pa) air. The impedance of the undoped SrHfO3 could not be measured because of its very high resistance. For illustration, the hodographs of SrHfxSn1–xO3–δ measured in dry air at temperatures of 700–800 °C are shown in Figure 5a–d. Figure 5e presents the fitting of the spectrum on the basis of the equivalent circuit consisting of three RQ elements connected in series (R denotes a resistance, Q is a constant phase element), and the results of DRT analysis for the sample with x = 0.08 at 700 °C. Taking into account the obtained char- acteristic capacitances, the high-frequency semicircle (C~10–11 F cm–2) can be assigned to the response of grain interior of a sample, the medium-frequency semicircle (C~10–8 F cm–2) is related to grain boundaries, and the low-frequency arc with a high capacitance (C~10–3 F cm–2) is caused by the electrode processes. As can be seen in Figure 5a–d, in the most of hodo- graphs the grain boundary response is small compared to that of the bulk, which makes it difficult to reliably sepa- rate the grain boundary resistance; therefore, only the total resistance of the SrHfxSn1–xO3–δ electrolytes was dis- cussed. The conductivity of the samples was calculated as follows: 𝜎 = ℎ 𝑅el𝑆 , (2) where h is the thickness of a sample, S is the area of elec- trodes, Rel is the electrolyte resistance. The Arrhenius dependences of conductivity presented in Figure 6 are nearly linear; the activation energy for all compositions is 1.5±0.1 eV. For comparison, the figure shows the conductivities of undoped strontium hafnate and strontium zirconate reported in [26, 36]. It can be seen that the substitution of hafnium by tin leads to an increase in conductivity by more than 2 orders of magnitude, e.g., the most conductive composition with x = 0.08 exhibits a con- ductivity ~830 times greater than undoped strontium haf- nate at 600 °C, despite the fact that the density of the SrHfxSn1–xO3–δ samples (~80%) is significantly lower than the density of the SrHfO3 sample (95%), which was sin- tered at a much higher temperature of 1750 °C [36]. Figure 7 demonstrates the conductivity dependences of SrHfxSn1–xO3–δ on the Sn concentration; as can be seen, the sample with x = 0.08 has the highest conductivity, which is equal to 4.1∙10–6 S cm–1 at 800 °C. However, the conduc- tivity of SrHf0.92Sn0.08O3–δ sample is still lower than that of SrZrO3 by about an order of magnitude, as can be seen in Figure 6. In general, an increase in ionic conductivity upon dop- ing may be due to an increase in the concentration of charge carriers (oxygen vacancies in the case of oxide-ion conductors) and/or an increase in their mobility. The iso- valent substitution Sn4+→Hf4+ should not lead to the for- mation of additional oxygen vacancies; therefore, it can be assumed that the increase in conductivity upon Sn doping is related to the higher mobility of oxygen vacancies. However, the smaller radius of Sn4+ ion compared to the Hf4+ ion is expected to lower the mobility because of the narrowing of channels for oxide-ions migration, although the size factor should not lead to a noticeable effect at low dopant concentrations. Probably, it is the chemical nature of tin, namely, its electron configuration, which influences interatomic bonding and, accordingly, the mobility of charge carriers in SrHfxSn1–xO3–δ. Table 1 Relative density of SrHf1–хSnxO3–δ ceramic samples. х 0 0.04 0.08 0.12 0.16 Relative density, % 75±2 82±2 79±2 79±2 82±2 https://doi.org/10.15826/chimtech.2023.10.1.13 Chimica Techno Acta 2023, vol. 10(1), No. 202310113 ARTICLE 5 of 8 DOI: 10.15826/chimtech.2023.10.1.13 Figure 5 Impedance spectra of SrHfxSn1–xO3–δ in dry air: x = 0.04 (a), x = 0.08 (b), x = 0.12 (c), x = 0.16 (d), impedance with fitting and DRT spectra for SrHf0.92Sn0.08O3–δ at 700 °C (e). Figure 6 Arrhenius plots of SrHfxSn1–xO3–δ conductivity in dry air. Solid lines show the conductivity of SrZrO3 [26] and SrHfO3 [36]. The open triangle is the conductivity of the sample with x = 0.08 measured by the four-probe DC method. Figure 7 Conductivity of SrHfxSn1–xO3–δ as a function of Sn-content in dry air. Open symbols are the conductivity of SrHfO3 reported in [36]. -11 -10 -9 -8 -7 -6 -5 -0.04 0 0.04 0.08 0.12 0.16 x lg σ , S ·c m -1 800 °С 600 °С 700 °С 750 °С 650 °С 550 °С https://doi.org/10.15826/chimtech.2023.10.1.13 Chimica Techno Acta 2023, vol. 10(1), No. 202310113 ARTICLE 6 of 8 DOI: 10.15826/chimtech.2023.10.1.13 In addition, it can be assumed that the very thin film of tin oxide is formed at the grain boundaries of doped stron- tium hafnate, which is undetectable by the conventional X- ray diffraction and microscopy methods. The impedance data are consistent with this assumption: the response of grain boundaries is not visible in the most of hodographs, which indicates that the contribution of boundaries to the total resistance of the samples is very small (Figure 5 a–d). Probably, the existence of the film of tin oxide results in a decrease of the resistance of grain boundaries, which is known to be one of the main reasons for the low conductivi- ty of the related perovskites based on strontium zirconate [26, 37], and, as a result, the total conductivity of the Sn- doped samples increases. However, further studies are needed to establish the mechanism of the effect of Sn dop- ing on the electrical conductivity of strontium hafnate. To determine the type of charge carriers, the conduc- tivity of SrHf0.92Sn0.08O3–δ sample, which exhibits the high- est conductivity, as a function of pO2 was studied using the four-probe DC method. Due to the high resistance of the sample, the measurements were carried out at a high tem- perature of 900 °C. As can be seen in Figure 8, the conduc- tivity increases with increasing pO2 under oxidizing condi- tions (рО2 > 10–5 atm), while a wide plateau is observed in reducing atmospheres (10–18–10–5 atm). The value of con- ductivity at the plateau is considered an oxide-ion conduc- tivity, which is independent of рО2. An increase in conduc- tivity at high рО2 is typically related to generation of oxy- gen holes upon oxygen incorporation into oxide lattice according to the generally accepted mechanism of defect formation. In this mechanism, the equilibrium between gaseous oxygen, oxygen ions, oxygen vacancies and holes in oxide-ion conducting oxides in oxidizing atmospheres can be represented in terms of Kröger-Vink notation as follows: 1 2 O2 + VO •• = OO 𝑥 + 2ℎ•, (3) The electroneutrality condition for reaction 3 can be written as follows: ℎ• = 2[VO ••]. (4) Taking into account the electroneutrality condition (Equation 4) and the mass action law for Equation 3 one can derive that the hole concentration is proportional to 𝑝O2 1 6⁄ . As long as the hole conductivity is proportional to the hole concentration, the slope of the log σ – log pO2 de- pendence is to be equal to 1/6. However, the slope of the experimentally obtained dependence shown in Figure 8 is lower, which can be caused by different reasons. First, in the transition region, in which the oxide-ion and hole con- ductivities are comparable, the slope may vary from zero to 1/6. Second, the tin oxide film, which is supposedly formed on the surface of grains in SrHfxSn1–xO3–δ may hin- der the oxygen incorporation (Reaction 3) and the hole generation. Figure 8 Conductivity of SrHf0.92Sn0.08O3–δ as a function of pO2 at 900 °C. Thus, based on the conductivity dependence on pO2, it can be concluded that in Sn-doped strontium hafnate ox- ide-ion conductivity dominates in a wide range of pO2, while the hole conductivity appears under oxidizing condi- tions. The transference number of ions ti can be evaluated from the log σ – log pO2 dependence as a ratio of the ionic conductivity, which is the conductivity on the plateau, to the conductivity at any value of pO2; e.g., for SrHf0.92Sn0.08O3–δ at 900 °C, ti = 0.35 in air and gradually increases with decreasing pO2, approaching unity at pO2 of 10–6 atm. 4. Limitations Sn doping improves the density of SrHfO3-based ceramic samples; however, the resulting ceramic is not sufficiently dense for application in electrochemical cells, such as fuel cells or electrolysis cells, which require the use of a gas- tight electrolyte. Our further research aims at the fabrica- tion of high-density ceramics by using nanopowders and sintering aids, and manufacturing and testing fuel and electrolysis cells. 5. Conclusions In the present research, the effect of isovalent substitution of hafnium by tin in strontium hafnate on sinterability and electrical conductivity was considered. SrHfxSn1–xO3–δ (x = 0–0.16) ceramic samples were obtained by the solid- phase method. All samples were found to be phase pure and have the orthorhombic structure of SrHfO3 with Pnma space group. The Sn doped samples sintered at a tempera- ture of 1600 °C for 5 hours exhibited a higher relative density (80±2%) compared to the undoped sample (75±2%). The grain size increased from ~100 nm to 1–5 μm upon the Sn-doping. The study of conductivity re- vealed that the substitution of hafnium by tin leads to an increase in conductivity, and the highest conductivity (4.1∙10–6 S cm–1 at 800 °C in dry air) is observed for the sample with x = 0.08. The conductivity of -6.0 -5.5 -5.0 -4.5 -20 -15 -10 -5 0 lg pO2, atm lg σ , S c m − 1 1/6 https://doi.org/10.15826/chimtech.2023.10.1.13 Chimica Techno Acta 2023, vol. 10(1), No. 202310113 ARTICLE 7 of 8 DOI: 10.15826/chimtech.2023.10.1.13 SrHf0.92Sn0.08O3–δ is ~830 times greater than that of the undoped strontium hafnate at 600 °C. The activation en- ergy of the conductivity of SrHfxSn1–xO3–δ is 1.5±0.1 eV. The dependence of conductivity on the oxygen partial pressure indicates that the oxide-ion conductivity dominates in a wide range of pO2, while the hole conductivity appears under oxidizing conditions. For SrHf0.92Sn0.08O3–δ composi- tion, the transference number of ions in air was 0.35 at 900 °C. It is not clear why the isovalent substitution gives a significant increase in conductivity; further research using the high-resolution electron microscopy and energy- dispersive X-ray spectroscopy methods is needed for un- derstanding the mechanism of the Sn-doping effect on the properties of strontium hafnate. ● Supplementary materials No supplementary materials are available. ● Funding This research had no external funding. ● Acknowledgments SEM and XRD experiments were done using the facilities of the shared access centre "Composition of compounds” (Institute of High-Temperature Electrochemistry, Ural Branch of the Russian Academy of Sciences). The authors are grateful to Artem Tarutin for the help with the meas- urements of conductivity by the DC 4-probe method. ● Author contributions Conceptualization: L.A.D. Visualization: A.S.K. Investigation: A.S.K., A.N.M. Methodology: L.A.D., A.S.K, A.N.M. Supervision: L.A.D. Writing – original draft: A.S.K. 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