Water uptake analysis of acceptor-doped lanthanum orthoniobates
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In this work, lanthanum orthoniobates doped with either antimony, calcium, or both have been synthesized and studied. The water uptake of the investigated materials has been analyzed by means of thermogravimetric studies. The results show the difference between the thermodynamics of hydration between the lanthanum orthoniobate system and other proton conducting ceramics. The relation between the water uptake and effective acceptor doping for the investigated system has been found, and the energetics of the water uptake relation are discussed.
KeywordsProton conductors Thermogravimetry Water uptake Hydration
The increased number of oxygen vacancies present in the oxide can promote ion transport by increasing charge carriers. The increase in proton concentration is linked to the hydration process, that is, to the water uptake of the oxide in humid atmospheres.
In the last decade, the hydration energetics of proton conductors have received much attention; however, studies have mostly been limited to the barium and strontium zirconate systems [7, 8, 9, 10]. For barium zirconate, a gradual rise of water uptake with decreasing temperature has been observed, showing enthalpy of hydration at levels of − 22 to − 40 kJ mol−1 . To the best of our knowledge, no directly measured data on water uptake are available for lanthanum orthoniobate and the only information on its hydration thermodynamics is based on transport measurements [3, 11, 12]. Haugsrud and Norby calculated enthalpy of hydration for 1% Ca-doped LaNbO4 to be − 115 kJ mol−1 , whereas Huse et al. reported even higher values based on modeling of titanium-doped material—approximately − 140 kJ mol−1 and − 170 kJ mol−1 in the case of non-associated and associated defect models, respectively . These numbers indicate that the difference in hydration energetics of barium zirconate and lanthanum orthoniobates is significant. The threefold difference between the hydration enthalpy values has to seriously affect the water uptake in these systems. In contrast, Ferrara et al. calculated the hydration enthalpy of lanthanum niobate from DFT studies for unstrained samples to be − 30 kJ mol−1 . Such discrepancies in reported values reflect the complexity of the energetics of hydration of this system.
In this work, we report the thermogravimetric determination of water uptake of doped lanthanum orthoniobates for the first time.
Room temperature water vapor sorption studies were undertaken on a Quantachrome iQ Autosorb apparatus in the relative pressure range p/p0 from 0.05 to 0.9. The surface of both powders and ceramics of antimony-doped lanthanum orthoniobate was characterized using X-ray photoelectron spectroscopy (XPS). The spectra were recorded on a Thermo Scientific K-Alpha + X-ray photoelectron spectrometer system operating at a base pressure of 2 × 10−9 mbar. The system incorporates a monochromatic, microfocused Al Kα X-ray source (hν = 1486.6 eV) and a 180° double focusing hemispherical analyzer with a 2D detector. The X-ray source was operated at 6 mA emission current and 12 kV anode bias, and an X-ray spot size of 400 µm was used. Data were collected at 200 eV pass energy for survey and 20 eV pass energy for core-level spectra, and a flood gun was used to minimize sample charging. Spectra were aligned assuming the C 1s core line to be at the binding energy of 285.0 eV. All data were analyzed using the Avantage software package.
Results and discussion
All reflections observed in the collected X-ray diffraction data for the different compositions could be indexed with a monoclinic fergusonite phase (I2/c). For samples co-doped with 2% antimony and calcium, additional small reflections of a secondary phase have been identified in the diffractogram. More information on the in-depth structural analysis of the investigated compounds can be found in previous works [5, 14, 17].
Selected properties of studied materials
Total conductivity at 1073 K, S/cm
1.0 × 10−3
3.0 × 10−4
2.3 × 10−4
3.4 × 10−5
1.7 × 10−5
3.1 × 10−4
1.3 × 10−4
One of the most interesting properties observed in Fig. 6 is the highest water uptake for the La0.98Ca0.02Nb0.9Sb0.1O4−δ sample. The reason for such a large difference between the water uptake values of sole calcium- and calcium and antimony-substituted samples is the difference between the crystal structures of the materials. The data shown in Fig. 6 were obtained at a relatively low temperature of 300 °C, which is below the structural phase transition for calcium-doped samples but above the one for antimony-doped ones. The transition between the high-temperature tetragonal scheelite to the monoclinic fergusonite structure occurs at approximately 773 K  and 473 K  in the samples doped only with calcium and the co-doped samples with 10 mol% of antimony, respectively.
Electrical measurements of the compositions investigated here have shown tendencies which do not fit the hydration data in the past. The highest achieved conductivity values for doped lanthanum orthoniobates of 10−3 S cm−1 at 1223 K in atmospheres containing ca 2% H2O have been reached in 1 mol % calcium doping in studies by Haugsrud and Norby . In contract, the value at 1073 K for antimony-substituted samples was of the order of 10−4 S cm−1 . In comparison, results for BaZr0.8Y0.2O3 investigated previously by the authors as well as Yamazaki et al.  show 5 × 10−2 for BaZr0.8Y0.2O3 versus maximum of 3 × 10−3 for La0.98Ca0.02Nb0.9Sb0.1O4−δ proton concentration at 300 °C. One of the main differences between the two compounds is the nature of their charge transport. In the case of barium zirconate, proton conduction is accompanied by oxygen ion transfer, while lanthanum orthoniobate is a pure proton conductor .
The results illustrated in Fig. 7 show the minimum of the total water uptake at 773 K. For lanthanum orthoniobate, this temperature is significant because of its structural phase transition , which changes the mobility of the charge carriers [26, 27]. It is possible that the coexistence of two separate orthoniobate phases, occurring during phase transition , can influence the water uptake by changing proton defect pathways within the structure.
Li et al. studied strontium-substituted LaNbO4, which showed a low value of diffusion coefficient of protons in this structure . The work suggests that the formation of stoichiometric impurities might have eliminated the surface oxygen vacancies, thus blocking water intake pathways between the gas phase and the oxide surface. A similar phenomenon may be present in our sample. This would help explain why even though the energetics of water uptake are thermodynamically favorable, the net water uptake is relatively small. The number of vacancies available for hydration is simply too small for the material to hydrate in an extensive way. This phenomenon can be either related to the segregation or the low surface diffusivity, which can be an intrinsic property of this system. This is in accordance with the results presented in Fig. 5 for different dopants and dopant concentrations and with the rise of the total water uptake with nominal acceptor content. Possible surface segregation can also be a reason for differences in room temperature water vapor sorption for materials with different dopant content (Fig. 8). For samples with higher acceptor dopant content, the surface is much more prone to adsorption. However, for all investigated samples, the total adsorbed water content is relatively low in comparison with other materials, especially nanocrystalline powders . Therefore, at elevated temperatures, one can be sure that water uptake observed in lanthanum orthoniobates is due to bulk proton incorporation not surface phenomena. Presented results along with previous studies of electrical properties of doped lanthanum orthoniobates lead to the conclusion that introduction of antimony on the niobium site promotes water uptake in these compounds, since it enhances the proton conductivity in relation to the undoped material . It is also possible that the proton defect mobility is different for a structure with antimony dopant. It is well known that antimony alters the structure and lowers the structural phase transition temperature, leading to changes in unit cell parameters. It was suggested that in antimony-doped lanthanum orthoniobate, the (Nb, Sb)–O bond type is more covalent than that of Nb–O , which can influence the proton diffusion pathways within the crystal lattice [5, 14]. Slow diffusion can be a limiting factor for the hydration phenomena in this case. Since the lack of strong temperature dependence indicates a low hydration enthalpy, the low diffusivity can explain the slow hydration kinetics of the investigated system.
The water uptake of antimony- and calcium-doped lanthanum orthoniobate has been analyzed by thermogravimetric studies. The relation between water uptake, material stoichiometry, and temperature has been studied. The weak temperature dependency of water uptake for all of the studied materials has been observed leading to the conclusion that the hydration reaction for lanthanum orthoniobate is favorable from the thermodynamic point of view. The low water uptake values have been assigned to possible low diffusivity of protonic defects in the structure. Further studies of the effect of surface segregation on the diffusion process are needed to fully understand the occurring phenomena.
The research was financially supported by the Ministry of Science and Higher Education, Poland Grant No. IP2015 051374. The preparation of the samples for XPS examination has been supported by National Science Centre, Poland, by Grant No. 2015/17/N/ST5/02813. D.J.P. acknowledges support from the Royal Society (UF100105 and UF150693). D.J.P. and A.R. acknowledge support from the EPSRC (EP/M013839/1 and EP/M028291/1).
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