Drastic change of electrical conductivity in Pr₂CuO₄ by isovalent La doping

Abstract

How to affect the transport properties of complex oxides was considered using the uncommon example of Pr_2−xLa_xCuO₄. It was established that appropriate La doping in Pr₂CuO₄ leads to a noticeable increase in high-temperature electrical conductivity. The reason for such changes are discussed from the viewpoint of possible charge redistribution in Pr_2−xLa_xCuO₄.

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Complex transition metal oxides attract much attention as a consequence of their wide variety of physical and chemical properties, which stipulate their application as functional materials in different electrochemical devices such as solid-state sensors, oxygen permeation membranes and solid oxide fuel cells (SOFCs).¹ During the last years, the interest in SOFC development and commercialization keeps on rising due to their high efficiency, fuel flexibility and economic expediency. According to this background, lowering of operating temperature is an essential step in further SOFC implementation, which provides new opportunities for improving long-term stability and using cheaper sealing materials. Reduction of the main energy losses at lower temperatures associated with cathode performance is possible via implementation of new materials with high catalytic activity in oxygen reduction reaction and compatibility with solid electrolyte.^2–5

Ln₂CuO₄ oxides, where Ln is a rare-earth element, commonly La–Sm, successfully proved themselves as promising cathodes for SOFCs operating below 750 °C.^6,7 These oxides with an exception of La₂CuO₄ have so-called T′-structure, which can be presented as the alternation of Ln₂O₂ slabs with fluorite structure and CuO₂ sheets.⁸ Since the best cathode characteristics such as high conductivity, catalytic activity and compatibility with commonly used Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte among the considered cuprates had been demonstrated by Pr₂CuO₄, attempts were made to improve efficiency of the cathode material by means of the heterovalent substitution of strontium for praseodymium, which was intended to increase of oxygen mobility in the crystal structure.⁹ However, such heterovalent substitution in Pr₂CuO₄ leads to dramatic changes in the crystal structure depending on a doping level and, as a result, in physical and chemical properties of Pr_2−xSr_xCuO₄ oxides.⁹ Moreover, strontium-containing materials, as a rule, display higher thermal expansion coefficients (TECs) in comparison with their analogues without strontium and are less resistive to CO₂. As a consequence, those materials do not provide required long-term stability and must be replaced with more effective ones.¹⁰ In this communication the uncommon way to affect transport properties, notably, electrical conductivity is considered with the example of Pr_2−xLa_xCuO₄ system. Changes in electrical conductivity with the dopant concentration in Pr_2−xLa_xCuO₄ complex oxides as well as reasons for these changes are reported for the first time to our knowledge. The choice of La as a dopant was inspired by an aspiration to extend an Ln₂O₂ fluorite slab by means of large cation incorporation in Pr₂CuO₄ without dramatic changes of structure type and physical properties. However, as it was revealed, electrical conductivity of Pr_2−xLa_xCuO₄ is quite sensitive to lanthanum content and can be enhanced under an appropriate doping level.

In order to ensure the attainment of phase equilibrium in synthesized solid solutions, all Pr_2−xLa_xCuO₄ powder samples were additionally annealed at 1000 °C for 50 hours. X-ray powder diffraction (XRPD) analysis confirmed that all La-doped samples (x = 0.05; 0.1; 0.2; 0.3) as well as undoped Pr₂CuO₄ were single-phase and had the T′-structure with a tetragonal symmetry (space group I4/mmm). No additional peaks were detected on the XRPD pattern of Pr_2−xLa_xCuO₄ (ESI†). The calculated lattice parameters of Pr_2−xLa_xCuO₄ as a function of La-content are presented in Fig. 1a. As is clear from Fig. 1a, unit cell parameters of Pr_2−xLa_xCuO₄ change unsteady with La content. Since non-linear dependences of the parameters vs. the solid solution composition (x) were observed, both c/a parameter ratio and cell volume were calculated and plotted together (Fig. 1b) to clarify this tendency. The unit cell volume rises monotonically with increase in La-content as it is expected for solid solution where a smaller cation is substituted with a larger one (r(Pr³⁺) = 1.28 Å, r(La³⁺) = 1.32 Å for 8-coordinated cations).¹¹ Otherwise, even small La addition induces elongation of the unit cell along the c-axis. The substitution of 2.5 at% Pr (i.e., Pr_1.95La_0.05CuO₄ contains only 1 atom of lanthanum per 10 unit cells) provokes elongation along the c axis at 0.03 Å, whereas further increasing of La content up to 15 at% results in elongation along the c axis at approximately the same value. The c/a ratio for the La-containing cuprates is close, but the c/a ratio for undoped Pr₂CuO₄ considerably differs. According to this background, the end-member (x = 0) of Pr_2−xLa_xCuO₄ solid solution falls out from the principal trend.

Fig. 1 (a and b) Variation of (a) unit cell parameters and (b) both c/a ratio and cell volume with La content (x) in Pr_2−xLa_xCuO₄ at room temperature.

High-temperature X-ray powder diffraction (HT XPRD) analysis of Pr_2−xLa_xCuO₄ reveals no phase transitions in the temperature range of 25–800 °C (ESI†). TECs calculated from HT XRPD vary within narrow limits of 11.9 ÷ 12.1 × 10⁻⁶ K⁻¹ (Table 1), which satisfies requirements for SOFC cathode materials.¹²

Table 1 Size variance and thermal expansion coefficients of Pr_2−xLa_xCuO₄

Composition	σ, Å^2	TEC × 10^6, K^−1
		V^1/3	Along a axis	Along c axis
Pr2CuO4	0	11.9	13.0	9.5
Pr1.95La0.05CuO4	3.90 × 10^−5	11.9	13.1	9.3
Pr1.9La0.1CuO4	7.60 × 10^−5	12.1	13.4	9.2
Pr1.8La0.2CuO4	1.44 × 10^−4	11.8	13.1	8.9
Pr1.7La0.3CuO4	2.04 × 10^−4	11.7	13.1	8.6

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High-temperature conducting properties of Pr_2−xLa_xCuO₄ (0 ≤ x ≤ 0.3) were examined by a conventional DC four-probe technique in temperature range of 100–900 °C in air. The synthesis technique, temperature and time conditions were completely unified during a sample preparation process so that effect of ceramic microstructure on transport properties should be identical. Analysis of cross-section SEM images did not reveal any substantial difference in samples microstructure (ESI†). Fig. 2a shows temperature dependencies of electrical conductivity of Pr_2−xLa_xCuO₄ (0 ≤ x ≤ 0.3). All studied materials exhibit thermoactivated (semiconducting) behavior in the low-temperature region (100–350 °C). In the high-temperature range of 350–900 °C conductivity of the materials tends to attainment of saturation. Temperature dependence of electrical conductivity in the low-temperature range can be satisfactory fitted by Arrhenius-like law modified for small polaron hopping:

where T is absolute temperature, k – the Boltzmann's constant, A – a pre-exponential factor, E_a – activation energy. The activation energy calculated in the temperature range of 100–350 °C is 0.37 eV for Pr₂CuO₄ and is a bit smaller (0.34 ± 0.02 eV) for all La-doped samples. Thus, substitution of La for Pr does not significantly influence activation barrier.

Fig. 2 (a and b) Electrical conductivity of Pr_2−xLa_xCuO₄ samples in air: (a) – as a function of temperature and (b) – as a function of La content (x) at different temperatures. The conductivity of Pr_1.95Ce_0.05CuO₄ as a function of temperature is additionally inserted to Fig. 2a for comparison.

Fig. 2b shows how electrical conductivity of Pr_2−xLa_xCuO₄ varies with La content (x) at different temperature. The highest conductivity is achieved for the composition with the lowest La-content used in this work (2.5 at%) at each temperature.

In many cases, oxygen stoichiometry, in particular, presence of interstitial oxygen atoms or vacancies, has a strong influence on different physical properties, especially, on magnetic ordering and electronic transport in complex oxides, as it was demonstrated.¹³ In spite of the fact that the apical oxygen atoms should not exist in the ideal Pr₂CuO₄ structure, their existence was proved by a number of investigators.^14,15 The presence of apical oxygen is extremely critical for Pr₂CuO₄ at low temperature (T < 25 °C).¹⁶

According to the results of the iodometric titration used for δ determination in Pr_2−xLa_xCuO_4±δ, oxygen content matches the stoichiometric value within a measurement error. No correlations between La content and oxygen stoichiometry were observed. Thermogravimetric studies also revealed no changes in oxygen stoichiometry under heating up to 950 °C in air. Therefore, oxygen content is not markedly varies neither with La content nor with temperature, and, as a result, cannot be a factor, which is responsible for conductivity rise.

Whereas the highest conductivity is attained for the composition in which 2.5 at% of Pr positions occupied with La atoms, it is worth assuming that so small dopant concentration can induce local structure distortion only. Elimination of mutual interactions of La atoms is clear enough under the made assumption. However, it can be suggested that electron density redistribution expressing in changes in praseodymium and copper formal oxidation states occurs. This phenomenon can be described as follows. Substitution of Pr³⁺ by La³⁺ leads to appearance of local strains in the structure due to a size mismatch between Ln₂O₂ fluorite slab and CuO₂ sheets. One of possible ways to compensate the strains is an increase of a praseodymium formal oxidation state from +3 to +4 that results in depressing of praseodymium radius (r(Pr⁴⁺) = 1.13 Å).¹¹ A simple calculation shows that change in the oxidation state only of ∼0.5% praseodymium atoms is enough to maintain the average cation radius in the fluorite slab in case of Pr_1.95La_0.05CuO₄. This process, more likely, has thermoactivation nature. Supposing increase of the Pr oxidation state and taking into account absence of changes in oxygen stoichiometry, reduction of copper is transpired due to electron transfer from praseodymium to a CuO₂ sheet. Eventually, this leads to increase of charge carrier concentration in a CuO₂ sheet, where, according to commonly accepted model, charge transfer is realized by small polaron hopping.¹⁷

In order to confirm the assumption about charge redistribution in Pr_2−xLa_xCuO₄ high-temperature properties of isostructural Ce-doped analogues of Pr_2−xCe_xCuO₄ were investigated. Substitution of Ce for Pr is heterovalent and leads to electron transfer into CuO₂ sheet.^14,18 It is expected that La- and Ce-doped praseodymium cuprates should demonstrate similar behavior.

The study of high-temperature properties of Pr_2−xCe_xCuO₄ shows that electrical conductivity has maximum for composition of x = 0.05 (Fig. 3) as well as in case of Pr_2−xLa_xCuO₄. According to the data obtained (Fig. 2a), temperature dependencies of conductivity for the La- and Ce-doped (x = 0.05) cuprates are identical above 350 °C in comparison with undoped Pr₂CuO₄. In case of Pr_2−xCe_xCuO₄ electron doping occurs. Similarity of the dependencies of electrical conductivity vs. La(Ce) content indicates that charge carrier generation in both cases should have the same nature that confirms the possible redistribution of electron density in Pr_2−xLa_xCuO₄.

Fig. 3 Electrical conductivity of Pr_2−xCe_xCuO₄ as a function of Ce content (x) at different temperatures in air.

Experimental studies reveal that the highest conductivity is achieved for La-doped samples with 0.05 ≤ x ≤ 0.2. However, increasing of amount of La leads to reduction of electrical conductivity. Size variance σ²(r_A) can be used for explanation of observed physical properties for systems with isovalent doping.¹⁹ Since systematic trends like as decrease of the normal state carrier density with σ²(r_A) increasing were observed in A₂CuO₄ superconductors,²⁰ one may conclude that isovalent substitution (in our case, substitution of La³⁺ for Pr³⁺) leading to increase of σ²(r_A) (Table 1) should cause a reduction of conductivity. Traditionally, heterovalent substitution is used to influence transport properties of complex transition metal oxides. Nevertheless, the isovalent substitution, as shown in the present work, also has effect on conductivity due to possible redistribution of electron density between cations possessing in different oxidation state. Thus, “isovalent” substitution for cations, which can exist in various oxidation states, is an alternative approach to modifying properties of complex oxides. It is expected to be possible to implement this approach not only in the case of praseodymium cuprate, but in the case of other complex oxides of transition metals.

The proposed strategy of conductivity enhance can be effectively used to improve the electrochemical performance of SOFC cathode materials. Preliminary studies of polarization resistance of an electrode/electrolyte interface in the intermediate temperature range of 500–750 °C have showed that area specific resistance of the Pr_1.9La_0.1CuO₄/GDC interface is nearly twice as low as that for the undoped Pr₂CuO₄ electrode under the same conditions.⁷ Detailed investigation of electrochemical behavior of the La-doped Pr₂CuO₄ electrode materials will be presented in subsequent reports.

Acknowledgements

The authors thank Dr L. I. Ryabova for useful discussions. This work is supported by Skolkovo Institute of Science and Technology (Center of electrochemical energy), Russian Foundation for Basic Research (grant no. 15-38-20247) and MSU-development Program up to 2020. L. M. K. is grateful to Haldor Topsøe A/S for the financial support.

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Footnote

† Electronic supplementary information (ESI) available: Experimental methods, X-ray diffraction data and TECs for Pr_2−xLa_xCuO₄. See DOI: 10.1039/c5ra17763d

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