High-temperature stability, structure and thermoelectric properties of phases
Introduction
Transition metal oxides are currently of significant interest for the development of renewable energy technologies such as solid oxide fuel cells, thermoelectric (TE) modules and high-temperature superconductors [1], [2]. Manganates with a perovskite-type structure represent a family of oxides with various remarkable properties, such as ferromagnetism, metallicity and spin/charge ordering phenomena [3].
Several -related perovskite-type phases have been the subject of numerous studies exploring diverse cationic substitutions and/or oxygen deficiencies [4], [5], [6] and studying their physical, chemical and thermoelectric properties [7], [8], [9], [10]. crystallizes at room temperature in the orthorhombic symmetry with Pnma S.G., i.e. with the cell parameters ( refers to the cubic crystal structure), involving successive cooperative rotations of octahedra around the 〈1 1 1〉 crystallographic axis. At high temperatures (T > 1180 K), a structural transition of yields the cubic crystal structure, i.e. a higher space group symmetry [11]. One main structural divergence between the cubic and the orthorhombic symmetry concerns the buckling bond angles (B–O–B), expressed as , which decrease below 180° in the orthorhombic crystal structure while remaining equal to 180° in the cubic symmetry [12]. The deviations of the (B–O–B) bond angle from 180° modify the transition metal d bandwidth characterized by the integral hopping W, since [13]. For instance, a lower bending of the (B–O–B) bond angle from 180° implies a smaller overlap of the O 2p and the transition metal 3d orbitals. This results in a narrowing of the d band and therefore influences the electronic transport properties of perovskites. Twinned domains are frequently observed in orthorhombic perovskite-type structures [14], [15], [16]. Twinning phenomena arise when orthorhombic unit cells grow in different directions accommodating the slight discrepancy between lattice parameters, i.e. in the case of Pnma S.G. ( and refer to the orthorhombic crystal structure), associated with a low octahedral rotation angle value [17]. Hence, thermally induced structural transitions which involve changes in space group symmetry results in modifications of the electronic structure and of the microstructure in the perovskite phases. In manganate phases, high-temperature structural transitions are often related to the formation of oxygen vacancies upon heating [18], [19]. Oxygen-deficient manganate phases have been studied in detail regarding their structures [20], [21] and microstructures [4], emphasizing the role of the oxygen vacancies in the modification of the crystal structure. The electrical transport properties of phases vary with the extent of the oxygen deficiencies, as previously reported by Taguchi [22].
Among the manganate phases, A- and B-site substituted exhibit a rich phase diagram [23], implying mixed-valence and cations with and electronic configurations, respectively. is an antiferromagnetic insulator and exhibits large Seebeck coefficients, i.e. at 300 K [24]. A- or B-site aliovalent substitutions in the system induce the creation of cations in the matrix, resulting in a n-type conduction. Good TE materials should display a high figure of merit , i.e. a large Seebeck coefficient S, a low electrical resistivity and a low thermal conductivity [25]. Recent studies on new promising n-type manganate phases, (with ), reveal a ZT value of 0.32 at 1070 K as a result of a low lattice thermal conductivity [26]. The study shows that specific internal boundaries and interfaces, which are induced by the presence of twinned domains, might decrease the thermal conductivity due to phonon scattering while keeping the electronic properties largely undisturbed. To our knowledge, the electrical and thermal transport properties of both A- and B-site substituted have been largely studied only up to [8], [24], [27], [28], [29], [30], [31]; the higher temperature range (T > 1100 K) has not been explored so far with regard to their TE properties. Since oxide materials are evaluated regarding their high-temperature thermoelectric applications, the thermal stability of the studied phases is of main interest. The present paper reports on the study of the phases (with and 0.10) with respect to its crystal structure, microstructure and thermal stability at high temperatures. The study focuses on the influence of the high-temperature structural transition, the oxygen vacancy formation, and the effects on the electrical and thermal transport properties in the temperature range of 600 K < T < 1250 K.
Section snippets
Experimental
Polycrystalline (with and 0.10) perovskite-type phases were synthesised by a ”chimie douce” synthesis method (abbreviated SC, for soft chemistry method) [32]. The SC synthesis process has already proven to be a successful method for synthesizing cobaltate- [33], titanate- [34] and manganate- [10] phases at relatively low temperatures (T < 1000 K). The SC synthesis route is based on the thermal decomposition of complex polymeric precursors, where the cations are
Structural characterization
Figs. 1a and b present the refined high-temperature XRPD patterns of at 773 and 1173 K, respectively. The reflections observed at 773 K can be indexed in the orthorhombic unit cell (Pnma S.G., N° 62), while the XRPD data recorded at 1173 K can be refined using the cubic structural model ( S.G., N° 221). The orthorhombic perovskite structure is generally described as a pseudo-cubic framework of corner-sharing octahedra with the A cations in cuboctahedral coordination[12].
Conclusion
The present study reveals that the phases undergo a thermal reduction and a structural transition from orthorhombic to cubic symmetry at high temperatures, both phenomena being reversible upon cooling. Increasing the Nb content in the structure results in a shift of the thermal reduction onset temperature to higher temperatures. Thus, the phases are thermally stable up to a high-temperature limit, i.e. for to for . The
Acknowledgements
Financial support from the Swiss Federal Office of Energy (BfE) and the Ministry of Education of the Czech Republic (project LC 523) is gratefully acknowledged. The authors thank Dr. L. Castaldi and Prof. C. Baerlocher for their invaluable help and the use of the high-temperature diffractometer at the Laboratory of Crystallography, Swiss Federal Institute of Technology, ETHZ, Switzerland, as well as Dr. S. Hébert for the fruitful discussions.
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Perovskites – modern and ancient
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