Cobaltites as perspective thermoelectrics
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Cobaltites as perspective thermoelectrics J. Hejtmánek1, M. Veverka1, K. Knížek1, H. Fujishiro2, S. Hebert3, Y. Klein,3, A. Maignan3, C. Bellouard4 and B. Lenoir4 1
Institute of Physics of ASCR, Na Slovance 2, 182 21 PRAHA 8, Czech Republic Dept. of Materials Science and Technology , Faculty of Engineering, Iwate University,Morioka 020-8551, Japan 3 Laboratoire CRISMAT, UMR 6508 associée au CNRS et ISMRA, 6 Bd du Maréchal Juin, 14050 Caen Cedex, France 4 LPM, Ecole Nationale Supérieure des Mines de Nancy, Parc de Saurupt ,54042 Nancy,France 2
Abstract Aiming the thermoelectric applications at high temperatures the Co3+/Co4+ oxides with 1D, 2D and 3D dimensionality of CoO6 octahedral building network were studied. This comparative experimental study of electrically conducting complex cobaltites is based on magnetic, transport and thermal characterization in a wide range of temperatures – from 10 K up to 1100 K. Namely we present and discuss the thermoelectric power, specific heat, magnetic and magnetotransport data acquired on Ln1−xAxCoO3 (Ln = La, Y, rare-earth, A = alkaline-earth), Na0.7CoO2., Ca3Co4O9 ,Pb-Sr-Ca-Co misfits and 1D Ca3Co2O6 and Sr6Co5O15. A relatively small thermopower, positive Hall coefficient and huge electronic contribution to the specific heat is observed in ferromagnetic metallic perovskites with Co3+/Co4+~ 1 while the large positive thermopower and similar large electronic contribution to the specific heat characterizes the most promising group of thermoelectric oxides- layered NaxCoO2 bronzes and Ca3Co4O9 and Pb-Sr-ca-Co misfits. Finally, spin fluctuations are shown to be the efficient scatters for thermal conductivity in cobalt parovskites. Introduction The recent material research of mixed cobalt oxides is strongly motivated by the potential of some of them to be used as chemically stable high temperature thermoelectric material. This fact intensified both their theoretical and experimental research. Nonetheless, despite the intensive investigations of the prototype materials represented by 3D perovskites Ln1−xAxCoO3 (Ln = La, Y, rare-earth, A = alkaline-earth) and 2D cobaltites of NaxCoO2 type, the concise physical background of their transport and magnetic properties remain still a matter of debate. This is likely due to a fact that cobalt ions can be stabilized either in low-spin state (diamagnetic for “pure” Co3+), with filled t2g levels and empty eg states, or magnetic ones, with filled eg states. As the energy difference between respective states is, due to comparable strength of crystal field and Hund’s energies , rather small, the thermodynamically most stable groundstate, with eventually different character of charge carriers, can be critically influenced by an interplay of additional degrees of freedom - orbital and charge. The challenge for unifying theoretical model represents the thermoelectric power where ambiguous models based either on “classical” approach, associated with diffusion of itinerant charge carriers, or less traditional- based on
configurational
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