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Role of theA-site size and oxygen stoichiometry in charge ordering commensurability of Ln 0.50 Ca 0.50 MnO 3 manganites J. Appl. Phys. 84, 5506 (1998); 10.1063/1.368313Cation size-temperature phase diagram of the manganites Ln0.5Sr0.5MnO3

Two new compounds of the Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O systems have been prepared. Their structure is built up from the intergrowth of four rock-salt-type layers and one [CoO2 ] hexagonal layer. Both cobaltites exhibit large thermopower values (S300 K ∼ 140 µV K−1), low resistivity values (ρ300 K = 40–60 mΩ cm) and small thermal conductivities (κ300 K ∼ 1 W K−1 m−1). Furthermore, these compounds exhibit a negative magnetoresistance, (MR = ρH − ρH = 0/ρH = 0), reaching, at 2.5 K, − 85% in 7 T for the Bi/Ca/Co/O misfit cobaltite. A large negative magnetothermopower is also found for these cobaltites in the same temperature range. A qualitative explanation of the observed behaviour is proposed.

Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic (T_N = 24 K) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t_2g orbitals of Cr^3+ and that spin polarization is predicted with 3 mu_B per Cr^3+. The structural characterizations of CuCr1-xMgxO2 reveal a very limited range of Mg^2+ substitution for Cr^3+ in this series. As soon as x = 0.02, a maximum of 1% Cr ions substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01. This explains the saturation of the Mg^2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01. Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr^4+ "holes", created by the Mg^2+ substitution, in the matrix of high spin Cr^3+ (S = 3/2) are responsible for the transport properties of these compounds.Comment: 9 pages, 11 figures, more information at http://www.physik.uni-augsburg.de/~eyert

The magnetic, transport, and thermoelectric properties of a Tl-based misfit cobaltite have been investigated. Although this material exhibits a metallic behavior, the thermopower ͑Seebeck͒ is large, with room temperature values close to ϩ90 V/K. Compared to previously studied misfit cobalt oxides ͓Ca 3 Co 4 O 9 ,͑Bi, Pb͒SrCoO͔ this Tl-based misfit is the only compound of this family which remains metallic and paramagnetic down to 2 K. On the one hand, this behavior emphasizes the importance of the edge-shared octahedra in the CoO 2 layer for the thermoelectric properties of these materials. On the other hand, the absence of a magnetically ordered state at low temperature explains the absence of low temperature negative magnetoresistance. It gives support to a mechanism based on quantum fluctuations stabilizing the paramagnetic state rather than a magnetically ordered state.Materials with a large thermopower ͑TEP͒ are potential candidates for applications to convert heat into energy. In this respect, transition metal oxides have been the focus of recent investigations after the discovery of high figure of merit Z in NaCo 2 O 4 single crystals 1 ͑ZϭS 2 / where S, , and are for Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively͒. The NaCo 2 O 4 structure consists of the stacking of CoO 2 layers with the Cdl 2 type structure, in which the edge-shared CoO 6 octahedra form a layer of triangular cobalt. 2 In the separating sodium layer the occupation of the crystallographic site is only 50% and random. Several explanations have been proposed to interpret the large figure of merit of NaCo 2 O 4 . 3-6 First, the random and partial filling of the Na site may help in the reduction of the thermal conductivity, necessary to obtain large Z, by analogy with the so-called ''phonon glass.'' 3 Second, the thermopower is unusually large for a metal and several mechanisms have been proposed as the origin of such a large S. The degeneracy of the Co 3ϩ and Co 4ϩ low-spin ͑LS͒ electronic states, induced by the competition of Hund's rule coupling and crystalline field, together with strong electron correlation could be responsible for the large S. 4 Thermopower should also be enhanced in these cobalt oxides because of the triangular geometry of the CoO 2 planes which favors magnetic frustration, as emphasized by Merino et al. 5 Lastly, by using density functional calculations within the local density approximation ͑LDA͒, the electronic structure of NaCo 2 O 4 has been proposed. 6 The three t 2g of LS cobalt species orbitals are further splitted by the rhombohedral crystal field in one a 1g and two e g Ј orbitals and by applying the TEP formula to this band structure, realistic values of S are obtained.Interestingly, the misfit-layered cobaltites are also built of similar CdI 2 type layers as in NaCo 2 O 4 , but they are stacked with rock-salt type separating layers and the structural discordance of their two kinds of sublattices make them composite crystals. 7-9 This class of cobalt oxides exhibits als...

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