Electric transport and magnetic properties of perovskites LaMn_1−xCo_xO₃up to 900 K

Abstract

The structural, magnetic and transport properties of the LaMn 1−x Co x O 3 system were investigated over a wide temperature range, 10-900 K. Two structural types, depending on x, were detected-orthorhombic Pbnm (x 0.5) and rhombohedral R 3c (x > 0.6), separated by the bi-phasic sample x = 0.6. At both ends of the LaMn 1−x Co x O 3 system, the respective substituents are unambiguously characterized as Co 2+ (x 0.1) and Mn 4+ (x 0.9). On the Mn-rich side, up to x < 0.5, we infer on the basis of high temperature transport and magnetic data the gradual increase of the Co 2+ valence towards the prevailing Co 3+ , while for the Co-rich samples (0.6 x 0.8), the Co 3+ states tend to disproportionate at high temperatures to Co 2+ + Co 4+ , which probably controls both the transport and magnetic properties. At low temperatures, the long range ferromagnetic order was confirmed by means of neutron diffraction for the x = 0.4 sample. A non-uniform magnetic state was detected at higher cobalt substitution up to x = 0.8 and was associated with FM clusters that are formed below ∼220 K and coagulate below ∼150 K.

Low-temperature sintering and enhanced thermoelectric properties of LaCoO3 ceramics with B2O3–CuO addition

“…Furthermore, BCu addition does not affect the high temperature limit of S of samples, all approaching $30 lV K À1 as T reaches around 500°C. The temperature behavior of S obtained here is basically consistent with previous studies for LaCoO 3 [25,26]. It was reported that S of doped LaCoO 3 could be estimated by the modified Heikes formula [27]:…”

Local electronic structure and magnetic properties ofLaMn0.5Co0.5O3studied by x-ray absorption and magnetic circular dichroism spectroscopy

“…Further discussion is also raised by the work of Joy and coworkers, 21,22 who have synthesized two different single phases of LaMn 0.5 Co 0.5 O 3 and inferred from a combination of magnetic susceptibility and x-ray photoelectron spectroscopy measurements that the phase with the higher T C contains high- 11,26,27 Interesting is that the magnetization of polycrystalline samples of LaCo 0.5 Mn 0.5 O 3 does not saturate in magnetic fields up to 7 T, 16 and that there are indications for a large magnetic anisotropy.…”

EXAFS study of compounds

“…The effective backscattering amplitude F eff , the phase correction gðkÞ and the total central atom phase shift x were calculated for each scattering path with the FEFF8 program. The structural data obtained from X-ray diffraction measurements [1] served as the input parameters for calculations and the starting values for fitting. Total w function was constructed according to the equation:…”

“…The LaMn 1Àx Co x O 3 series exhibits a change of structure from orthorhombic, space group Pbnm, to rhombohedral, space group R-3c [1]. This change occurs around x ¼ 0:6 and is accompanied with a jump in the unit cell volume.…”