Cobalt perovskites
The project is directed to the rare earth (RE) based perovskites with metal ions of paramagnetic (low-spin Co
4+) or diamagnetic (low-spin Co
3+, Ga
3+) character. It aims in an experimental and theoretical evidence for a stabilization of the mixed valence of selected RE ions in the calcium doped RE
1-xCa
xMO
3 perovskites. This leads in some cases to a first order metal-insulator (M-I) transition that is associated with a charge transfer between the M and RE ions and to the change of their ionic configurations. We plan to identify and characterize new cobaltites and gallates (M = Co, Ga) with rare earths in the mixed valence, including those with occurrence of M-I transition, and investigate them by using different means. The focus is given to the heat capacity at very low temperatures (0.4 - 2 K) and neutron spectroscopy measurements that will serve as powerful local probes as concerns the identification of non-Kramers ions (as Pr
3+) and Kramers ions (as Pr
4+). The results will be confronted with the ab-initio and effective Hamiltonian calculations.
The cobaltites derived from LaCoO
3 are prospective materials for their electronic and catalytic properties and represent also fundamental interest because of the temperature and pressure driven transitions between Co(III) ionic states of different spin numbers. The aim of the project is to investigate novel phenomenon where itinerant charge carriers convert the surrounding diamagnetic Co(III) states into magnetic ones, forming large spin polarons. In the frame of this project, new electron and hole doped cobaltites, including the mixed Co/Ni and Co/Cu systems, will be prepared and their structural, magnetic and electric transport properties will be studied over the range of low and high temperatures. The observed data will be confronted with theoretical band structure calculations treating the polarons as impurity using the supercell or impurity models. It is expected that this project will provide a better microscopic description of transport phenomena in mixed-valency oxides close to the boundary between the regime of localized and itinerant electrons.
The cobaltites of the perovskite types form a class of materials with a sequence of phase transitions that are accompanied with a marked change of electric and thermal transport properties. The temperature range of these transitions includes low temperatures and also the technologically important region up to 500°C. The aim of the project is to investigate in detail the atomic-level origins of the physical phenomena in systems Ln
1-xA
xCoO
3 and Ln
1-xA
1+xCoO
4 (Ln = La, Y, rare earths, A = alkali earths), eventually with Mn partial substitution for Co. The project involves technology of ceramic and single-crystal samples, their crystallographic and complex physical characterization. The character of the spin states in systems with Co
3+/Co
4+ ions will be determined by the nuclear magnetic resonance on
59Co nuclei, eventually
139La or
141Pr. The experiments will be complemented by ab-initio electronic structure calculations. Attention will be paid to the orbital degeneracy in the intermediate and high-spin cobalt states and its lifting by the Jahn-Teller effect, as well as to specific phenomena associated with the presence of orbital momentum.
Magnetism and the insulator-metal transition in the Co3+/Co4+ perovskites.
The cobaltites of perovskite or perovskite related types exhibit unusual magnetic and transport anomalies that are related to the temperature, magnetic-field or pressure induced changes of the orbital and spin state of cobalt ions. Entropy associated with the spin degree of freedom becomes important factor in the charge carrier transport, and some complex cobaltites are thus considered as prospective materials for thermoelectric applications. The project focuses on open problems in cobaltites Ln
1-xAe
xCoO
3 and Ln
2-xAe
xCoO
4 (Ln = La, Y, rare earth, Ae = alkaline earth), such as a possibility of the orbital order or charge and spin density segregation due to intrinsic inhomegeneities, and on question of the localized vs. itinerant character of charge carriers. By means of the structural, magnetic and electric transport experiments, combined with the ab-initio and effective Hamiltonian based calculations, the proposed study aims to a better microscopic understanding of observed phenomena.
