Contribution of the Multiplicity Fluctuation in the Temperature Dependence of Phonon Spectra of Rare-Earth Cobaltites.

We have studied, both experimentally and theoretically, the unusual temperature dependence of the phonon spectra in NdCoO, SmCoO and GdCoO, where the Co ion is in the low-spin (LS) ground state, and at the finite temperature, the high-spin (HS) term has a nonzero concentration nHS due to multiplicity fluctuations. We measured the absorption spectra in polycrystalline and nanostructured samples in the temperature range 3-550 K and found a quite strong breathing mode softening that cannot be explained by standard lattice anharmonicity. We showed that the anharmonicity in the electron-phonon interaction is responsible for this red shift proportional to the nHS concentration.

Effect of Multiplicity Fluctuation in Cobalt Ions on Crystal Structure, Magnetic and Electrical Properties of NdCoO and SmCoO.

The structural, magnetic, electrical, and dilatation properties of the rare-earth NdCoO and SmCoO cobaltites were investigated. Their comparative analysis was carried out and the effect of multiplicity fluctuations on physical properties of the studied cobaltites was considered. Correlations between the spin state change of cobalt ions and the temperature dependence anomalies of the lattice parameters, magnetic susceptibility, volume thermal expansion coefficient, and electrical resistance have been revealed.

Methane oxidation over A-site ordered and disordered Sr(0.8)Gd(0.2)CoO(3-δ) perovskites.

A tetragonal phase Sr0.8Gd0.2CoO3-δ with ordered Gd(3+)/Sr(2+) ions and oxygen vacancy sites is found to be about five times less active in the reaction of methane combustion than a quenched cubic perovskite phase with randomly distributed (disordered) Gd(3+)/Sr(2+) ions over the A-sites of the crystal lattice.