Lay summary
The main objective of this proposal is to study the layered cobaltite compounds, mainly by means of nuclear and macroscopic techniques:
Rare earth magnetism in layered cobaltites PrBaCo2O5.5 by muyon spin rotation and neutron measurements.
In contrast to the other layered cobaltites the PrBaCo2O5.5 shows a pronounced decrease of the magnetic susceptibility at temperatures blow 30 K. Zero field muyon spin relaxation measurements show that existing spin fluctuations present at higher temperatures slow down below 50 K, giving rise to a broad maximum in the longitudinal relaxation rate. We attribute this behaviour to a slowing down of the Pr moment since the Co subsystem already persists static magnetic order below 250 K. Neutron scattering measurements at DMC diffractometer will make it possible to test our hypothesis by resolving the magnetic structure below 50 K.

NMR and Moessbauer spectroscopy of YBaCo2O5.5 and YBaCo1.98Fe0.02O5.47

With these local probe techniques it should be possible to obtain separate information on the different Co species in the sample (octahedral andpyramidal) because they can be distinguished by their different quadrupolar splitting. In principle it should be possible to obtain the valence and the spin-state separately for the two species. Also, we would like to measure two samples with different oxygen concentrations in the compound with δ close to 0.5 where a phase separation is expected.With these experiments we will be able to verify our conclusions about the muyon sites, since the occupation of muyon sites near to Co-octahedra and Co-pyramids should change accordingly.

Effect of oxygen ordering on structural and magnetic properties of layered cobaltites LnBaCo2O5+δ (Ln= Gd, Y)
In layered cobaltites compounds the oxygen stoichiometry can be changed ina relatively wide range depending on the ionic radius of the rare earthelement. This aspect plays an important role for the magnetic and electronic properties. We plan to investigate the possible changes of these properties due to the synthesis history since there is a strong tendency of the oxygen ions and vacancies to order in the compound. In this sense, the preparation of samples with different oxygen content (showing long range oxygen ordering) will be prepared. The obtained samples will be investigated in a similar way like for the quenched samples and the results compared. Such work is planed to be performed for the YBaCo2O5+δ compound since the Y3+ is a non-magnetic.

Thermodynamics of oxygen at lower oxygen partial pressures

Using TG measurements and a defect chemistry model for the two compoundsLnBaCo2O5+δ (Ln =Pr, La) we were able to study the thermodynamics LnBaCo2O5+ofoxygen in a partial pressure range of 10-3 - 0.5 bar O2. From these experiments it was observed that in order to obtain still lower oxygen content for these compounds partial pressures smaller than 10-3 bar are requested. In our TG measurements a precise tuning of such low partial pressures became technically difficult. We plan therefore, to extend thepO2 range in the future, using coulometric titration and results interpreted using the already existing defect chemistry model.