RESUMEN
We report a (139)La NMR investigation of low-doped insulating manganite samples (LaMnO(3+delta) and La(1-y)Ca(y)MnO(3+delta)) as a function of temperature. A volume fraction with fast nuclear relaxations was revealed by the inhomogeneous loss of the NMR signal over a broad temperature interval. Comparison with muon spin rotation data demonstrates that the wipeout of the (139)La signal is mainly due to slowly fluctuating electric field gradients. This provides strong evidence for the slow diffusion of lattice excitations, identified with Jahn-Teller small polarons.
RESUMEN
After reviewing microstructural studies on superconducting materials showing T, T', and T* structural types, results are presented on the microstructure of some n-type superconductors and related materials prepared with accurate control of the oxygen stoichiometry. Electron microscopy is used to describe the ordering of interstitial oxygen defects in T-type La2NiO4 + delta leading to the formation of the n = 2 term of a homologous series with the general formula La8nNi4nO16n + 1. Structural transitions and superstructure formation in the Pr2-x-yCexSryCuO4-delta system are reported, where T, T', and T* phases are isolated as a function of both Ce and Sr content.
