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The electrical conductivity of lithium-manganese spinels is analyzed in samples with different starting Li cationic fraction x. Li-rich spinels, resulting from Li-Mn substitution around the stoichiometric value (x = 0.333), show conductivity values higher than that observed in stoichiometric LiMn2O4. Besides, a conductivity drop, associated with a structural phase transition at about 290 K in LiMn2O4, progressively disappears by decreasing x, while it is absent in Li-rich samples. Stoichiometry effects on the concentration of charge carriers and on the available sites for the hopping transport process are evaluated, as well as the effects due to coexisting insulating phases. The role of the Jahn-Teller effect on the conductivity behaviour of stoichiometric and Li-poor spinels is also considered.

The coexistence of LiMn2O4and Li2MnO3phases from MnO/Li2CO3mixtures with lithium cationic fraction 0.33≤x≤0.53 was studied by X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) measurements. XRD quantitative phase analysis was carried out using the Rietveld profile refinement procedure, while the Li2MnO3percentage was determined directly by comparing the EPR intensities with the intensity of the pure Li2MnO3spectrum. The experimental results show the presence of an Li-rich spinel phase and allow an estimation of the lithium excess, since the structural analysis of pure Li2MnO3samples evidenced a nearly stoichiometric composition (x=0.663) with very limited cationic disorder. © 1997 Academic Press.

X-ray diffraction, magnetic susceptibility, and electron paramagnetic resonance (EPR) measurements have been carried out on mixed nickel-lithium oxides as a function of the lithium content. The experimental results allow us to analyze the cationic order and the magnetic interactions. All the data are consistent with a model that admits coexistence of different structural phases in a wide composition range, previously not accounted for in the discussion of the physical properties of these mixed oxides. © 1996 The American Physical Society.

EPR data of ordered solid solutions of lithium nickel oxides are reported as a function of the lithium content. The features of the signal and the EPR centre density are analysed by a model of dynamical trapping of holes in [Ni2+-O-Ni2+)-h+] complexes. The possible origin of the interactions responsible for the magnetic ordering and some features of the transport properties are also discussed.

Polycrystalline samples and thin films of the x=0.8 member of the CexGd1-xO2-y solid solution have been successfully prepared. Their structural and transport properties have been studied by means of X-ray diffraction and electrical conductivity measurements as a function of preparation route, grain size, substrate nature and film thickness. Our data shows that the substrate roughness plays a relevant role in affecting the final sigma; however, by optimizing the thin films preparation conditions we showed that the conductivity of CGO material is always greater than that of the state-of-the-art oxide conductor, the YSZ 8 mol%, even though an electronic contribution ranging from 10% to 20% was found.

The dopant role on the electric and dielectric properties of the perovskite-type CaCu3Ti4O12 (CCTO) compound is evidenced. Impedance spectroscopy measurements show that the relevant permittivity value attributed to sintered CCTOis due to grain boundary (g.b.) effects. The g.b. permittivity value of the pure CCTOcan be increased of 1–2 orders of magnitude by cation substitution on Ti site and/or segregation of CuO phase, while the bulk permittivity keeps values 90

AC impedance spectroscopy (IS) measurements were performed in the 15–700 K temperature range on pure and Ni, Fe and Co doped CaCu3Ti4O12 (CCTO) materials. Capacitance values were also confirmed by direct current measurements at room temperature. Thermoelectric power measurements showed that the electrons are involved in the conduction process of the semiconducting bulk region. The IS results evidenced a dielectric behaviour in the grain boundary region, giving a permittivity of about 3400 for the pure sample, so CCTO can be considered an internal barrier layer capacitance (IBLC) material. The giant permittivity of CCTO can be strongly increased to values ofw150 000 by Co doping on Ti site. The IBLC behaviour, together with the giant permittivity and the opportunity to combine capacitance and resistance values in an R//C circuit, evidence the applicability of this material as an integrated resonant element for the electronic industry.