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Abstract: A structural and electrical characterization of the system NiNb2− x Ta x O6 (0≤x≤2) is presented. For x≤0.25 materials with the columbite-type structure typical of NiNb2O6 have been obtained whereas for x≥1 tri-rutile-like oxides were obtained. The electrical properties are similar in both cases; they are semiconducting with very low electrical conductivity and very high activation energy, though slight differences were found as a function of Ta content. Improvement of conductivity by reducing the stoichiometric materials could not be achieved due to decomposition. In this connection, partial substitution of Nb or Ta by Ti has been carried out in order to create oxygen vacancies. Tantalum was partially replaced by Ti to a significant extent in the tri-rutile structure inducing a slight increasing of conductivity. However, for the columbite case neither Nb nor Ta could be partially replaced. This behavior is quite different from that reported for other similar columbites such as MnNb2O6− δ , which exhibits high electrical conductivity upon substitution of niobium by titanium. [Copyright &y& Elsevier]

Abstract: We describe the synthesis, crystal structure, magnetic and electrochemical characterization of new rock salt-related oxides of formula, Li3 M 2RuO6 (M=Co, Ni). The M=Co oxide adopts the LiCoO2 (R-3m) structure, where sheets of LiO6 and (Co2/Ru)O6 octahedra are alternately stacked along the c-direction. The M=Ni oxide also adopts a similar layered structure related to Li2TiO3, where partial mixing of Li and Ni/Ru atoms lowers the symmetry to monoclinic (C2/c). Magnetic susceptibility measurements reveal that in Li3Co2RuO6, the oxidation states of transition metal ions are Co3+ (S=0), Co2+ (S=1/2) and Ru4+ (S=1), all of them in low-spin configuration and at 10K, the material orders antiferromagnetically. Analogous Li3Ni2RuO6 presents a ferrimagnetic behavior with a Curie temperature of 100K. The differences in the magnetic behavior have been explained in terms of differences in the crystal structure. Electrochemical studies correlate well with both magnetic properties and crystal structure. Li-transition metal intermixing may be at the origin of the more impeded oxidation of Li3Ni2RuO6 when compared to Li3Co2RuO6. Interestingly high first charge capacities (between ca. 160 and 180mAhg−1) corresponding to ca. 2/3 of theoretical capacity are reached albeit, in both cases, capacity retention and cyclability are not satisfactory enough to consider these materials as alternatives to LiCoO2. [Copyright &y& Elsevier]