Search

A series of spinel type derivatives has been obtained from doping the parent compound, Li4/3Ti5/3O4, with Cr3+ giving rise to the phases of general formula Li(1+x)/2TixCr(5−3x)/2O4 (1.07⩽x⩽1.6). X-ray and neutron diffraction refinements confirm that all these phases crystallise in the Fd3m space group, in which Cr3+ ions occupy octahedral sites whereas Ti4+ and Li+ are distributed in both octahedral and tetrahedral sites. Conductivity measurements on polycrystalline samples show that higher substituted samples behave as ionic conductors whereas for the lower limits of substitution the materials are predominantly electronic conductors. Magnetic susceptibility results indicate that the upper doped phases do not show magnetic cooperative interactions but these are significant for the less substituted materials, in which the effective magnetic moment is higher. [Copyright &y& Elsevier]

New phases which arise from partial substitution of Ti4+ by Cr3+ and Li+ of the compound La2/3TiO3 have been obtained, giving rise to the series La1.33LixCrxTi2−xO6 (x=0.66, 0.55 and 0.44). These phases adopt a perovskite-type structure as deduced from their structural characterization. Rietveld''s analyses of neutron diffraction data show that it is orthorhombic (S.G. Pbnm) with ordered domains. Conductivity has been examined by complex impedance spectroscopy and it increases with increasing lithium and chromium content. These materials behave as mixed conductors with low activation energies. Magnetic susceptibility variation with temperature shows antiferromagnetic interactions at the lowest temperatures. [Copyright &y& Elsevier]

Two new phases in the (SrTiO 3 )-(BiMnO 3 ) perovskite type system were studied. These SrBiMn 2−x Ti x O 6 ( x =0.5 and 0.75) ceramics were prepared by the liquid-mix method as polycrystalline powders. They were characterized by neutron diffraction (ND), electrical and magnetic measurements and these results were analysed in connection with the x =0.25 phase, previously described by us. Their crystal structures were fitted in the tetragonal I 4/ mcm space group, with additional superstructure reflections suggesting local ordering. Electron microscopy results revealed these ordering effects are actually present concerning both perovskite sublattices. The electrical response pointed to a mechanism consisting in variable range hopping of polarons. A relaxor ferroelectric behaviour dependent on the pellets thickness was found with T m in the 140–240 K range. The magnetic structures imply an incommensurable modulation of the magnetic moments along the c or a directions, depending on the substitution degree x . A cluster picture in terms of chemically different nanoregions responsible of the macroscopic response is discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] The Li x Mn3− x − y Fe y O4 spinels show a versatile electronic behavior. [•] Optimal compositional ranges for different application fields are proposed. [•] Frustrated ferromagnetic response is compositionally driven. [•] High ε′ values are obtained when B sites are occupied by both Mn and Fe cations. [•] High lithium contents are linked to the electrochemical behavior. [ABSTRACT FROM AUTHOR]

Abstract: New spinel-type phases of general formula LiMMnO4 (M=Cr, Ti), derived from LiMn2O4 by substitution of Mn3+ by Cr3+ or Mn4+ by Ti4+, have been obtained and characterised. Neutron diffraction refinements confirm that both phases crystallise in the space group, giving the cation distributions [Li]8a [CrMn]16d O4 and [Li0.66Ti0.34]8a [Li0.34MnTi0.66]16d O4. Electrical conductivity has been examined by various techniques showing that these materials behave as semiconductors. The electrochemical behaviour indicates different oxidation–reduction steps in both cases concomitant with the insertion/deinsertion of lithium in non-reversible processes. X-ray diffraction patterns show that the above process is topotactic in LiCrMnO4. Magnetic data and neutron diffraction measurements show that no long-range magnetic ordering is present, suggesting a spin-glass transition for M=Cr at low temperature, while for M=Ti the presence of non-magnetic ions in the octahedral sublattice provokes an inherent magnetic frustration. [Copyright &y& Elsevier]

Abstract: Manganites are materials that show remarkable phenomena related to charge orbital ordering (CO/OO) and it is extremely important to understand the fundamental nature of this behaviour. This paper reports on the structural, electronic and magnetic behaviour of the new SrBiMn1.75Ti0.25O6 manganite and the dependence of these properties with temperature. A detailed structural analysis has been carried out by electron, X-ray, neutron diffraction between 4 and 700K. The electron diffraction patterns obtained at room temperature (RT) evidence that the average structure ( and c ∼2a p ) presents a modulation that doubles the a and c lattice parameters. A very high charge ordering (CO) transition temperature of 510K, similar to that found for the non-doped material, SrBiMn2O6, is observed. Above this temperature the symmetry changes from orthorhombic to tetragonal. The conductivity and susceptibility data show a strong interdependence with the structural transition. The activation energy decreases from 0.24eV to 0.12eV below and above 510K, respectively. On the other hand, the high value of the magnetic moment (μ eff =9.34 μ B), observed between 300 and 500K, is interpreted in terms of the existence of Mn4 magnetic clusters. Finally, extra maxima appearing in neutron diffraction patterns at lower temperatures are interpreted considering the stabilization of AFM interactions, consistent with an A-type magnetic ordering. The results are discussed in the framework of charge ordering/orbital ordering and cluster models. [Copyright &y& Elsevier]

Abstract: Microcrystalline powders of Sr1−x □ x LaMnTiO6−δ (0≤ x ≤0.15) perovskites have been synthesized by the sol–gel method. Structural characterization from neutron diffraction data shows tetragonal symmetry for all samples in the whole temperature range explored (5–700K) and HRTEM analysis confirms it at the micro scale. Structural refinements suggest that Sr/La and Mn/Ti cations are placed at random in A and B sublattices, respectively, being the vacancies exclusively located on the former sites. Doping with vacancies in the A sites does not provoke any extra structural ordering or distortion but modifies the electronic scenario at the B-sites, leading to an enhancement of Griffiths phase as the amount of vacancies, x, increases. Semiconductor behaviour has been observed, without any transition to metallic state, with typical activation energies of 0.15eV. The intrinsic magnetic frustrated behaviour is described in terms of a cluster-glass model. [Copyright &y& Elsevier]