M.L. López

Electrical properties of La1.33−xLi3xTi2O6 (0.1<x<0.3)

Abstract Structural characterization and ionic conductivity properties of three members of the La1.33−xLi3xTi2O6 (being x=0.29, 0.21 and 0.09, respectively) perovskite-type series are reported. These phases are described as tetragonal (S.G. P4/mmm) and the unit cell parameters (a≈3.87 and c≈7.78 A) are related to the cubic ABO3 perovskite one. X-ray diffraction data were analyzed by the Rietveld method and an interesting feature is that the cation vacancies are concentrated on alternating A-planes (i.e. for z= 1 2 ) of the structure. Therefore, cation mobility seems to be due to a 2-D mechanism in such planes. Electrical properties have been measured by a.c. techniques and the higher conductivity results were obtained for the intermediate composition, x≈0.21. The activation energies of the three phases are quite similar.

Crystal structure and electrical properties of LiFeTiO4 spinel

Abstract The complex oxide LiFeTiO 4 was prepared by an usual solid state method and its crystal structure has been refined by Rietvelds analysis of powder X-ray diffraction data. These results indicate a spinel-type structure in which all titanium cations are in octahedral sites whereas lithium and iron cations are distributed on tetrahedral and octahedral sites, nearly in the same ratio, that could be expressed by the formula: (Li 0.47 Fe 0.53 )[Li 0.53 Fe 0.47 Ti]O 4 . The ionic conductivity measured on polycrystalline samples has been obtained by ac techniques in the temperature range of 473–873 K.

Surface characterisation of spinels with Ti(IV) distributed in tetrahedral and octahedral sites

The differences of the binding energy of the Ti 2p 3/2 electron obtained by X-ray photoelectron spectroscopy (XPS) in ternary spinels such as LiFeTiO 4 , LiMnTiO 4 and LiCrTiO 4 have been studied. X-ray diffraction patterns for these spinels and neutron powder diffraction data for LiFeTiO 4 , LiMnTiO 4 and LiCrTiO 4 reveal that Fe(III) is in octahedral and tetrahedral sites, and Cr(III) and Mn(III) are exclusively in octahedral sites. However, Ti(IV) cations are distributed 100% in octahedral sites for LiFeTiO 4 , 66% in octahedral sites and 34% in tetrahedral sites for LiMnTiO 4 , and 88% in octahedral sites and 12% in tetrahedral sites for LiCrTiO 4 . XPS studies confirm the oxidation state of the cations Fe(III), Cr(III), Mn(III) and Ti(IV); and using high resolution XPS (5.85 eV pass energy) it is possible to decompose the Ti 2p 3/2 signal in two peaks at 458.0 eV (68%) and at 458.7 eV (32%) assigned to octahedral and tetrahedral Ti(IV), respectively for LiMnTiO 4 . In the case of LiCrTiO 4 , the Ti 2p 3/2 signal could be decomposed in two peaks at 458.4 eV (87%) and 459.3 eV (13%) with a similar assignation than that observed for LiMnTiO 4 . A single Ti 2p 3/2 peak at 458.1 eV and corresponding to octahedral Ti(IV) was observed for LiFeTiO 4 .

Structure and magnetic properties of LiMVO4 (M = Co, Ni, Cu) spinels

The two-dimensional antiferromagnetic correlations found for LiCuVO4 below 30 K have been explained by considering the ordered distribution of lithium and copper in the octahedral sites.

The monoclinic perovskite La2LiSbO6. A rietveld refinement of neutron powder diffraction data

Abstract The crystal structure of La 2 LiSbO 6 has been refined by neutron diffraction at 295 K.This compound adopts a distorted perovskite structure with an ordered distribution of Li and Sb atoms in B-sites and the La atoms are located in A-sites. The tilting of (Sb, Li)O 6 octahedra is responsible of the irregular coordination in the LaO 8 polyhedra. The bond valence calculations for metal atoms are in agreement with the experimental bond distances.

Structural and electrical characterization of new materials with perovskite structure

Abstract The series M 1 − 3 x La 1 + x (MgW)O 6 , where M = Na, K and x = 0.0, 0.11, and 0.22, was obtained by solid state reacti air at 723 and 1273 K. The formulae were inferred from different techniques and they were in good agreement with the structure determinations. The structures could be referred to a perovskite-type and show an ordered arrangement of MgO 6 and WO 6 octahedra in the B-sites and also Na and La cations are ordered in layers into the A-sites. The electrical behaviour of these phases show that the activation energies are relatively high and they could be related to the previous structural analysis.

Relationship between crystal structure and electric properties for lithium-containing spinels

Abstract The crystal structure of the spinel LiZnNbO 4 has been refined by Rietvelds analysis of powder X-ray diffraction data. The structure has tetragonal symmetry and the cell parameters are: a = 6.0824(1), c = 8.4035(7) A , space group P 4122 (Z = 4). The atomic positions are: Zn (4 c ), Li (4 b ), Nb (4 a ), 01 (8 d ) and 02 (8 d ). A different ionic conductivity behaviour for the isostructural spinels LiZnNbO 4 and Li 2 TeO 4 is detected and it is discussed on the basis of the mobile ion sites in both compounds.

Ionic conductivity of a new defect pyrochlore

Abstract The synthesis and crystal structure of the new oxide Rb 3 CrTe 3 O 12 are reported. This compound can be considered as derived from a cubic pyrochlore (A 2 B 2 O 7 ) of formula Rb 3 2 Cr 1 2 Te 3 2 O 6 in which both A-sites (Rb) and anionic ones are partially occupied (A 1.5 B 2 O 6 ). This kind of structure enables the mobility of Rb + cations through well defined 3D-tunnels. Electrical conductivity measurements agree with this assumption and the experimental data are interpreted on these bases.

Magnetic behaviour governed by Co spin transitions in LaCo1−xTixO3 (0 ≤ x ≤ 0.5) perovskite oxides

Perovskite-type cobaltites LaCo1−xTixO3 (0 ≤ x ≤ 0.5) synthesized by the liquid mix technique were characterized by x-ray diffraction, neutron diffraction, XPS, magnetic susceptibility and magnetization versus magnetic field measurements. Structural symmetry changes from rhombohedral (S.G. ) for 0.05 ≤ x ≤ 0.20 to orthorhombic (S.G. Pbnm) for 0.25 ≤ x ≤ 0.40. The bond distances dependence on the composition, x, has been analysed from x-ray and neutron diffraction data. Both the structural and the spectroscopic (from XPS) results suggest that Ti4+ substitution implies the stabilization of low-spin CoIII ions and the evolution of magnetic moments in the paramagnetic zone is also coherent with this assumption. For small values of x both FM responses and the absence of saturation in the M versus H curves at 2 K are interpreted in terms of a magnetic frustration. For x > 0.25 the cobalt cations seem to stabilize as CoIII (rather than Co3+) coexisting with Co2+. In these more substituted perovskites, AFM Co2+–O–Co2+ interactions become predominant, leading to a well established AFM ordering for x = 0.4 and 0.5.

Transport properties of new Ti manganites: Sr2?xLaxMnTiO6 (0.25 ? x ? 1)

Sr2−xLaxMnTiO6 (0.25 ≤ x ≤ 1) orthorhombic perovskite-like system has been synthesized by the ceramic method. Structural characterization from x-ray and neutron diffraction shows a random distribution of Sr/La and Mn/Ti cations, in A- and B-sublattices, respectively, in all the cases. These materials show complex magnetic behaviour which can be interpreted in terms of frustrated FM interactions, considering a cluster-glass picture. M–I transitions have been observed for x = 0.75, 1 and negative MR responses in the ranges of 1000% and 90% have been detected at 40 K and 210 K, respectively. Magnetization (ZFC and FC) measurements in ac and dc are interpreted considering the existence of superparamagnetic clusters embedded in a paramagnetic matrix.