M.L. Veiga

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.

Structural and electrical study of the alluaudites (Ag1−xNax)2FeMn2(PO4)3 (x=0, 0.5 and 1)

Abstract Three compounds with the alluaudite-type structure (Ag 1− x Na x ) 2 FeMn 2 (PO 4 ) 3 , with x =0, 0.5 and 1 have been synthesised by solid state reactions. These phases crystallise in the monoclinic system (space group C2/c, and Z =4), and the structure is formed by layers of edge-sharing FeO 6 and MnO 6 octahedra linked together via common corners of PO 4 tetrahedra, yielding to 1D channels in which monovalent cations are located. Complex impedance measurements were made in the frequency range from 1 Hz to 10 MHz, at temperatures between 373 and 873 K, and reveal the predominately ionic character of the electrical conductivity in all these compounds. The activation energies show that the silver derivatives ( x =0 and 0.5) are better conductors than the sodium one ( x =1) and a phase transition about 625 K is observed for the first of them.

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 .

New polymorphs of A2VP2O8 (A=Na, Rb):: Structure determination and ionic conductivity

Abstract New polimorphs of the compounds Na 2 VP 2 O 8 and Rb 2 VP 2 O 8 have been prepared by a direct fusion of the reactants. Na 2 VP 2 O 8 (NVP) crystallises in the tetragonal P4mb space group with a =8.108(4) A and c =4.943(6) A. Rb 2 VP 2 O 8 (RVP) is orthorhombic, space group Pnma, a =12.790(3) A, b =7.09(3)(8) A and c =9.1786 A. Both compounds are formed by the association of P 2 O 7 groups and VO 5 square pyramids. NVP is isotypical with K 2 VP 2 O 8 and consists of [VP 2 O 8 ] ∞ layers parallel to the ab plane intercalated by Na atoms that are located into pentagonal tunnels running along the c -axis. RVP is isotypical with Cs 2 VP 2 O 8 and formed by the assemblage of [VP 2 O 10 ] units leading to the two-dimensional [VP 2 O 7 ] ∞ layers laying in the bc plane; the Rb + are located between the layers in the interstitial spaces defined by the seven-sided windows which do not give rise to tunnels. The second part of this paper is devoted to the ionic conductivity properties of these materials using the complex impedance method, the measurements were made in the frequency range 10 MHz–1 Hz. The activation energies calculated were E a (NVP)=0.49 eV and E a (RVP)=0.85 eV.

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.

Synthesis, structural characterization, and two-dimensional antiferromagnetic ordering for the oxides Ti3(1−x)NixSb2xO6 (1.0 ≥ x ≥ 0.6)

A new family of mixed oxides of general formula Ti{sub 3(1{minus}x)}Ni{sub x}Sb{sub 2x}O{sub 6} for 1.0 {ge} x {ge} 0.6 has been synthesized, by solid state reaction in air, and the crystallographic data for these oxides have been determined. The structures of these compounds are related to the rutile or trirutile type structure. Magnetic susceptibility measurements, between 4.2 and 300 K, show the existence of a broad maximum centered at about 35 K for the NiSb{sub 2}O{sub 6} oxide which has been fitted by using the high temperature series expansion method (HTSE) and the estimated value of the exchange integral is {minus}7 K. This bidimensional antiferromagnetic ordering is drastically lowered when the x value increases for the different doped phases in the system Ti{sub 3(1{minus}x)}Ni{sub x}Sb{sub 2x}O{sub 6}.