P.H. Domingues

Magnetization of lithiated magnetite, LixFe3O4

Abstracts are not published in this journal

Magnetic and quadrupolar hyperfine anisotropy in single crystal rubidium dithioferrate

Abstract This letter reports Mossbauer absorption of 57 Fe in monocrystalline RbFeS 2 perpendicular to the c axis of the crystal which suggests that the c axis is the axis of magnetisation and yields a negative sign for the EFG.

Mössbauer studies on high-temperature lithiated magnetite, LixFe3O4

Abstract The influence of the insertion of lithium ions in magnetite by high-temperature techniques are studied by Mossbauer spectroscopy at room temperature. The results show: (i) the existence of two non-interacting phases, one corresponding to the pure magnetite and the other to a paramagnetic lithiated phase; (ii) that the mechanisms of electron hopping remain practically unaffected by the insertion of Li up to x ≈ 0.20. The results, when compared with those obtained by electrochemical Li insertion, show good agreement.

Magnetic properties of lithium ferrite doped with aluminum and gallium

Measurements are reported on high‐field magnetization of powder samples of ordered lithium ferrite doped with Ga and Al [LiFe4.5(Ga,Al)0.5O8]. We found a linear behavior of the spontaneous magnetization with the difference in Ga‐Al concentrations. The data were interpreted with the Neel model to give the cation distribution and with the method of Rado–Folen to estimate the molecular field parameters (n, α, β) and the bilinear exchange integrals ( Jij).

The Observation of Phase Transition in LiF3O4 by Mössbauer Spectroscopy

The compound LiFe3O4 is a novel material derived from magnetite by the insertion of Li+ in the spinel structure. As a consequence of the insertion, there is a change of the crystal structure from spinel to rock salt type, with all octahedral sites filled and the tetrahedral ones empty. A model is proposed in which the electron hopping between B-sites is little affected by the Li insertion. Our results also indicate some interesting features: (a) the compound is paramagnetic at room temperature and the Mössbauer spectra can be fitted with four different sites (as expected from the crystallography); (b) it undergoes a kind of structural phase transition in the temperature region 80 K<T<100 K manifested by relaxation and a large broadening of the linewidths. For temperatures between 100 and 125 K there is anomalous behavior of the hyperfine parameters. For 80 K one sees again the same pattern observed for temperatures above the transition region, i.e., the presence of four well-resolved paramagnetic doublets, characterizing the region ranging from 80 to 125 K as a reentrant one. The antiferromagnetic phase transition is expected to occur around 50 K. This reentrant region where the phase transition occurs is about the same as the region where the Verwey transition in the magnetite is observed. Measurements with other techniques confirm this assumption. To our knowledge, it is the first time that such a behavior is observed in materials related to magnetite.

Mössbauer studies of the solid solutions of synthetic hureaulite, [FexMn5-x][PO4H]2[PO4]2[H2O]4

Abstract Nine samples of synthetic hureaulites, [Fe x Mn 5 -x ][PO 4 H] 2 [PO 4 ] 2 [H 2 O] 4 , in the interval 1 ≤ x ≤5, were prepared by hydrothermal methods and analysed by Mossbauer spectroscopy in order to determine the occupation of the sites by Fe 2+ as a function of iron concentration, as well as the influence of these substitutions on the hyperfine parameters.

Magnetism in the series (FexMn1−x)5(PO4)2(PO4H)2(H2O)4

Abstract Differential magnetic susceptibility measurements in synthetic polycrystals of the hureaulite series (FexMn1−x)5(PO4)2(PO4H)2(H2O)4 (0 ≤ x ≤ 1) are reported for the first time. Its structure consists essentially of a condensation of pentamers formed by distorted edge-sharing octahedra whose centres are occupied by Mn2+ or Fe2+ ions and disposed in three different crystallographic sites in the proportion (III):(II):(I) = 2:2:1. The results from high temperature susceptibility measurements show an effective magnetic moment ranging from 11.89 to 13.53 μB/f.u., suggesting that the spins in the pentamers are all parallel except one, which was attributed to the central one, that would have its spin reversed. The compound with x = 1 (ferrous compound) behaves as a normal canted antiferromagnet down to the Neel temperature, 12.7 K. Below this temperature the susceptibility tends to zero with the temperature. It was shown that the replacement of Fe2+ by Mn2+ introduces ferromagnetic correlations in the neighbourhood of the critical temperature due to anisotropic exchange interactions (weak ferromagnetism). The compound for x = 0 (manganous compound) is a weak ferromagnet with Neel temperature of 6.0 K. It was also observed that for x ≤ 0.40 a second phase transition begins to appear due, possibly, to a rearrangement of spins.

High-temperature substitutional lithiation of magnetite

The reaction products of Li2CO3 and Fe2O3 have been studied by X-ray diffraction for the compositional range 0≤LiFe≤0.2. Under oxidizing conditions (750°C in air) the products were LiFe5O8 and Fe2O3. Reduction in H2 at 450°C yielded a single, apparently cation—deficient spinel phase for 0≤LiFe≤0.05; the spinel phase was mixed with a rocksalt phase for 0.1≤LiFe≤0.2. Further reduction (780°C in Ar in a carbon furnace) yielded Fe3O4 and a rocksalt phase. The spinel/rocksalt fraction extrapolates to zero at a rocksalt composition LiFe2.5O4. This LiFe2.5O4 phase, with space group Fm3m, differs from the product of room—temperature lithiation of Fe3O4, which retains the [Fe2]O4 spinel framework with space group Fd3m and so prevents a completely random distribution of Li on the octahedral sites.

The role of Al and Ga ions in lithium ferrite LiFe4.5AlxGayO8

In this work magnetisation and lattice parameter data obtained for aluminium and gallium substituted lithium ferrite (90% Fe) are presented. Some trends of the diamagnetic impurities distributions for the tetrahedral and octahedral sublattices are derived. The lattice parameter and the saturation magnetisation of the double substituted lithium ferrite are linear functions of the difference in the concentrations of gallium and aluminium.

Mössbauer studies of hydrogenated potassium di-thioferrate

Abstract This letter reports Mossbauer studies of hydrogenated potassium dithioferrate which suggests the formation of new Fe sites after hydrogen absorption.