Showing papers on "Charge ordering published in 1989"
TL;DR: In this article, the Verwey transition temperature of magnetite Fe 3(1−δ) O 4 was determined from temperature changes of magnetization measured along the principal crystal axes.
25 citations
TL;DR: In this article, the Verwey transition in magnetite has been simulated by adopting elements of order-disorder theory, and the model of Strassler and Kittel has been used to rationalize the electronic and thermodynamic properties of magnetite.
Abstract: The principal features of the Verwey transition in magnetite have been simulated by adopting elements of order-disorder theory. In certain limiting cases we obtain the model of Strassler and Kittel which had previously been used to rationalize the electronic and thermodynamic properties of magnetite. According to the present microscopic model the discontinuous Verwey transition in magnetite is driven by a change in a highly correlated electron system with temperature from a charge-ordered small-polaron state associated with local lattice deformations to a disordered state in which electrons resonate between Fe2+ and Fe3+ ions located on the octahedral cationic sites.
15 citations
TL;DR: The magnetocrystalline anisotropy of magnetite with varying metal to oxygen ratio was investigated in this article, where homogeneous single-crystal specimens of Fe3(1−δ)O4 with 0 ⩽ δ were obtained.
14 citations
TL;DR: A Mossbauer investigation of powered magnetite, Fe 3(1−δ) O 4, of controlled metal-to-oxygen ratio indicates a sudden change in spectra at the Verwey transition for closely stoichiometric samples ( δ = 0), and a gradual alteration in Spectral Changes for highly cation-deficient samples (λ = 0.01) as mentioned in this paper.
13 citations
TL;DR: In this paper, the experimental structure factors S (Q ) have been successfully simulated from model calculations and there is evidence of long-range charge ordering in the molten state and of local order based on the ZnS structure with a coordination number close to 4.
Abstract: Neutron diffraction measurements have been made on molten LiCl, LiBr and LiI. The experimental structure factors S ( Q ) have been successfully simulated from model calculations. There is evidence of long-range charge ordering in the molten state and of local order based on the ZnS structure with a co-ordination number close to 4.
9 citations
TL;DR: In this article, a careful Mossbauer investigation of the influence of Co substitution on the Verwey transition of the spinel system Co x Fe 3−x O 4 with 0 ≤ x ≤ 0.6 has been carried out, showing a decreasing transition temperature with increasing composition x.
6 citations
01 Feb 1989
TL;DR: In this article, a mixed valence serpentine with ideal formula {Fe22+,Fe3+}[Si,Fe3+,]O5(OH)4, has been examined by X-ray and neutron diffraction, thermopiezic analysis, magnetization and susceptibility measurements and Mossbauer spectroscopy.
Abstract: Two samples of cronstedtite, a mixed valence serpentine with ideal formula {Fe22+,Fe3+}[Si,Fe3+]O5(OH)4, have been examined by X-ray and neutron diffraction, thermopiezic analysis, magnetization and susceptibility measurements and Mossbauer spectroscopy. The conductivity is thermally activated, with activation energies of 0.25 eV in the basal plane and 0.37 eV in the perpendicular direction. The shape of paramagnetic Mossbauer spectra above 200 K is influenced by charge fluctuations in octahedral sites and fits of spectra at temperatures up to 410 K with a stochastic relaxation model give an activation energy of 0.19 eV. Static charge ordering sets in progressively below about 100 K. Cronstedtite orders antiferromagnetically below 12 K in a structure with antiferromagnetic octahedral sheets and moments perpendicular to the a-axis. Magnetic and charge-ordered structures are proposed for the ideal composition.
5 citations
TL;DR: In this article, the specific heat and magnetization around the Verwey transition of Fe3-xCo4O4 and Fe3yO4 single crystals were measured and it was shown that the driving forces for the two anomalies are not connected.
Abstract: To test our previous conclusions about the appearance of two separate peaks in the neighbourhood of the Verwey transition in single-crystal magnetite, we have measured the specific heat and magnetization around the Verwey transition of Fe3-x CoxO4, and Fe3-yO4 single crystals. Our present results confirm our previous conclusions: for Fe3-xCo4O4 with 0 ≤ X ≤ 0·04, two transitions appear in the vicinity of the Verwey transition, and, for Fe3-yO4 with y ≥ 0·037 no transition is observed. The driving forces for the two anomalies are not connected.
4 citations
TL;DR: In this paper, the electron-vibrational bands of neutron scattering by mixed valence (MV) molecular crystals containing clusters of transition metal ions in different states of oxidation as the structural units of lattice are considered.
2 citations
01 Jan 1989
TL;DR: In this paper, the electronic structure and properties of high Tc transition metal oxides (La2-xMxCuO4 and YBa2Cu3O7−δ) were discussed.
Abstract: This lecture describes and discusses the electronic structure and properties of the new high Tc transition metal oxides (La2-xMxCuO4 and YBa2Cu3O7−δ) as determined from highly precise all-electron local density calculations. The important contributions of charge transfer excitations make excitons a likely candidate for their high Tc.