Abstract: The Verwey transition in magnetite is rationalized in terms of an electronic model involving correlated electron occupancy of nearest-neighbor octahedral site pairs. This formalism can be restated in terms of order-disorder theory. The site pair representation of the lattice is analyzed in terms of possible occupation states that represent respectively the ground state of the system as a trapped electron, a first excited state in which an electron can resonate between the two constituents of a site pair, and a second excited state involving two electrons on neighboring sites. The free energy of this representative assembly is then minimized to obtain the equilibrium configuration. The Verwey transition is driven by the Coulomb repulsive interaction between electrons on neighboring sites. In certain limiting cases one obtains results equivalent to the phenomenological Strassler-Kittel model, which had been successfully used on an empirical basis to analyze both the first- and second-order Verwey transitions that have been experimentally encountered with increasing departures from the ideal 4/3 ratio of oxygen/iron in magnetite.

Abstract: Liquid-glass transition in binary charged hard-spheres is studied within the mode-coupling theory in terms of Debye-Waller and Lamb-Mosbauer factors. It is found that the addition of a Coulomb tail to the hardcore interaction allows for glassification at much lower density than in a pure hardsphere system. The wave-vector dependence of Lamb-Mosbauer factors is qualitatively similar to the neutral-hard-spheres system while the Debye Waller factors show a different behaviour due to charge ordering effects.

Abstract: The normal reflectivity of Ti 4 O 7 ( T = 290 K ) is measured in the range of 0.1 to 3 eV. The reflectivity shows a minimum which corresponds to the collective resonance of all Ti 3+ valences. The optical absorption as calculated from the diffuse reflectivity of Ti 4 O 7 is investigated in the spectral range of 0.5 to 2.5 eV and at temperatures between 10 and 300 K. A broad asymmetrical absorption band with a maximum at about 0.7 eV and an absorption sideband at about 1.5 eV is observed. The overall line shape is explained by the polaronic absorption mechanism. From room temperature down to the metal-insulator transition at T > m ≈ 150 K the total polaronic absorption cross section remains constant. It drops down in the intermediate phase (125–150 K) and then increases with decreasing temperature below the Verwey transition at T v ≈ 125 K. The increasing absorption cross section below T v as well as the shift of the peak position to lower energy is in accordance with the gap opening at ≈ 0.2 eV previously reported (Kaplan et al. [1]) and is explained by the separation of single polaronic from bipolaronic energy levels. The magnetic susceptibility of Ti 4 O 7 is discussed with respect to an exchange pairing mechanism above and below T m . It is suggested that a melting of the bipolaronic state during the metal-insulator transition results in a fluctuating valence bond system above T m .

Abstract: We discuss our thorough optical investigations on the 3d transition metal compounds Fe 3 O 4 and Ti 4 O 7 within the bipolaron model of Chakraverty. Of particular interest are the sharp lines overlapping the phonon modes. These are ascribed to the fingerprints of a charge ordering phase. Futhermore, we present the phonon spectrum of rutile, in order to review the vibrational states of magnetite and above all of Ti 4 O 7 .

Abstract: A theory is presented for the charge ordering observed in many inorganic compounds; equations have been derived for the Vervey temperatures of binary and ternary compounds with the CdI2 structure. The temperature dependence of the electronic-order parameter has been related to the composition, the temperature, and the deformation parameter. The theoretical calculations agree qualitatively with experiment for TiSe2, Ti1−cVcSe2, Ti1−cZrcSe2.

Abstract: Molecular dynamics calculations have been used to study structural relaxation and dynamical correlations near the glass transition in the system [Ca(NO3)2]0.4[KNO3]0.6. As in a typical molten salt, the overall structure is determined by charge ordering. However, the radial distribution function for Ca2+ ions is unusual in that even at high temperatures it shows a split first peak due to specific spatial correlations of the cations with the nitrate anions. Structural relaxation that accompanies cooling of the system has been characterized with the aid of the van Hove real‐space correlation functions Gs (r,t) for the constituent atoms (Ca, K, N, O). The relaxation of the incoherent structure factor Fs (k,t), with a wave vector k near the peak of the static structure factor, has been investigated as a function of temperature. The results clearly reveal both the α and β relaxation processes; the former can be well represented by a master curve with a stretched exponential shape. An analysis of the susceptibil...

Abstract: Effects of the hydrostatic pressure as well as the rare earth substitution are examined on the two-step Verwey transition of the stoichiometric YFe 2 O 4 , by the electric and the magnetic measurements. The upper and the lower transition temperatures decrease with the compression by 2.6 and 10.5 K/kb, respectively. Substitution of Y by Lu decreases whereas that by Dy increases the transition temperature. At room temperature in the high conductivity phase, lattice constants in the hexagonal frame, a and c , are measured for substituted crystals and for the stoichiometric YFe 2 O 4 under pressure. It is shown that the upper and the lower transition temperatures are independent from c but decrease 10 and 42 K, respectively, by 10 -3 compression of the lattice along the a axis. The character of the transition is discussed briefly.

Abstract: The ordering of the ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions on the octahedral sites of magnetite (${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$) at temperatures below the Verwey metal-insulator transition has been studied by quantitative high-energy transmission electron diffraction. We find that there are ten independent charge-ordering models (including the Verwey model) for the low-temperature structure that satisfy the Anderson condition if the symmetry is Cc (monoclinic). Dynamical electron diffraction patterns are simulated and compared with experiment for these charge-ordering models, using atomic coordinates obtained from neutron diffraction work. We find that one of these ten charge-ordering models agrees best with experiment and that the electrons in this model form a characteristic wave. Our calculations of electron correlation energy show that this model has the second lowest energy, while the Verwey model has the lowest. This indicates the importance of electron-phonon interactions in stabilizing the structure.

Abstract: A low temperature structural phase transition in La2-xBaxCuO4 was recently discovered, and has been shown to severely depress the superconducting transition temperature. It is suggested that this structural transition is associated with a charge density wave in the CuO2 planes which was predicted to play a major role in high-Tc oxide superconductivity.