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[en] Recent studies demonstrated that there may appear different novel states in correlated systems close to localized-itinerant crossover. Especially favourable conditions for that are met in low-dimensional and in frustrated systems. In this paper I discuss on concrete examples some of such novel states. In particular, for some spinels and triangular systems there appears a 'partial Mott transition', in which first some finite clusters (dimers, trimes, tetramers, heptamers) go over to the itinerant regime, and the real bulk Mott transition occurs only later. Also some other specific possibilities in this crossover regime are shortly discussed, such as spin-Peierls-Peierls transition in TiOCl, spontaneous charge disproportionation in some cases, etc.
[en] The effect of a conductive agent on the structural and electrochemical properties of Li4Ti5O12(LTO) spinel was investigated through neutron diffraction during Li intercalation and electrochemical measurements. The charging process of LTO is known as transformation of the white (Li3)8a[LiTi5]16dO12 into a dark-colored (Li3-X)8a[LiX+Y]16c[LiTi5]16dO12 by incorporating the inserted Li into octahedral 16c sites, and the Li in tetrahedral 8a sites shifted to 16c sites. The occupancy of the tetrahedral 8a site varied with the existence of carbon in the electrode. Without carbon, the lattice parameter and cell volume of LTO decreased more notably than in the carbon-containing LTO electrode during Li insertion process. These phenomena might be attributed that the Li occupancy of the tetrahedral 8a of the LTO electrode without carbon was less than that of the carbon-containing LTO electrode
[en] It is shown that magnetoreflectance of natural light up to +4% exists in magnetostrictive ferrimagnetic spinel CoFe2O4 single crystal; this effect is associated with a change of the fundamental absorption edge, the impurity absorption band, and the phonon spectrum under the action of a magnetic field. The correlation between the field dependences of magnetoreflectance and magnetostriction has been established. The physical mechanisms responsible for the spectral and field peculiarities of magnetoreflection have been explained. It is shown that the magnetorefractive effect in CoFe2O4, which is associated with magnetoelastic properties of the spinel, amounts to +1.5 × 10–3 in magnetic fields exceeding the saturation field. Analysis of magnetooptical and magnetoelastic data has made it possible to estimate deformation potential as Ξu = 20 eV for the valence band of the spinel.
[en] Transition-metal spinels (general formula AB2X4) have been, for many years, the subject of intense experimental and theoretical activity. Structurally, the most interesting feature of these systems is the fact that the B cation occupies the nodes of a pyrochlore lattice, which is known to be geometrically frustrated. Therefore, one can explore how the natural tendency of the transition metals to order in the charge, magnetic and orbital sectors is affected by geometrical frustration. Recently, orbital ordering has become a topical subject in a variety of both non-frustrated systems, such as manganites and other perovskites, and in the spinels. In this paper, I review the recent experimental activity on the subject of orbital ordering in transition-metal spinels and relate this to models of orbital ordering that are being developed by theoreticians
[en] This paper deals with the problem of determining the heat capacity anomaly associated with a first-order transition when using relaxation calorimetry. A method of data recording and analysis is proposed, which is shown to be well suited to investigate such a feature, including its hysteretical character. This technique is applied to spinel vanadates, allowing us to shed light on a recent controversy about the double-transition which takes place in these oxides.
[en] The paper is presenting the first attempt of examination carrying out of high-current pulse beams action effects by low-power electrons on oxide ceramic structures. In capacity of examination object was lithium-titanium ceramics with chemical composition Li0.649Fe1.598Ti0.5Zn0.2Mn0.05104. Irradiation has been carried out in vacuum (P=10-2 Pa) by single pulses of beam of lo power electrons with following parameters: E=15 keV, current density in the pulse 12-25 A/cm2, pulse duration 50 μs, frequency of pulses succession 1 Hz. It is determined, that electron treatment does not provoke noticeable changes on the ceramics phase composition, which preserves its spinel structure
[en] The review briefly outlines the results of authors’ theoretical research into unique hyperkagome atomic and orbital order in geometrically frustrated iridium spinel-like structures. The structural mechanism of formation of the hyper-kagome atomic order is established. It is shown that closed loops of Ir—Ir bonds represent a fundamental feature of the iridium spinel-like structures. Each loop consists of ten atoms, thus being a decagon. We assume that decagons are responsible for abnormal chemical and physical properties of crystals. A prediction is made of structural types of crystals in which the hyper-kagome atomic order can exist.
[en] We performed ultrasound velocity measurements in the geometrically-frustrated ferrite spinel ZnFe_2O_4. In the low-temperature paramagnetic phase, the elastic moduli exhibit softening with a characteristic minimum with decreasing temperature. This elastic anomaly suggests the coupling of lattice to molecular-spin excitations