Phase transitions that occur in materials, driven, for instance, by changes in temperature or pressure, can dramatically change the materials' properties. Discovering new types of transitions and understanding their mechanisms is important not only from a fundamental perspective, but also for practical applications. Here we investigate a recently discovered Fe4O5 that adopts an orthorhombic CaFe3O5-type crystal structure that features linear chains of Fe ions. On cooling below ∼150 K, Fe4O5 undergoes an unusual charge-ordering transition that involves competing dimeric and trimeric ordering within the chains of Fe ions. This transition is concurrent with a significant increase in electrical resistivity. Magnetic-susceptibility measurements and neutron diffraction establish the formation of a collinear antiferromagnetic order above room temperature and a spin canting at 85 K that gives rise to spontaneous magnetization. We discuss possible mechanisms of this transition and compare it with the trimeronic charge ordering observed in magnetite below the Verwey transition temperature.
A slight minimum in the zero-field resistivity of a single crystalline La0.8Sr0.2MnO3 and shallow one for ceramic La0.8Sr0.2MnO3 samples were observed at T∼4 K and at T∼25–30 K, respectively. The minimum for the ceramic shifts towards lower temperatures, flattens with increasing magnetic fields (H), and vanishes at some critical H. The above effects are accompanied by an appreciable negative magnetoresistance (MR). On the other hand, the minimum for the single-crystalline sample is almost field independent and the MR in the relevant temperature range is very small. Two different mechanisms were found to account for the results observed in the single crystal and the polycrystalline samples: (i) mesoscopic corrections to the bulk resistivity that include Coulomb interaction and weak localization and (ii) intergranular tunneling. The resistivity of the large-grain ceramic sample comprises both types of behavior. The minimum of the bulk contribution becomes clearly seen under H, which suppresses the ceramic-type minimum.
An oxide semiconductor (perovskite-type Mn2 O3 ) is reported which has a narrow and direct bandgap of 0.45 eV and a high Vickers hardness of 15 GPa. All the known materials with similar electronic band structures (e.g., InSb, PbTe, PbSe, PbS, and InAs) play crucial roles in the semiconductor industry. The perovskite-type Mn2 O3 described is much stronger than the above semiconductors and may find useful applications in different semiconductor devices, e.g., in IR detectors.
An orthorhombic polymorph of titanium oxide (Ti(2)O(3)) has been synthesized at high pressure-high temperature (HP-HT) conditions. It has been refined in the Pnma space group and the Th(2)S(3) structural type with the unit cell parameters as follows: a = 7.8248(6) Å, b = 2.8507(4) Å, c = 8.0967(3) Å, V = 180.61(1) Å(3) and Z = 4. The samples of Pnma-Ti(2)O(3) were of a golden colour, in contrast to the conventional black corundum-structured Ti(2)O(3). The structural stability of this polymorph has been examined by simultaneous Raman and x-ray diffraction studies under high pressure over 70 GPa and high temperature over 2200 K. No phase transformations or chemical reactions have been established. The electrical resistivity of Th(2)S(3)-structured Ti(2)O(3) samples showed a semiconducting behaviour and, at ambient conditions, was equal to 0.20-0.46 Ω cm. Conventional near-infrared absorption spectroscopy established the absence of energy gaps above 0.25 eV.
The effect of radiation-induced disordering in a nuclear reactor (fast neutrons fluence Φ = 5 · 10 19 cm 2 , Tirr = 340 K) on resistivity ρ, superconducting transition temperature TC and upper critical field HC 2 of polycrystalline MgCNi3 samples was investigated. It was found that TC decreases under irradiation from 6.5 to 2.9 K and completely recovers after annealing at 600 • C. Temperature dependences ρ(T ) are characteristic of compounds with strong electron-phonon interaction. The dHC 2 /dT behaviour testifies to a considerable decrease in density of electronic state at Fermi level N (EF ) in the course of disordering.Radiation-induced disordering caused by irradiation with high-energy particles is a unique method of investigating the properties of superconducting and normal states of ordered crystals [1,2]. Even in broad-band metals, such as intermetallic compounds with A15 structure, long-range ordering loss leads to considerable rearrangement of the electronic spectrum, resulting in disappearance of individual features of the electronic structure. Disordering causes decrease in densities at Fermi level N (E F ) and respective noticeable drop of T C in compounds with high initial N (E F ) (Nb 3 Sn or V 3 Si), and considerable (from 1.5 to 7 K) rise of T C in compounds with low N (E F ) and T C due to growth of N (E F ) (Mo 3 Si and Mo 3 Ge) [3,4,5]. In type HTSC compounds, disordering leads to more significant changes in properties: fast and complete T C degradation is accompanied with N (E F ) decrease and metal-insulator transition [2]. Thus investigation of response of a system to radiation-induced disordering serves as a kind of a test to reveal the characteristic features of its electron states. It was shown in recent papers [6,7] that T C drop from 38 to 5 K observed at MgB 2 under radiation-induced disordering is connected mainly with considerable drop of N (E F ), similar to Nb 3 Sn or V 3 Si compounds. In our investigation, we concentrated on the effect of disordering on the properties of superconducting compound MgCNi 3 (T C ∼ 8 K) with perovskite cubic structure of type SrTiO 3 , unconventional for intermetallides [8]. Our interest in this system was explained by the fact that its ground state is close to ferromagnetic due to the presence of a narrow peak in N (E) located 45 meV below the Fermi level [9]. This allowed us to regard it as a candidate for an unconventional (possibly triplet) superconductivity, similar to Sr 2 RuO 4 compound. It is known that in Sr 2 RuO 4 , as distinct from conventional superconducting compounds (intermetallides), T C undergoes anomalously strong suppression even under a slight disorder [10]. In MgCNi 3 , maximum T C is achieved at excess of carbon content only (nominal composition MgC 1.5 Ni 3 ), even though, according to neutron diffraction study, the actual composition is closer to Mg 0.96 CNi 3 , and excess carbon occupies the region between sample grain boundaries [11].In the sample preparation, fine powders Mg, C and Ni with purity better than 99.5% were used...
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