We synthesized single and polycrystals of iron oxide with an unconventional Fe4O5 stoichiometry under high-pressure high-temperature (HP-HT) conditions. The crystals of Fe4O5 had a CaFe3O5-type structure composed of linear chains of iron with octahedral and trigonal-prismatic oxygen coordinations. We investigated the electronic properties of this mixed-valence oxide using several experimental techniques, including measurements of electrical resistivity, the Hall effect, magnetoresistance, and thermoelectric power (Seebeck coefficient), X-ray absorption near edge spectroscopy (XANES), reflectance and absorption spectroscopy, and single-crystal X-ray diffraction. Under ambient conditions, the single crystals of Fe4O5 demonstrated a semimetal electrical conductivity with nearly equal partial contributions of electrons and holes (σn ≈ σp), in line with the nominal average oxidation state of iron as Fe2.5+. This finding suggests that both the octahedral and trigonal-prismatic iron cations contribute to the electrical conductivity of Fe4O5via an Fe2+/Fe3+ polaron hopping mechanism. A moderate deterioration of crystal quality shifted the dominant electrical conductivity to n-type and considerably worsened the conductivity. Thus, alike magnetite, Fe4O5 with equal numbers of Fe2+ and Fe3+ ions can serve as a prospective model for other mixed-valence transition-metal oxides. In particular, it could help in the understanding of the electronic properties of other recently discovered mixed-valence iron oxides with unconventional stoichiometries, many of which are not recoverable to ambient conditions; it can also help in designing novel more complex mixed-valence iron oxides.

Ein gemischtvalentes Eisenoxid mit ungewöhnlicher Fe5O6-Stöchiometrie wurde synthetisiert, wie S. V. Ovsyannikov et al. in ihrer Zuschrift auf S. 5681 beschreiben. Nahe 275 K durchläuft das Material einen Ladungsordnungsübergang des Verwey-Typs, der einhergeht mit einer Dimerisierung in den Eisenketten und der Bildung neuer Fe-Fe-Bindungen. Dieses einzigartige Merkmal zeigt, dass Fe5O6 vielversprechend für die Nutzung in innovativen Anwendungen ist.

Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next-generation nanoelectronic devices. To date, vanadium dioxide (VO2) is the only known simple transition-metal oxide that demonstrates a near-room-temperature metal-insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed-valent iron oxide with an unconventional Fe5O6 stoichiometry. Near 275 K, Fe5O6 undergoes a Verwey-type charge-ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe-Fe chemical bonds. This unique feature highlights Fe5O6 as a promising candidate for the use in innovative applications. We established that the minimal Fe-Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge-ordering phenomena in transition-metal oxides in general.

In this work, we systematically investigated the electrical resistivity, Hall, and magnetoresistance effects of Al-doped Mg2Si thermoelectrics, irradiated with a fluence of fast neutrons and consequently isochronally annealed at moderate high temperatures up to 500 degrees C. We found that the fast neutron bombardment itself only slightly modified the electronic properties. Meanwhile, a series of the postirradiation high-temperature anneals affected the properties dramatically. Thus, after annealing at temperatures of 275-325 degrees C, Mg2Si:Al showed pronounced jumps in both the electrical resistivity and the Hall constant values by several orders of magnitude. This unexpected electronic transition corresponded to a significant variation in the carrier concentration. We proposed that this unusual electronic transition may be related to temperature-assisted chemical bonding of the radiation defects that can involve free charge carriers in the sample, thereby tuning dramatically its transport properties. We proposed a simple model for Mg2Si:Al thermoelectrics, which links the chemical bonding of the interstitial Mg ions with the electronic properties of this material. This finding suggests a new avenue to tuning of electronic properties and unconventional electronic transitions in materials with a high concentration of defects.

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.

We experimentally investigated the electronic transport properties of four iron pnictide crystals, namely, EuFe2As2, SrFe2As2, and CaFe2As2 parent compounds, and superconducting CaFe1.94Co0.06As2 at ambient and high pressures up to 20 GPa. At ambient pressure we examined the electrical resistivity, Hall and magnetoresistance effects of the samples in a temperature range from 1.5 to 380 K in high magnetic fields up to 13.6 T. In this work we carried out the first simultaneous investigations of the in-plane and out-of-plane Hall coefficients, and found new peculiarities of the low-temperature magnetic and structural transitions that occur in these materials. In addition, the Hall coefficient data suggested that the parent compounds are semimetals with a multi-band conductivity that includes hole-type and electron-type bands. We measured the pressure dependence of the thermoelectric power (the Seebeck effect) of these samples up to 20 GPa, i.e. across the known phase transition from the tetragonal to the collapsed tetragonal lattice. The high-pressure behavior of the thermopower of EuFe2As2 and CaFe2As2 showing the p-n sign inversions was consistent with the semimetal model described above. By means of thermopower, we found in single-crystalline CaFe2As2 direct evidence of the band structure crossover related to the formation of As–As bonds along the c-axis on the tetragonal → collapsed tetragonal phase transition near 2 GPa. We showed that this feature is distinctly observable only in high-quality samples, and already for re-pressurization cycles this crossover was strongly smeared because of the moderate deterioration of the sample. We also demonstrated by means of thermopower that the band structure crossover that should accompany the tetragonal → collapsed tetragonal phase transition in EuFe2As2 near 8 GPa is hardly visible even in high-quality single crystals. This behavior may be related to a gradual valence change of the Eu ions under pressure that leads to an injection of free electrons and the steady shift of the conduction to n-type.

Perovskite-type Mn2O3 described by S. V. Ovsyannikov and co-workers on page 8185, is a unique, hard semiconductor having a narrow and direct bandgap of 0.45 eV and switchable p-n electrical conduction. Pv-Mn2O3, comprising nontoxic and inexpensive elements, is much stronger than traditional semiconductors, and promises applications in the semiconductor industry, e.g., in IR detectors and other devices.

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.

We report results of investigations of electronic transport properties and lattice dynamics of Al-doped magnesium silicide (Mg2Si) thermoelectrics at ambient and high pressures to and beyond 15 GPa. High-quality samples of Mg2Si doped with 1 at. % of Al were prepared by spark plasma sintering technique. The samples were extensively examined at ambient pressure conditions by X-ray diffraction studies, Raman spectroscopy, electrical resistivity, magnetoresistance, Hall effect, thermoelectric power (Seebeck effect), and thermal conductivity. A Kondo-like feature in the electrical resistivity curves at low temperatures indicates a possible magnetism in the samples. The absolute values of the thermopower and electrical resistivity, and Raman spectra intensity of Mg2Si:Al dramatically diminished upon room-temperature compression. The calculated thermoelectric power factor of Mg2Si:Al raised with pressure to 2–3 GPa peaking in the maximum the values as high as about 8 × 10−3 W/(K2m) and then gradually decreased with further compression. Raman spectroscopy studies indicated the crossovers near ∼5–7 and ∼11–12 GPa that are likely related to phase transitions. The data gathered suggest that Mg2Si:Al is metallized under moderate pressures between ∼5 and 12 GPa.

Resistivity ρ(T), Hall coefficient RH(T), superconducting transition temperature Tc and slopes of the upper critical field dHc2/dT were studied in (Ba1-xKx)Fe2As2 (x = 0.218, 0.356, 0.531) single crystals irradiated with fast neutrons. It is found that dTc/dρSC-the rate of decreasing Tc as a function of the ρSC (ρSC is the resistivity at T = Tc)-linearly increases with concentration x. Slow changes in the Hall coefficient RH, as well as the quadratic electronic contribution to the resistivity, show that there are no substantial changes in the topology of the Fermi surface caused by irradiation. The slopes of the upper critical field dHc2/dT in ab and c directions as a function of ρSC determined by Hall measurements show a reasonable agreement with a model that suggests constancy of the band parameters.

Silicon is a key technological material, and its controlled doping is one of the simple and effective ways which are applied for creation of new advanced materials with tunable optoelectronic properties. Boron was known to be a dopant that can dramatically change the properties of silicon. However, a limited solubility of boron atoms in silicon matrix strongly restricted creation of bulk diamond-type structured Si-B alloys with the high boron content exceeding 0.5-1 at. 96. In this work we show that bulk Si1-xBx alloys with a rather high boron content (e.g., 2.4 at. 96) may be fabricated by alloying of boron and silicon at high temperatures above the melting point of silicon and high pressure. We extensively investigated the electronic transport and optical properties of these alloys using several techniques, including electrical resistivity, Hall effect, magnetoresistance, Raman, IR and optical spectroscopy, and X-ray diffraction. We found that Si1-xBx solid solutions are metals that possess very unusual optical properties, e.g., they demonstrate the antiresonant Raman spectra and the loss of the reflectivity in the near-IR range. Our work indicates new perspectives in creation and applications of Si1-xBx solid solutions with the diamond-type structure.

A new perovskite, CaCo(2+)3V(4+)4O12, has been synthesized at high-pressure and high-temperature (HP-HT) conditions. The properties of this perovskite were examined by a range of techniques. CaCo3V4O12 was found to adopt a double-perovskite cubic lattice [a = 7.3428(6) Å] with Im3 symmetry. We have established that this new perovskite is stable at ambient conditions, and its oxidation and/or decomposition at ambient pressure begins above 500 °C. It undergoes an abrupt antiferromagnetic transition around 98 K. Electrical resistivity data suggest semimetallic conductivity in the temperature range of 1.6-370 K. We have established that the Co(2+) ions in CaCo3V4O12 are in the high-spin state with a sizable orbital moment, even though their square-planar oxygen coordination could be more suitable for the low-spin state, which is prone to Jahn-Teller distortion. Electrical resistivity curves also exhibit a distinct steplike feature around 100 K. CaCo3V4O12 is a first example of perovskite in which the sites A' are fully occupied by Co(2+) ions, and hence its synthesis opens the door to a new class of double perovskites, ACo3B4O12, that may be derived by chemical substitution of the A sublattice by lanthanides, sodium, strontium, and bismuth and by other elements and/or of the B sublattice by some other transition metals.

Raman and IR spectroscopy and thermal conductivity were employed to investigate the effects of simultaneous vacuum annealing on defect transformation processes in diamonds irradiated by fast neutrons and nanocrystalline diamond films (DF) implanted with O or P ions. A correlation between the thermal conductivity at room temperature and the absorption coefficient near 13 μm was found in diamonds irradiated by neutron fluence 2⋅1019 cm–2. It was found that two-phonon absorption in diamond IR spectra was sensitive to destruction of long-range ordering in the diamond crystal lattice. An absorption band with a shape characteristic of localized centers in the band gap (activation energy ~0.25 eV) was identifi ed in IR spectra of DF irradiated by neutrons and annealed at temperatures <900°C. It was shown that the optical and transport properties of the studied radiation-damaged DF were determined by multi-vacancy complexes and inclusions of a non-diamond phase.

AbstractThe new title perovskite is synthesized from mixtures of Ca(VO3)2, Co(VO3)2, and Co (14—18 GPa, 900—1300 °C, 0.5—8 h).

The behavior of electrical resistivity rho(T), temperature of superconducting transition Tc, and upper critical field Hc2(T) of polycrystalline YNi2B2C after irradiation with thermal neutron and subsequent high-temperature isochronous annealings in the temperature range Tann = 100 - 1000C has been studied.

Recently, a golden colored, dense polymorph of titanium sesquioxide, Ti2O3 with a Th2S3-type structure, has been synthesized at high-pressure high-temperature conditions. In this paper, we present results of investigations of structural, optical, and electronic transport properties of this unusual golden polymorph of Ti2O3 under high pressure. Several experimental techniques were used, including x-ray diffraction studies using synchrotron radiation, Raman spectroscopy, electrical resistivity, and thermoelectric power. The structural studies showed that the Th2S3-type lattice is conserved under pressure, while it is subjected to an isostructural phase transition with a similar to 0.7% volume drop at 38.5 GPa. We speculated that this transition could be driven by the s -> d electron transfer in the Ti atoms. For the Th2S3-type Ti2O3, we have established a bulk modulus value, B-0 = 258.3 GPa at B-0(1) = 4.1. A full profile analysis of the diffraction patterns allowed us to discover anomalies in the compression behavior of the Th2S3-type structure. The bond valence sums method suggested that at ambient conditions the Ti cations have predominantly Ti3+ oxidation state, but applied pressure stimulates a partial charge disproportionation between the Ti-1 and Ti-2 sites achieving the maximal effect-reduction of the Ti-1 cations to similar to Ti2.5+ and oxidation of the Ti-2 ones to similar to Ti3.5+ near 14 GPa. Pressure evolution of Raman spectra across the above crossovers showed distinct changes corroborating the above findings. The high-pressure electronic transport studies confirmed that the Th2S3-type Ti2O3 remains semiconducting up to 21 GPa at ambient and low temperatures down to 4.2 K. These studies found additional features, e. g., in the activation energy curve near 7 GPa, that is accompanied by inversion of the dominant conductivity type from electron to hole. The intriguing high-pressure behavior of Ti2O3 with the Th2S3-type structure can contribute to better understanding of high-pressure properties of transition-metal sesquioxides.