Electronic properties of single-crystalline Fe4O5.

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.

Novel Class of Rhenium Borides Based on Hexagonal Boron Networks Interconnected by Short B2 Dumbbells.

Transition metal borides are known due to their attractive mechanical, electronic, refractive, and other properties. A new class of rhenium borides was identified by synchrotron single-crystal X-ray diffraction experiments in laser-heated diamond anvil cells between 26 and 75 GPa. Recoverable to ambient conditions, compounds rhenium triboride (ReB3) and rhenium tetraboride (ReB4) consist of close-packed single layers of rhenium atoms alternating with boron networks built from puckered hexagonal layers, which link short bonded (∼1.7 Å) axially oriented B2 dumbbells. The short and incompressible Re-B and B-B bonds oriented along the hexagonal c-axis contribute to low axial compressibility comparable with the linear compressibility of diamond. Sub-millimeter samples of ReB3 and ReB4 were synthesized in a large-volume press at pressures as low as 33 GPa and used for material characterization. Crystals of both compounds are metallic and hard (Vickers hardness, H V = 34(3) GPa). Geometrical, crystal-chemical, and theoretical analysis considerations suggest that potential ReB x compounds with x > 4 can be based on the same principle of structural organization as in ReB3 and ReB4 and possess similar mechanical and electronic properties.

Verwey-Type Charge Ordering and Site-Selective Mott Transition in Fe4O5 under Pressure.

The metal-insulator transition driven by electronic correlations is one of the most fundamental concepts in condensed matter. In mixed-valence compounds, this transition is often accompanied by charge ordering (CO), resulting in the emergence of complex phases and unusual behaviors. The famous example is the archetypal mixed-valence mineral magnetite, Fe3O4, exhibiting a complex charge-ordering below the Verwey transition, whose nature has been a subject of long-time debates. In our study, using high-resolution X-ray diffraction supplemented by resistance measurements and DFT+DMFT calculations, the electronic, magnetic, and structural properties of recently synthesized mixed-valence Fe4O5 are investigated under pressure to ∼100 GPa. Our calculations, consistent with experiment, reveal that at ambient conditions Fe4O5 is a narrow-gap insulator characterized by the original Verwey-type CO. Under pressure Fe4O5 undergoes a series of electronic and magnetic-state transitions with an unusual compressional behavior above ∼50 GPa. A site-dependent collapse of local magnetic moments is followed by the site-selective insulator-to-metal transition at ∼84 GPa, occurring at the octahedral Fe sites. This phase transition is accompanied by a 2+ to 3+ valence change of the prismatic Fe ions and collapse of CO. We provide a microscopic explanation of the complex charge ordering in Fe4O5 which "unifies" it with the behavior of two archetypal examples of charge- or bond-ordered materials, magnetite and rare-earth nickelates (RNiO3). We find that at low temperatures the Verwey-type CO competes with the "trimeron"/"dimeron" charge ordered states, allowing for pressure/temperature tuning of charge ordering. Summing up the available data, we present the pressure-temperature phase diagram of Fe4O5.

Giant Room-Temperature Power Factor in p-Type Thermoelectric SnSe under High Pressure.

Materials that can efficiently convert heat into electricity are widely utilized in energy conversion technologies. The existing thermoelectrics demonstrate rather limited performance characteristics at room temperature, and hence, alternative materials and approaches are very much in demand. Here, it is experimentally shown that manipulating an applied stress can greatly improve a thermoelectric power factor of layered p-type SnSe single crystals up to ≈180 µW K-2 cm-1 at room temperature. This giant enhancement is explained by a synergetic effect of three factors, such as: band-gap narrowing, Lifshitz transition, and strong sample deformation. Under applied pressure above 1 GPa, the SnSe crystals become more ductile, which can be related to changes in the prevailing chemical bonding type inside the layers, from covalent toward metavalent. Thus, the SnSe single crystals transform into a highly unconventional crystalline state in which their layered crystal stacking is largely preserved, while the layers themselves are strongly deformed. This results in a dramatic narrowing in a band gap, from Eg = 0.83 to 0.50 eV (at ambient conditions). Thus, the work demonstrates a novel strategy of improving the performance parameters of chalcogenide thermoelectrics via tuning their chemical bonding type, stimulating a sample deformation and a band-structure reconstruction.

Structural Diversity of Magnetite and Products of Its Decomposition at Extreme Conditions.

Magnetite, Fe3O4, is the oldest known magnetic mineral and archetypal mixed-valence oxide. Despite its recognized role in deep Earth processes, the behavior of magnetite at extreme high-pressure high-temperature (HPHT) conditions remains insufficiently studied. Here, we report on single-crystal synchrotron X-ray diffraction experiments up to ∼80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe3O4 polymorphs, γ-Fe3O4 with the orthorhombic Yb3S4-type structure and δ-Fe3O4 with the modified Th3P4-type structure. The latter has never been predicted for iron compounds. The decomposition of Fe3O4 at HPHT conditions was found to result in the formation of exotic phases, Fe5O7 and Fe25O32, with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe3+ in the pressure interval between 43 and 51 GPa. Our experiments demonstrate that HPHT conditions promote the formation of ferric-rich Fe-O compounds, thus arguing for the possible involvement of magnetite in the deep oxygen cycle.

Structural Stability and Properties of Marokite-Type γ-Mn3O4.

We synthesized single crystals of marokite (CaMn2O4)-type orthorhombic manganese (II,III) oxide, γ-Mn3O4, in a multianvil apparatus at pressures of 10-24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-Mn3O4 is a semiconductor with an indirect band gap Eg of 0.96 eV and two antiferromagnetic transitions (TN) at ∼200 and ∼55 K. The phase stability of the γ-Mn3O4 crystals was examined in the pressure range of 0-60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-Mn3O4 was determined to be B0 = 235.3(2) GPa with B' = 2.6(6). The γ-Mn3O4 phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO6 polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn3+ ions in the octahedra and the Mn2+ ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.

Synthesis of Ilmenite-type ε-Mn2O3 and Its Properties.

In contrast to the corundum-type A2X3 structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX3 ilmenite-type structure comprises two different octahedrally coordinated positions that are usually filled with differently charged ions (e.g., in Fe2+Ti4+O3 ilmenite). Here, we report a synthesis of the first binary ilmenite-type compound fabricated from a simple transition-metal oxide (Mn2O3) at high-pressure high-temperature (HP-HT) conditions. We experimentally established that, at normal conditions, the ilmenite-type Mn2+Mn4+O3 (ε-Mn2O3) is an n-type semiconductor with an indirect narrow band gap of Eg = 0.55 eV. Comparative investigations of the electronic properties of ε-Mn2O3 and previously discovered quadruple perovskite ζ-Mn2O3 phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in ε-Mn2O3 below 210 K. The synthesis of ε-Mn2O3 indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A2O3, and potentially various mixed-valence polymorphs of these oxides, for example, with ilmenite-type, LiNbO3-type, perovskite-type, and other structures, could be stabilized at HP-HT conditions.

Author Correction: Discovery of Fe7O9: a new iron oxide with a complex monoclinic structure.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

A Room-Temperature Verwey-type Transition in Iron Oxide, Fe5 O6.

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 Fe5 O6 stoichiometry. Near 275 K, Fe5 O6 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 Fe5 O6 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.

Pressure tuning of charge ordering in iron oxide.

A Verwey-type charge-ordering transition in magnetite at 120 K leads to the formation of linear units of three iron ions with one shared electron, called trimerons. The recently-discovered iron pentoxide (Fe4O5) comprising mixed-valent iron cations at octahedral chains, demonstrates another unusual charge-ordering transition at 150 K involving competing formation of iron trimerons and dimerons. Here, we experimentally show that applied pressure can tune the charge-ordering pattern in Fe4O5 and strongly affect the ordering temperature. We report two charge-ordered phases, the first of which may comprise both dimeron and trimeron units, whereas, the second exhibits an overall dimerization involving both the octahedral and trigonal-prismatic chains of iron in the crystal structure. We link the dramatic change in the charge-ordering pattern in the second phase to redistribution of electrons between the octahedral and prismatic iron chains, and propose that the average oxidation state of the iron cations can pre-determine a charge-ordering pattern.

Spin-induced multiferroicity in the binary perovskite manganite Mn2O3.

The ABO3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn2O3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn2O3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties.

Structural and Magnetic Transitions in CaCo3V4O12 Perovskite at Extreme Conditions.

We investigated the structural, vibrational, magnetic, and electronic properties of the recently synthesized CaCo3V4O12 double perovskite with the high-spin (HS) Co2+ ions in a square-planar oxygen coordination at extreme conditions of high pressures and low temperatures. The single-crystal X-ray diffraction and Raman spectroscopy studies up to 60 GPa showed a conservation of its cubic crystal structure but indicated a crossover near 30 GPa. Above 30 GPa, we observed both an abnormally high "compressibility" of the Co-O bonds in the square-planar oxygen coordination and a huge anisotropic displacement of HS-Co2+ ions in the direction perpendicular to the oxygen planes. Although this effect is reminiscent of a continuous HS → LS transformation of the Co2+ ions, it did not result in the anticipated shrinkage of the cell volume because of a certain "stiffing" of the bonds of the Ca and V cations. We verified that the oxidation states of all the cations did not change across this crossover, and hence, no charge-transfer effects were involved. Consequently, we proposed that CaCo3V4O12 could undergo a phase transition at which the large HS-Co2+ ions were pushed out of the oxygen planes because of lattice compression. The antiferromagnetic transition in CaCo3V4O12 at 100 K was investigated by neutron powder diffraction at ambient pressure. We established that the magnetic moments of the Co2+ ions were aligned along one of the cubic axes, and the magnetic structure had a 2-fold periodicity along this axis, compared to the crystallographic one.

Tuning the electronic and vibrational properties of Sn2P2Se6 and Pb2P2S6 crystals and their metallization under high pressure.

External stimuli enabling either a continuous tuning or an abrupt switching of the basic properties of materials that are utilized in various industrial appliances could significantly extend their range of use. The key characteristics of semiconductors are basically linked to their electronic and optical properties. In this study, we experimentally demonstrated that two kindred wide-band-gap semiconductors, ferroelectric Sn2P2Se6 and paraelectric Pb2P2S6, which are commonly used in optical technologies, have remarkably different and unusual responses in their electronic band structures to applied moderate pressures. The electrical resistance of Sn2P2Se6 smoothly decreased with pressure by about eight orders of magnitude to 10 GPa, thereby suggesting a progressive shrinkage in its band gap; whereas, the resistance of Pb2P2S6 was only insignificantly lowered with pressure to 20 GPa. By means of Raman spectroscopy, we observed several distinct crossovers in the compression behaviour of both crystals and attributed them to phase transitions. These Raman studies provided evidence for the metallization of Sn2P2Se6 at 29 GPa and Pb2P2S6 at 49 GPa. We inferred that, namely, the metal cations in these crystals control the pressure responses of their band structures and proposed that the other M2P2X6 compounds, those already known and those not yet reported (e.g., with M = Cu, In, Fe, Co, Mn, Cr, Ca, Sr, and Mg), could also exhibit the diverse and non-trivial pressure responses of their electronic band structures. Thus, our study has revealed the significant potential for the stress-related technologies of this poorly-studied class of materials, thereby stimulating both the synthesis and investigation of new members.

Dramatic Changes in Thermoelectric Power of Germanium under Pressure: Printing n-p Junctions by Applied Stress.

Controlled tuning the electrical, optical, magnetic, mechanical and other characteristics of the leading semiconducting materials is one of the primary technological challenges. Here, we demonstrate that the electronic transport properties of conventional single-crystalline wafers of germanium may be dramatically tuned by application of moderate pressures. We investigated the thermoelectric power (Seebeck coefficient) of p- and n-type germanium under high pressure to 20 GPa. We established that an applied pressure of several GPa drastically shifts the electrical conduction to p-type. The p-type conduction is conserved across the semiconductor-metal phase transition at near 10 GPa. Upon pressure releasing, germanium transformed to a metastable st12 phase (Ge-III) with n-type semiconducting conductivity. We proposed that the unusual electronic properties of germanium in the original cubic-diamond-structured phase could result from a splitting of the "heavy" and "light" holes bands, and a related charge transfer between them. We suggested new innovative applications of germanium, e.g., in technologies of printing of n-p and n-p-n junctions by applied stress. Thus, our work has uncovered a new face of germanium as a 'smart' material.

Discovery of Fe7O9: a new iron oxide with a complex monoclinic structure.

Iron oxides are fundamentally important compounds for basic and applied sciences as well as in numerous industrial applications. In this work we report the synthesis and investigation of a new binary iron oxide with the hitherto unknown stoichiometry of Fe7O9. This new oxide was synthesized at high-pressure high-temperature (HP-HT) conditions, and its black single crystals were successfully recovered at ambient conditions. By means of single crystal X-ray diffraction we determined that Fe7O9 adopts a monoclinic C2/m lattice with the most distorted crystal structure among the binary iron oxides known to date. The synthesis of Fe7O9 opens a new portal to exotic iron-rich (M,Fe)7O9 oxides with unusual stoichiometry and distorted crystal structures. Moreover, the crystal structure and phase relations of such new iron oxide groups may provide new insight into the cycling of volatiles in the Earth's interior.

Charge-ordering transition in iron oxide Fe4O5 involving competing dimer and trimer formation.

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 hard oxide semiconductor with a direct and narrow bandgap and switchable p-n electrical conduction.

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.

Crystal structure and thermal expansion of Mn(1-x)Fe(x)Ge.

A series of temperature-dependent single-crystal and powder diffraction experiments has been carried out using synchrotron radiation in order to characterize the monogermanides of Mn, Fe and their solid solutions. The MnGe single crystal is found to be enantiopure and we report the absolute structure determination. The thermal expansion, parametrized with the Debye model, is discussed from the temperature-dependent powder diffraction measurements for Mn(1-x)Fe(x)Ge (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.75, 0.8, 0.9). Whereas the unit-cell dimension and the Debye temperature follow a linear trend as a function of composition, the thermal expansion coefficient deviates from linear dependence with increasing Mn content. No structural phase transformations have been observed for any composition in the temperature range 80-500 K for both single-crystal and powder diffraction, indicating that the phase transition previously observed with neutron powder diffraction most probably has a magnetic origin.

New antiferromagnetic perovskite CaCo3V4O12 prepared at high-pressure and high-temperature conditions.

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.

Anomalous compression and new high-pressure phases of vanadium sesquioxide, V2O3.

We report results of a powder x-ray diffraction (XRD) study of vanadium sesquioxide, V2O3, under pressurization in a neon pressure-transmitting medium up to 57 GPa. We have established a bulk modulus value for corundum-type V2O3 of B0 = 150 GPa at B' = 4. This bulk modulus value is the lowest among those known for the corundum-type-structured oxides, e.g. Al2O3, α-Fe2O3, Cr2O3, Ti2O3, and α-Ga2O3. We have proposed that this might be related to the difference in the electronic band structures: at room temperature V2O3 is metallic, but the above corundum-structured sesquioxides are semiconducting or insulating. Around ∼21-27 and ∼50 GPa we registered changes in the XRD patterns that might be addressed to phase transitions. These transitions were sluggish upon room-temperature compression, and hence we additionally facilitated them by the laser heating of one sample. We have refined the XRD patterns of only the first high-pressure phase in an orthorhombic lattice of a Rh2O3(II)-type. Our findings significantly extend the knowledge of the P-T phase diagram of V2O3 and advance the understanding of its properties. We speculate that the elastic properties of V2O3 can be closely linked to its electronic band structure and, consequently, we propose that slightly doped V2O3 (e.g. with Cr) could be a potential candidate for systems in which the bulk modulus value may be remarkably switched by moderate pressure or temperature.