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

Unveiling the role of inorganic nanoparticles in Earth's biochemical evolution through electron transfer dynamics

This article explores the intricate interplay between inorganic nanoparticles and Earth's biochemical history, with a focus on their electron transfer properties. It reveals how iron oxide and sulfide nanoparticles, as examples of inorganic nanoparticles, exhibit oxidoreductase activity similar to proteins. Termed "life fossil oxidoreductases," these inorganic enzymes influence redox reactions, detoxification processes, and nutrient cycling in early Earth environments. By emphasizing the structural configuration of nanoparticles and their electron conformation, including oxygen defects and metal vacancies, especially electron hopping, the article provides a foundation for understanding inorganic enzyme mechanisms. This approach, rooted in physics, underscores that life's origin and evolution are governed by electron transfer principles within the framework of chemical equilibrium. Today, these nanoparticles serve as vital biocatalysts in natural ecosystems, participating in critical reactions for ecosystem health. The research highlights their enduring impact on Earth's history, shaping ecosystems and interacting with protein metal centers through shared electron transfer dynamics, offering insights into early life processes and adaptations.

Inorganic Fe-O and Fe-S oxidoreductases: paradigms for prebiotic chemistry and the evolution of enzymatic activity in biology

Oxidoreductases play crucial roles in electron transfer during biological redox reactions. These reactions are not exclusive to protein-based biocatalysts; nano-size (<100 nm), fine-grained inorganic colloids, such as iron oxides and sulfides, also participate. These nanocolloids exhibit intrinsic redox activity and possess direct electron transfer capacities comparable to their biological counterparts. The unique metal ion architecture of these nanocolloids, including electron configurations, coordination environment, electron conductivity, and the ability to promote spontaneous electron hopping, contributes to their transfer capabilities. Nano-size inorganic colloids are believed to be among the earliest 'oxidoreductases' to have 'evolved' on early Earth, playing critical roles in biological systems. Representing a distinct type of biocatalysts alongside metalloproteins, these nanoparticles offer an early alternative to protein-based oxidoreductase activity. While the roles of inorganic nano-sized catalysts in current Earth ecosystems are intuitively significant, they remain poorly understood and underestimated. Their contribution to chemical reactions and biogeochemical cycles likely helped shape and maintain the balance of our planet's ecosystems. However, their potential applications in biomedical, agricultural, and environmental protection sectors have not been fully explored or exploited. This review examines the structure, properties, and mechanisms of such catalysts from a material's evolutionary standpoint, aiming to raise awareness of their potential to provide innovative solutions to some of Earth's sustainability challenges.

Orthogonal magnetic structures of Fe4O5: representation analysis and DFT calculations

The magnetic and electronic structures of Fe4O5 have been investigated at ambient and high pressures via a combination of representation analysis, density functional theory (DFT+U) calculations, and Mössbauer spectroscopy. A few spin configurations corresponding to the different irreducible representations have been considered. The total-energy calculations reveal that the magnetic ground state of Fe4O5 corresponds to an orthogonal spin order. Depending on the magnetic propagation vector k, two spin-ordered phases with minimal energy differences are realized. The lowest energy magnetic phase is related to k = (0, 0, 0) and is characterized by ferromagnetic ordering of iron magnetic moments at prismatic sites along the b-axis and antiferromagnetic ordering of iron moments at octahedral sites along the c-axis. For the k = (1/2, 0, 0) phase, the moments in the prisms are antiferromagnetically ordered along the b-axis and the moments in the octahedra are still antiferromagnetically ordered along the c-axis. Under high pressure, Fe4O5 exhibits magnetic transitions with the corresponding electronic transitions of the metal-insulator type. At a critical pressure PC ∼ 60 GPa, the Fe ions at the octahedral sites undergo a high-spin to low-spin state crossover with a decrease in the unit-cell volume of ∼4%, while the Fe ions at the prismatic sites remain in the high-spin state up to 130 GPa. This site-dependent magnetic collapse is experimentally observed in the transformation of Mössbauer spectra measured at room temperature and high pressures.

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.

Magnetism and the Trimeron Bond

A review of progress in understanding the Verwey transition in magnetite (Fe₃O₄) over the past decade is presented. This electronic and structural transition at T _V ≈ 125 K was reported in 1939 and has since been a contentious issue in magnetism. Long range Fe²⁺/Fe³⁺ charge ordering has been confirmed below the transition from crystal structure refinement, and Fe²⁺ orbital ordering and formation of trimerons through weak bonding of Fe²⁺ states to two Fe neighbors has been discovered. This model has accounted for many spectroscopic observations such as the ⁵⁷Fe NMR frequencies. The trimeron lifetime has been measured, and trimeron soft modes have been observed. The origin of the first to second order crossover of Verwey transitions in doped magnetites has been revealed by a nanoparticle study. Electronic and structural fluctuations are found to persist to temperatures far above T _V and local structural distortions track the bulk magnetization, disappearing at the 850 K Curie transition. New binary mixed-valent iron oxides discovered at high pressure are found to have electronic transitions and orbital molecule ground states similar to those of magnetite.

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 E_g = 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.

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 E_g of 0.96 eV and two antiferromagnetic transitions (T_N) 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.

Guest-Controlled Incommensurate Modulation in a Meta-Rigid Metal-Organic Framework Material

Structural transitions of host systems in response to guest binding dominate many chemical processes. We report an unprecedented type of structural flexibility within a meta-rigid material, MFM-520, which exhibits a reversible periodic-to-aperiodic structural transition resulting from a drastic distortion of a [ZnO₄N] node controlled by the specific host-guest interactions. The aperiodic crystal structure of MFM-520 has no three-dimensional (3D) lattice periodicity but shows translational symmetry in higher-dimensional (3 + 2)D space. We have directly visualized the aperiodic state which is induced by incommensurate modulation of the periodic framework of MFM-520·H₂O upon dehydration to give MFM-520. Filling MFM-520 with CO₂ and SO₂ reveals that, while CO₂ has a minimal structural influence, SO₂ can further modulate the structure incommensurately. MFM-520 shows exceptional selectivity for SO₂ under flue-gas desulfurization conditions, and the facile release of captured SO₂ from MFM-520 enabled the conversion to valuable sulfonamide products. MFM-520 can thus be used as a highly efficient capture and delivery system for SO₂.

Stabilized Charge, Spin, and Orbital Ordering by the 6s2 Lone Pair in Bi0.5Pb0.5MnO3

Bi and Pb ions with charge degree of freedom depending on 6s² and 6s⁰ electronic configurations were combined with the Mn ion in a perovskite oxide. Comprehensive theoretical and experimental investigations revealed the Bi³⁺_0.5Pb²⁺_0.5Mn³⁺_0.5Mn⁴⁺_0.5O₃ charge ordered state with CE-type spin and dz² orbital orderings as observed in La_0.5Ca_0.5MnO₃, Nd_0.5Sr_0.5MnO₃, and Bi_0.5Sr_0.5MnO₃. The charge and orbital orderings were preserved above 500 K owing to the stereochemical activity of Bi³⁺ and Pb²⁺ ions which stabilized the structural distortion.

Local Structure and Magnetism of Fe2O3 Maghemite Nanocrystals: The Role of Crystal Dimension

Here we report on the impact of reducing the crystalline size on the structural and magnetic properties of γ-Fe2O3 maghemite nanoparticles. A set of polycrystalline specimens with crystallite size ranging from ~2 to ~50 nm was obtained combining microwave plasma synthesis and commercial samples. Crystallite size was derived by electron microscopy and synchrotron powder diffraction, which was used also to investigate the crystallographic structure. The local atomic structure was inquired combining pair distribution function (PDF) and X-ray absorption spectroscopy (XAS). PDF revealed that reducing the crystal dimension induces the depletion of the amount of Fe tetrahedral sites. XAS confirmed significant bond distance expansion and a loose Fe-Fe connectivity between octahedral and tetrahedral sites. Molecular dynamics revealed important surface effects, whose implementation in PDF reproduces the first shells of experimental curves. The structural disorder affects the magnetic properties more and more with decreasing the nanoparticle size. In particular, the saturation magnetization reduces, revealing a spin canting effect. Moreover, a large effective magnetic anisotropy is measured at low temperature together with an exchange bias effect, a behavior that we related to the existence of a highly disordered glassy magnetic phase.

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 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.

Controlling the thermoelectric power of silicon–germanium alloys in different crystalline phases by applying high pressure

Si–Ge crystals are promising materials for use in various stress-controlled electronic junctions for next-generation nanoelectronic devices.

Design of Heteroanionic MoON Exhibiting a Peierls Metal-Insulator Transition

Using a first-principles approach, we design the heteroanionic oxynitride MoON to exhibit a first-order isosymmetric thermally activated Peierls-type metal-insulator transition (MIT). We identify a ground state insulating phase (α-MoON) with monoclinic Pc symmetry and a metastable high temperature metallic phase (β-MoON) of equivalent symmetry. We find that ordered fac-MoO_{3}N_{3} octahedra with edge and corner connectivity stabilize the twisted Mo-Mo dimers present in the α phase, which activate the MIT through electron localization within the 4d a_{1g} manifold. By analyzing the temperature dependence of the soft zone-boundary instability driving the MIT, we estimate an ordering temperature T_{MIT}∼900  K. Our work shows that electronic transitions can be designed by exploiting multiple anions, and heteroanionic materials could offer new insights into the microscopic electron-lattice interactions governing unresolved transitions in homoanionic oxides.

Single phase charge ordered stoichiometric CaFe3O5 with commensurate and incommensurate trimeron ordering

Mixed-valent transition metal compounds display complex structural, electronic and magnetic properties which can often be exquisitely tuned. Here the charge-ordered state of stoichiometric CaFe₃O₅ is probed using neutron powder diffraction, Monte Carlo simulation and symmetry analysis. Magnetic ordering is dominated by the formation of ferromagnetic Fe³⁺-Fe²⁺-Fe³⁺ trimers which are evident above the magnetic ordering transition. Between T_N= 289 K and 281 K an incommensurate magnetically ordered phase develops due to magnetic frustration, but a spin Jahn-Teller distortion lifts the frustration and enables the magnetic ordering to lock in to a charge-ordered commensurate state at lower temperatures. Stoichiometric CaFe₃O₅ exhibits single phase behaviour throughout and avoids the phase separation into two distinct crystallographic phases with different magnetic structures and Fe valence distributions reported recently, which likely occurs due to partial Fe²⁺ for Ca²⁺ substitution. This underlines the sensitivity of the magnetism and chemistry of these mixed-valent systems to composition.

Complex Cation and Spin Orders in the High-Pressure Ferrite CoFe3O5

A ferrite in the Sr₂Tl₂O₅-type MFe₃O₅ family with M = Co has been synthesized at 12 GPa pressure. Neutron diffraction shows the sample to be Co deficient with composition Co_0.6Fe_3.4O₅. The Co/Fe cation distribution is found to be profoundly different from those of MFe₃O₅ analogs and lies between normal and inverse limits, as Co²⁺ substitutes across trigonal prismatic and one of the two octahedral sites. CoFe₃O₅ shows complex magnetic behavior with weak ferromagnetism below T_C1 ≈ 300 K and a second transition to ferrimagnetic order at T_C2 ≈100 K. Spin scattering of carriers leads a substantial increase in the hopping activation energy below T_C1, and a small negative magnetoresistance is observed at low temperatures.

A new mathematical approach to finding global solutions of the magnetic structure determination problem

Determination of magnetic structure is an important analytical procedure utilized in various fields ranging from fundamental condensed-matter physics and chemistry to advanced manufacturing. It is typically performed using a neutron diffraction technique; however, finding global solutions of the magnetic structure optimization problem represents a significant challenge. Generally, it is not possible to mathematically prove that the obtained magnetic structure is a truly global solution and that no solution exists when no acceptable structure is found. In this study, the global optimization technique called semidefinite relaxation of quadratic optimization, which has attracted much interest in the field of applied mathematics, is proposed to use as a new analytical method for the determination of magnetic structure, followed by the application of polarized neutron diffraction data. This mathematical approach allows avoiding spurious local solutions, decreasing the amount of time required to find a tentative solution and finding multiple solutions when they exist.

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.

The charge order transition of commonly known magnetite has only recently been unraveled. Here, the measurement of the low-temperature high-pressure phase diagram of a related material (Fe₄O₅) elucidates the interplay of average oxidation state and charge-ordering phenomena in the iron oxide family.

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.

Multiferroic binary oxides with the perovskite structure have been very rare. Here, Cong et al. report magnetically-driven ferroelectricity and a large magnetoelectric effect in a binary perovskite compound Mn₂O₃ at low temperatures.

Long range electronic phase separation in CaFe3O5

Incomplete transformations from ferromagnetic to charge ordered states in manganite perovskites lead to phase-separated microstructures showing colossal magnetoresistances. However, it is unclear whether electronic matter can show spontaneous separation into multiple phases distinct from the high temperature state. Here we show that paramagnetic CaFe₃O₅ undergoes separation into two phases with different electronic and spin orders below their joint magnetic transition at 302 K. One phase is charge, orbital and trimeron ordered similar to the ground state of magnetite, Fe₃O₄, while the other has Fe²⁺/Fe³⁺charge averaging. Lattice symmetry is unchanged but differing strains from the electronic orders probably drive the phase separation. Complex low symmetry materials like CaFe₃O₅ where charge can be redistributed between distinct cation sites offer possibilities for the generation and control of electronic phase separated nanostructures.

Electronic phase separation is an important feature of many correlated perovskite compounds but hasn’t been seen in other complex oxides with similar physical behaviour such as magnetite. Hong et al. find phase separation between a magnetite-like charge ordered phase and a charge averaged phase in CaFe₃O₅.

Unusual ground states in [Formula: see text] (R = rare earth; T = Rh, Ir; and X = Si, Ge, Sn): a review

Rare earth compounds of the type [Formula: see text] (R  =  rare earth; T  =  Rh, Ir, and X  =  Si, Ge, Sn) display a variety of phase transitions towards exotic states, including charge density waves (CDW), local moment magnetism, antiferromagnetism in the heavy fermion state, superconductivity and giant positive magnetoresistance. They support strongly correlated electron systems. In particular, R ₅Ir₄ [Formula: see text] (R  =  Dy-Lu) exhibit strong coupling CDWs with high transition temperatures, and superconductivity or magnetic ordering at lower temperatures. [Formula: see text] [Formula: see text] (R  =  Gd-Tm; T  =  Co, Rh, Ir) show multiple magnetic transitions with large magnetoresistance below the magnetic transitions. Finally, the light rare earth series [Formula: see text] [Formula: see text] (R  =  Ce, Pr, Nd; T  =  Rh, Ir) display heavy fermion behaviour (for Ce and Pr) or possess giant positive magnetoresistance (for Nd) at low temperatures. This review provides a comprehensive overview of compounds, crystal structures and phase transitions. This is followed by an in-depth discussion of the mechanisms of the phase transitions and the properties of the ordered states.

Stability of iron-bearing carbonates in the deep Earth's interior

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth's lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth's geotherm at pressures to ∼50 GPa FeCO₃ partially dissociates to form various iron oxides. At higher pressures FeCO₃ forms two new structures-tetrairon(III) orthocarbonate Fe₄³⁺C₃O₁₂, and diiron(II) diiron(III) tetracarbonate Fe₂²⁺Fe₂³⁺C₄O₁₃, both phases containing CO₄ tetrahedra. Fe₄C₄O₁₃ is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth's lower mantle.

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.

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.