Structural Transition at 360 K in the CaFe5O7 Ferrite: Toward a New Charge Ordering Distribution

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.

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.

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.

Facile synthesis of CaFe2O4 for visible light driven treatment of polluting palm oil mill effluent: Photokinetic and scavenging study

In this paper, a facile synthesis method for CaFe₂O₄ is introduced that produces a catalyst capable of significant photocatalytic degradation of POME under visible light irradiation. The co-precipitation method was used to produce two catalysts at calcination temperatures of 550 °C and 700 °C dubbed CP550 and CP700. CP550 demonstrated the maximum COD removal of 69.0% at 0.75 g/L catalyst loading after 8 h of visible light irradiation which dropped to 61.0% after three consecutive cycles. SEM images indicated that the higher calcination temperature of CP700 led to annealing which reduced the pore volume (0.025 cm³/g) and pore diameter (10.3 nm) while simultaneously creating a smoother and more spherical surface with lower S_BET (9.73 m²/g). In comparison, CP550 had a rough hair-like surface with higher S_BET (27.28 m²/g) and pore volume (0.077 cm³/g) as evidenced by BET analysis. XRD data indicated the presence of CaFe₅O₇ in the CP550 composition which was not present in CP700. The presence of Wustite-like FeO structures in CaFe₅O₇ are likely the cause for lower photoluminescence intensity profile and hence better charge separation of CP550 as these structures in CaFe₂O₄ have been known to increase resistivity and electron localization. The COD removal of CP550 dropped from 69.0% to just 7.0% upon adding a small quantity of isopropanol into the reaction mixture indicating hydroxyl radicals as the primary reactive oxidative species.

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.

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.

Cation, magnetic, and charge ordering in MnFe3O5

The recently-discovered high pressure material MnFe3O5 displays a rich variety of magnetically ordered states on cooling. Fe spins order antiferromagnetically below a Néel transition at 350 K. A second transition at 150 K marks Mn spin order that leads to spin canting of some of the Fe spins and ferrimagnetism. A further transition at 60 K is driven by charge ordering of Fe2+ and Fe3+ over two inequivalent Fe sites, with further canting of all spins. Electrical resistivity measurements reveal semiconducting behaviour in MnFe3O5 with a change in activation energy at 285 K.