Spinel-type Crystal Structure Research Articles

Analysis of CNT and rGO Mediation for an Efficient Super-Capacitive Response in Cr-Doped Co3O4 Based Composites

The synthesis of novel and high capacitance electrode materials has attracted much attention over the last few decades to meet the needs of electrode materials in supercapacitors. Cobalt oxide, one of several vanadium oxides, has recently gained popularity due to its unique layered structure, phase transition, and applications in supercapacitors. Here, we present structural, morphological, and electrochemical analysis of Cr-doped Co3O4 nanostructures and their carbon nanotubes/reduced graphene oxide (CNT/rGO) based composites. Hydrothermal and solvothermal routes are followed to prepare the samples. The active material is developed via a polymer-based binder and is used as the electrode in a three-electrode electrochemical system. X-ray diffraction confirms the spinel-type cubic crystal structure, while the stoichiometric elemental contents are verified via an energy dispersive X-ray spectroscopy. Well-shaped layered growth of the nanocomposites is revealed by field emission scanning electron microscopy. The electrochemical analysis is performed using a 2 M KOH electrolyte solution and a three-electrode electrochemical setup. The pure and Cr-doped nanocomposite samples reveal a pseudo capacitive behavior in all samples. The systematic capacitive and resistive response of the samples has also been presented in this report. The aforementioned attributes make the synthesized specimen a potential candidate for an electrode material.

Harmonic response of Gd-doped Mn-ferrite nanoparticles under AC magnetic field and optimization of Gd doping for MPI applications

This study synthesized Gd-doped Mn-ferrite nanoparticles for magnetic particle imaging (MPI) applications with doping levels (x) of 0, 0.05, 0.07, and 0.10. Analyses using X-ray diffraction measurements unequivocally confirmed the presence of a spinel-type crystal structure in the synthesized particles and X-ray absorption fine-structure spectroscopy confirmed the incorporation of Gd atoms in place of Mn within the crystal lattice. Magnetization and harmonic measurements under an AC magnetic field revealed that particles with a high initial permeability and low coercivity showed a high response intensity. Notably, particles possessing a Gd doping level of x = 0.07 displayed the highest response and were considered promising as MPI materials.

Morphology-dependent supercapacitive properties of Co3O4 nanomaterials synthesized via coprecipitation and hydrothermal methods

The supercapacitive properties of Co3O4 nanocrystalline powders with two different morphologies synthesized by coprecipitation (referred to as Co3O4–C) and hydrothermal (referred to as Co3O4-H) methods were compared and studied. The samples were analyzed for their phase purity, crystal structure, surface morphology, and surface area. Both samples were found to be single-phase nanostructures with a normal spinel-type cubic crystal structure (space group Fd3¯m), as indicated by Raman and XRD (X-ray diffraction) data analyses. TEM (Transmission electron microscopy) images clearly show that the Co3O4–C sample exhibits spherical particles with a mean size of 10 nm. On the other hand, the Co3O4–H sample shows a flower-like assembly of particles. The Co3O4–C sample has a higher specific surface area than the Co3O4-H sample due to its smaller particle size. XPS (X-ray photoelectron spectroscopy) data were collected to analyze the chemical states and cation distribution of the samples, revealing a 2:1 ratio of Co3+ and Co2+ in both samples. Both samples displayed pseudocapacitive behaviour in CV (cyclic voltammetry) and GCD (galvanostatic charge–discharge) analyses. Despite having a smaller surface area, the Co3O4–H electrode exhibited a higher CS (specific capacitance) compared to the Co3O4–C electrode at all current densities when tested using 1 M KOH electrolyte. At a specific current density (0.5 A/g), the Cs values for Co3O4–C and Co3O4–H are found to be 366 F/g and 233 F/g, respectively. As the current density increases, the specific capacitance of both electrodes decreases, but this reduction is more prominent for Co3O4-C than Co3O4-H. The study indicates that besides surface area, the morphology of the sample also plays a crucial role in determining the capacitance of a material.

Crystal Structure and Bond-Valence Investigation of Nitrogen-Stabilized LiAl5O8 Spinels.

An in-depth insight into the effect of nitrogen substitution on structural stabilization is important for the design of new spinel-type oxynitride materials with tailored properties. In this work, the crystal structures of ordered and disordered LiAl5O8 obtained by slow cooling and rapid quenching, respectively, were analyzed by a X-ray diffraction (XRD) Rietveld refinement and OccQP program. The variation in the bonding state of atoms in the two compounds was explored by the bond valence model, which revealed that the instability of spinel-type LiAl5O8 crystal structure at room temperature is mainly due to the severe under-bonding of the tetrahedrally coordinated Al cations. With the partial substitution of oxygen with nitrogen in LiAl5O8, a series of the nitrogen-stabilized spinel LiyAl(16+x-y)/3O8-xNx (0 < x < 0.5, 0 < y < 1) was successfully prepared. The crystal structures were systematically investigated by the powder XRD structural refinement combined with 7Li and 27Al magic-angle spinning nuclear magnetic resonance. All the Li+ ions entered the octahedra, while the Al resonances may be composed of multiple non-equivalent Al sites. The structural stability of spinel LiyAl(16+x-y)/3O8-xNx at ambient temperature was attributed to the cationic vacancies and high valence generated by the N ions, which alleviated the under-bonding state of the tetrahedral Al-O bond. This work provides a new perspective for understanding the composition-structure relationship in spinel compounds with multiple disorders.

Deposition of cobalt oxide films by reactive pulsed magnetron sputtering

Cobalt oxide films were deposited with the help of reactive high power impulse magnetron sputtering (HiPIMS) and mid-frequency pulsed magnetron sputtering (PMS) in argon gas at an argon gas pressure of 1 Pa and with different oxygen admixtures. The HiPIMS discharge was operated at a repetition frequency f=100 Hz with a duty cycle of 1%. Deposition rate and ion fraction were measured. The intensity of plasma ions, in particular Co+ and O+, is enhanced during HiPIMS compared to PMS and extends to larger ion energies. Films deposited on glass substrates were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction. Crystal structure and electrical resistivity of as-deposited films were found to depend on the deposition conditions. Deposited films are compatible with a non-stoichiometric Co3O4 spinel type crystal structure with unoccupied cobalt sites. PMS produces films with a preferred lattice orientation. HiPIMS show a large internal stress which relaxes during annealing. Electrical resistivity is several orders of magnitude smaller for films deposited by HiPIMS compared to PMS.

Resistance-Temperature Characteristics of Cr Doped Ni&lt;sub&gt;0.7&lt;/sub&gt;Mn&lt;sub&gt;2-x&lt;/sub&gt;Cu&lt;sub&gt;0.3&lt;/sub&gt;Cr&lt;sub&gt;x&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; NTC Thermistors

Manganese-based solid solution ceramics of transition metal with spinel-type crystal structure have generated much interest as NTC thermistors in various domestic and industrial applications. Doping has been considered as one of the most important method to improve their electrical properties. In this work, Cr doped Mn-based solid solution NTC ceramics Ni0.7Mn2-xCu0.3CrxO4 (x = 0.1, 0.2, 0.3 and 0.4) were prepared by solid-state reaction method. X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray energy dispersive spectrometer (EDS) were applied to study the phase structure, surface morphology and surface element content. The results indicate that the resistance and B value increase as the substituted Cr content increases.

Discovery of Ternary Silicon Titanium Nitride with Spinel-Type Structure

Here we report on the discovery of a ternary silicon titanium nitride with the general composition (Si1−x,Tix)3N4 with x = 0 < x < 1 and spinel-type crystal structure. The novel nitride is formed from an amorphous silicon titanium nitride (SiTiN) precursor under high-pressure/high-temperature conditions in a large volume high-pressure device. Under the conditions of 15–20 GPa and 1800–2000 °C, spinel-type γ-Si3N4 and rock salt-type c-TiN are formed. In addition, crystals of the discovered nano-sized ternary phase (Si1−x,Tix)3N4 embedded in γ-Si3N4 are identified. The ternary compound is formed due to kinetically-controlled synthesis conditions and is analyzed to exhibit the spinel-type structure with ca. 8 atom% of Ti. The Ti atoms occur in both Ti3+ and Ti4+ oxidation states and are located on the Si sites. The ternary nano-crystals have to be described as (Si,Ti)3N4 with N-vacancies resulting in the general composition (Si4+1−x Ti4+x-δTi3+δ)3N4-δ.

Influence of doping Mg cation in Fe3O4 lattice on its oxygen storage capacity to use as a catalyst for reducing emissions of a compression ignition engine

Improving oxygen storage capacity (OSC) of metal oxides by doping with metal cations can produce a catalyst with superior properties to improve engine performance and reduce emissions. In this study, Mg cations were incorporated into a ferric oxide lattice to form Mg0.25Fe2.75O4 via the solution combustion method. The structure, texture, morphology, and oxygen storage capacity of the samples were deeply investigated. The catalytic activity of Mg0.25Fe2.75O4 was finally compared with Fe3O4 as a reference nanocatalyst in terms of its combustion emissions using a six-cylinder Cummins diesel engine. It was found that the doped catalyst presented high crystallinity containing a mixture of the spinel-type crystal lattice and α-Fe2O3 structure, which confirms the ability of the solution combustion method for the fabrication of well-crystalline catalysts. The crystalline structure, surface area, and porosities and vacancy of spinel structure of Mg doped catalyst compared to the inverse spinel structure of Fe3O4 affect OSC of the samples, such that a significant increase in OSC of Fe3O4 (7941 µmol/g) occurred by loading of Mg cations (8661 µmol/g). Based on the engine emissions results, synthesized nanocatalysts are beneficial for decreasing the hydrocarbon (HC), carbon monoxide (CO), and particle mass (PM1.0) emissions. More specifically, the effect of nanocatalysts OSC would be dominated by the impact of increased soot oxidation, leading to PM1.0 reduction.

Magnetic properties of biofunctionalized iron oxide nanoparticles as magnetic resonance imaging contrast agents.

Background: One of the future applications of magnetic nanoparticles is the development of new iron-oxide-based magnetic resonance imaging (MRI) negative contrast agents, which are intended to improve the results of diagnostics and complement existing Gd-based contrast media.Results: Iron oxide nanoparticles designed for use as MRI contrast media are precisely examined by a variety of methods: powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, Mössbauer spectroscopy and zero-field nuclear magnetic resonance (ZF-NMR) spectroscopy. TEM and XRD measurements reveal a spherical shape of the nanoparticles with an average diameter of 5–8 nm and a cubic spinel-type crystal structure of space group Fd−3m. Raman, Mössbauer and NMR spectroscopy clearly indicate the presence of the maghemite γ-Fe2O3 phase. Moreover, a difference in the magnetic behavior of uncoated and human serum albumin coated iron oxide nanoparticles was observed by Mössbauer spectroscopy.Conclusion: This difference in magnetic behavior is explained by the influence of biofunctionalization on the magnetic and electronic properties of the iron oxide nanoparticles. The ZF-NMR spectra analysis allowed us to determine the relative amount of iron located in the core and the surface layer of the nanoparticles. The obtained results are important for understanding the structural and magnetic properties of iron oxide nanoparticles used as T2 contrast agents for MRI.

Time- and temperature-resolved luminescence spectroscopy of LiAl4O6F:Mn red phosphors

Ceramic samples of LiAl4O6F phosphors doped with 1.0 mol. % manganese ions have been synthesized by high-temperature solid-state reaction technique and investigated as possible red-emitting light converters. It has been found that the emission spectrum of LiAl4O6F:Mn phosphors consists of a narrow band peaked at 661 nm and a broader band peaked in the region of 675 ÷ 720 nm depending on synthesis conditions. The intensity of the latter band increases and the peak of this band shifts to longer wavelengths with increasing the fraction of MgF2 used as a flux material. Besides that, these two emission bands have strongly different decay times, namely τ1 ~ 240 μs and τ2 ~ several ms, respectively. The decay kinetics and time-resolved spectra of these two emission components have been studied in the temperature range of 10 ÷ 290 K under pulsed laser excitation at 262 nm. The observed luminescence properties have been analyzed within a model which considers the presence of manganese ions having different valence states (Mn4+ and Mn2+) in the specific spinel-type crystal structure of the LiAl4O6F host matrix, which provides many possibilities for different local environment near the doping manganese ions.

Fabrication and characterization of monodispersed Mn0.8Ni0.2Co2O4 mesoporous microspheres for supercapacitor application

The monodispersed Ni doped MnCo2O4 mesoporous microspheres were synthesized through a simple ammonium bicarbonate-assisted solvothermal route. The spinel-type crystal structure with a lattice parameter of 8.199 Å for Mn0.8Ni0.2Co2O4 composition was obtained by using X-ray diffraction analysis. The Brunauer−Emmett−Teller (BET) specific surface area of the sample was found to be 75.78 m2 g−1 with an average pore diameter of 9.88 nm. Electron microscopy studies revealed that the stable mesoporous microspheres are constituted by well-connected aggregates of nanoparticles. The influence of Ni doping on the pseudo-capacitance of MnCo2O4 electrode was investigated by means of cyclic voltammetry in 6 M KOH electrolyte. We found that the spinel-type Mn0.8Ni0.2Co2O4 mesoporous microspheres exhibit specific capacitances of 1822 F g−1 at a scan rate of 5 mV/s. Furthermore, the electrochemical impedance spectroscopy analysis revealed the low resistance and good electrochemical stability of the sample.

AuPt/3DOM CoCr2O4: Highly Active Catalysts for the Combustion of Methane

Three-dimensionally ordered macroporous (3DOM) CoCr2O4 and its supported bimetallic Auy Pt nanocatalysts (xAuyPt/3DOM CoCr2O4, x = 0.49, 0.98, and 1.92 wt%; Au/Pt molar ratio (y) = 0.99, 0.98, and 1.01) were prepared using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction methods, respectively. Physicochemical properties of the samples were characterized by means of various techniques. Catalytic activities of the xAuyPt/3DOM CoCr2O4 samples were evaluated for methane combustion. It is found that the 3DOM CoCr2O4 support possessed a single-phase and cubic spinel-type crystal structure, the xAuyPt/3DOM CoCr2O4 samples displayed a high-quality 3DOM architecture and a surface area of 32-33 m2/g, and the AuyPt nanoparticles (NPs) with a size of 3-5 nm were well dispersed on the 3DOM CoCr2O4 surface. The loading of an appropriate amount of AuyPt NPs could enhance the adsorbed oxygen species concentration and low-temperature reducibility of the sample. Among all of the samples, 0.98Au0.98Pt/3DOM CoCr2O4 showed the best catalytic performance: the T10%, T50%, and T90% (temperatures required for achieving methane conversion of 10, 50, and 90 %, respectively) were 253, 315, and 354 oC at a space velocity of 20,000 mL/(g h). It is concluded that the good activity of 0.98Au0.98Pt/3DOM CoCr2O4 was associated with its highly dispersed Au0.98Pt NPs, high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between Au0.98Pt NPs and 3DOM CoCr2O4.

Pseudocapacitance of Mesoporous Spinel-Type MCo2O4 (M = Co, Zn, and Ni) Rods Fabricated by a Facile Solvothermal Route.

We present the structural properties and electrochemical capacitance of mesoporous MCo2O4 (M = Co, Zn, and Ni) rods synthesized by a facile solvothermal route without necessity to use templates. The Brunauer–Emmett–Teller specific surface areas of these mesoporous rods are found to be about 24, 54, and 62 m2 g–1 with major pore diameters of about 31, 15, and 9 nm for MCo2O4, M = Co, Zn, and Ni, respectively. X-ray photoelectron spectroscopy and X-ray diffraction studies reveal the phase purity of the samples with a predominant spinel-type crystal structure. The spinel crystal structure with lattice parameters of 8.118, 8.106, and 8.125 Å is obtained for MCo2O4, M = Co, Zn, and Ni, respectively. The transmission electron microscopy study reveals that the mesoporous rods are built by self-assembled aggregates of nanoparticles which are well-interconnected to form stable mesoporous rods. The electrochemical capacitor performance was investigated by means of cyclic voltammetry, galvanostatic charge/discharge cycling, and impedance spectroscopy in a three-electrode configuration. As a result, the spinel-type MCo2O4 rods exhibit high specific capacitances of 1846 F g–1 (CoCo2O4), 1983 F g–1 (ZnCo2O4), and 2118 F g–1 (NiCo2O4) at a scan rate of 2 mV/s. Furthermore, the mesoporous spinel-type metal oxides show desirable stability in alkaline electrolyte during long-term cycling with excellent cycling efficiency.

Hydrothermal synthesis of spherical NiCO2O4 nanoparticles as a positive electrode for pseudocapacitor applications

Hydrothermal method was adapted to synthesis NiCo2O4 nanoparticles by varying nickel and cobalt precursor concentration as 1:1, 1:2, and 1:3 ratios. X-ray diffraction (XRD) results revealed the spinel NiCo2O4 structure belongs to $${\rm{Fd}}\overline {\rm{3}} {\rm{m}}$$ space group system with face-centered cubic crystal structure. Raman characteristic peaks observed at 495 and 654 cm−1 explored Eg and A2g modes of spinel NiCo2O4 product. Photoluminescence (PL) results revealed the hole recombination of Ni2+/Co2+ ions from 3d-Eg and 3d-Tg electronic state of spinel NiCo2O4 material. The characteristic Fourier transform infrared spectroscopy (FTIR) metal–oxygen bands appeared at 658 and 558 cm−1 revealed the spinel-type crystal structure. SEM image revealed the NiCo2O4 spherical nanoparticles formation with an average particle size of around 500 nm. The cyclic voltammetry studies revealed the estimated average specific capacitance value of NC3 (NiCo2O4 spherical nanoparticles) as 542 F g−1 relatively higher than NC1 and NC2. The electro impendence spectroscopy results explored the small arc formation in high frequency range and very low charge transfer resistance (R ct), which resulted high conductive active materials. The estimated specific capacitance for NC3 exhibited superior galvanstatic charging and discharging (GCD) characteristics with high specific capacitance of 294 F g−1 at high current density of 1 A g−1 and revealed that the obtained electrode is suitable for supercapacitor applications. Hydrothermal synthesis using an excess of Co source leads to smaller and more uniform particle size. This particle size and the slightly larger crystallite size formed in the materials leads to the improved electrochemical performance of the particles.

New evidence on the correlation between lattice fringe with catalytic performance for suprafacial CO and intrafacial CH4 oxidations over Co3O4 by isotopic 18O2 exchange

Tricobalt tetroxide (Co3O4) with a typical spinel-type crystal structure using as oxidation catalyst has attracted considerable attention. In this study, series of Co3O4 samples with diverse morphologies including nanoslice (NS), nanoparticle (NP), nanoflower (NF), and nanopolyhedron (NPL) crystals faceted with (3 1 1), (2 2 0), and (1 1 1) surfaces have been synthesized. Physicochemical properties of these materials were characterized by means of numerous techniques [XRD, N2 sorption, SEM, HRTEM, XPS, H2-TPR, and oxygen isotopic exchange (OIE) reaction] as well as DFT calculation. Furthermore, their catalytic activities for CO and CH4 oxidation were comparatively evaluated. The NS and NP samples (respective SBET of 137 and 90m2/g) possess much higher adsorbed oxygen and better low-temperature reducibility with respect to the other samples. In 18O2 exchange reaction, the NS sample showed the best transfer ability for surface as well as bulk oxygen among the investigated samples. Accordingly, the excellent catalytic performance towards CO and CH4 oxidation over NS-Co3O4 samples was associated with the abundant surface oxygen species, good low-temperature reducibility, especially the best mobility for both surface and bulk oxygen.

The Preparation of Soft Magnetic Composites Based on FeSi and Ferrite Fibers

Abstract The fields of soft magnetic composites and powder metallurgy technologies have a powerful potential to redesign the way of electric motor preparation, and will continue to grow for years to come. A design of the novel soft microcomposite material composed of spherical FeSi particles and Ni0.3Zn0.7Fe2O4 ferrite nanofibers is reported together with a characterization of basic mechanical and electrical properties. The needle-less electrospinning method was used for a preparation of Ni0.3Zn0.7Fe2O4 ferrite nanofibers, which has a spinel-type crystal structure as verified by XRD and TEM analysis. The dielectric coating was prepared by mixing of nanofibers with glycerol and ethanol because of safe manipulation with fumed fibers and homogeneous distribution of the coating around the FeSi particle surface. The final microcomposite samples were prepared by a combination of the traditional PM compaction technique supplemented with a conventional sintering process of the prepared green compacts. The composition and distribution of the secondary phase formed by the spinel ferrite fibers were examined by SEM. It is demonstrated that the prepared composite material has a tight arrangement without any significant porosity, which manifest itself through superior mechanical properties (high mechanical hardness, Young modulus, and transverse rupture strength) and specific electric resistivity compared to the related composite materials including resin as the organic binder.

Preparation of spinel nickel-cobalt oxide nanowrinkles/reduced graphene oxide hybrid for nonenzymatic glucose detection at physiological level

Nickel-cobalt oxide nanowrinkles with spinel-type crystal structure supported on reduced graphene oxide (NiCo2O4 NWs-rGO) was prepared to develop a sensitive and stable nonenzymatic glucose sensor. The NiCo2O4 NWs-rGO hybrid were prepared by a facile one-pot hydrothermal reaction, and sequential calcination in air. The morphology, composition and crystal structure of the NiCo2O4 NWs-rGO hybrid were characterized by scanning electron microscope, transmission electron microscope, selected area electron diffraction, and energy-dispersive spectroscopy. The electrochemical behavior of the hybrid and its catalytic activity towards glucose oxidation were investigated by several electrochemical methods. Compared with single component NiO or Co3O4, spinel type NiCo2O4 NWs displayed higher catalysis towards glucose oxidation. Further integration of NiCo2O4 with graphene could reduce the overpotential and enhance the catalytic current due to the improved conductivity and dispersity of NiCo2O4. The NiCo2O4 NWs-rGO based glucose sensor showed a wide linear range of 0.005-8.6mM, a low detection limit of 2μM (S/N=3), and an improved stability. A satisfactory recovery was also obtained for glucose detection in human serum at physiological level. Our results indicate rationally combine spinel type mixed metal oxide with graphene is a good alternative to fabricate advanced metal oxide based electrochemical sensors.

Canted spin structure and the first order magnetic transition in CoFe2O4 nanoparticles coated by amorphous silica

The functional polymer (PMA-co-MAA) latex microspheres were used as a core template to prepare magnetic hollow spheres consisting of CoFe2O4/SiO2 composites. The spinel type crystal structure of CoFe2O4 ferrite is formed under annealing, whereas the polymer cores are completely removed after annealing at 450°C. Magnetic and Mössbauer spectroscopy measurements reveal very interesting magnetic properties of the CoFe2O4/SiO2 hollow spheres strongly dependent on the particle size which can be tuned by the annealing temperature. In the ground state of low temperatures, the CoFe2O4 nanoparticles are in antiferromagnetic state due to the canted magnetic structure. Under heating in the applied field, the magnetic structure gradually transforms from canted to collinear, which increases the magnetization. The Mössbauer data revealed that the small size CoFe2O4/SiO2 particles (2.2–4.3nm) do not show superparamagnetic behavior but transit from the magnetic to the paramagnetic state by a jump-like magnetic transition of the first order This effect is a specific property of the magnetic nanoparticles isolated by inert material, and can be initiated by internal pressure creating at the particle surface. The suggested method of synthesis can be modified with various bio-ligands on the silane surface, and such materials can find many applications in diagnostics and bio-separation.

A Novel Composite Material Designed from FeSi Powder and Mn0.8Zn0.2Fe2O4Ferrite

A design of the novel microcomposite material composed of spherical FeSi particles and Mn0.8Zn0.2Fe2O4ferrite is reported together with a characterization of basic mechanical and electrical properties. The sol-gel autocombustion method was used for a preparation of Mn0.8Zn0.2Fe2O4ferrite, which has a spinel-type crystal structure as verified by XRD and TEM analysis. The final microcomposite samples were prepared by a combination of the traditional PM compaction technique supplemented with unconventional microwave sintering process of the prepared green compacts. The composition and distribution of the secondary phase formed by the spinel ferrite were examined by SEM. It is demonstrated that the prepared composite material has a tight arrangement without any significant porosity, which manifests itself through superior mechanical properties (high mechanical hardness, Young modulus, and transverse rupture strength) and specific electric resistivity compared to the related composite materials including resin as the organic binder.

The Verwey Phase of Magnetite: A Long-Running Mystery in Ferrites

Magnetite (Fe3O4) is the original magnetic material and the parent of ferrite magnets, with modern applications ranging from spintronics to MRI contrast agents. At ambient temperatures magnetite has a cubic spinel-type crystal structure, but it undergoes a complex structural distortion and becomes electrically insulating below the 125 K Verwey transition. The electronic ground state of the Verwey phase has been unclear for over 70 years as the low temperature structure was unknown, but the full low temperature superstructure was recently determined by high energy microcrystal x-ray diffraction. There are 168 frozen phonon modes in the acentric (and hence multiferroic) low temperature magnetite structure. The ground state was found to be Fe2+/Fe3+ charge ordered and Fe2+ orbital ordered to a first approximation, but an unexpected localization of electrons in three-Fe ‘trimeron’ units was discovered. This description is supported by band structure calculations. This brief review will summarise recent progress on understanding the ground state structure of the Verwey phase of magnetite.