Metal-oxygen Bond Lengths Research Articles

Regulating the Ni-O bond length by constructing Ni(OH)2/CNTs heterostructure for enhanced nucleophile electrocatalytic oxidation

The electro-driven generation of the active Ni(OH)O species is crucial for the Ni(OH)2 to efficiently drive the nucleophilic oxidation reaction. However, the strategy to facilitate the generation of the Ni(OH)O species is only limited to the construction of the Pt/Ni(OH)2 heterostructures. In this study, carbon nanotubes (CNTs) are employed to reduce the dehydrogenation energy of the Ni(OH)2 by constructing Ni(OH)2/60CNTs heterostructures. The CNTs shorten the Ni-O bond length and increase the valence state of Ni atoms in Ni(OH)2, thus highly promoting the generation of Ni(OH)O. When the Ni(OH)2/60CNTs heterostructure is used for urea oxidation reaction, it displays highly enhanced catalytic activity relative to pure Ni(OH)2, attaining a current density of 10 mA cm−2 at 1.36 V versus reversible hydrogen electrode. This work provides a new strategy for reducing the dehydrogenation energy of the Ni(OH)2 and will inspire more research work on the recognition of the impact of metal–oxygen bond length on NOR activity.

Tartrate-gel synthesized BaFe12-xCuxO19 (0 ≤ x ≤ 1) nanoceramics: Magnetic and catalytic properties

Magnetic catalysts present distinct advantages over their non-magnetic counterparts, primarily due to their ease of recovery from the reaction mixture through magnetic separation. Herein, magnetic and catalytic properties of Cu-substituted barium hexaferrite (BaFe12-xCuxO19, 0 ≤ x ≤ 1) nanoceramics, synthesized via a tartrate-gel method, are evaluated. Using XRD, FTIR, Raman, SEM, XPS, and VSM techniques, the synthesized compositions were characterized. The powder XRD analysis revealed that all samples predominantly consisted of hexaferrite phase, with small impurity phases, α-Fe2O3, CuO and BaFe2O4. The structural parameters (a, b, c), extracted from Rietveld refinement analysis, increased with ‘x’, primarily due to substitution of smaller Fe3+ cation by larger Cu2+ cation. The FTIR and Raman spectra of the samples exhibited characteristic bands corresponding to the M-type hexaferrite. A red-shift occurred in these bands as the Cu amount increased in the Ba-hexaferrite lattice, owing to increase in metal-oxygen bond length. Due to sintering aid nature of copper, considerable increase in the particle size is observed with inclusion of Cu. XPS spectral analysis indicated the oxygen defects in the Cu-substituted sample. The magnetic parameters (MS, HC, and K1) are found to decrease upon incorporation of Cu ions into the Ba-hexaferrite crystal structure. Conversely, the Curie temperature (TC) showed a slight increase with Cu content ‘x’. The unsubstituted sample (x = 0) exhibited no catalytic activity in the reduction of nitrophenol, whereas both Cu-substituted samples displayed enhanced activity with comparable rate constants (0.02 min−1). This activity was attributed to the presence of Cu ions at the octahedral sites (2a or 4f2). Additionally, a magnetic separation process followed by a reusability test was conducted for the x = 0.5 sample over five successive cycles. The consistent activity observed in each cycle underscored the robustness of the engineered magnetic catalyst.

Short-range disorder mediated stability of Zn in rock-salt MgO beyond configurational entropy

In rock-salt Mg0.50TM0.25Zn0.25O, CuO is more effective in stabilizing rock-salt ZnO in MgO, compared to CoO and NiO. Cu incorporation creates a wide metal–oxygen bond length distribution and short-range disorder and enhances Zn stabilization.

Aluminum and Zinc Co-substituted Cobalt Ferrite: Structural, Magnetic, and Magnetostrictive Properties

We report on the influence of Al and Zn co-substitution on the structural, magnetic, and magnetostrictive properties of cobalt ferrite (Co1–xZnxFe2–xAlxO4, 0 ≤ x ≤ 0.15; CZFAO) materials, which were made using a glycine-nitrate autocombustion process. The cubic spinel structure is present in both the as-synthesized and the sintered CZFAO samples, where it was evident that the crystallite size and lattice parameter reduced with increasing x due to the incorporation of smaller Al3+ ions in place of larger Fe3+ ions in spinel ferrite. Co-substitution induced changes in the metal–oxygen bond lengths, causing both the tetrahedral and octahedral infrared and Raman spectroscopic bands to shift toward higher wavenumbers. The electron microscopy analyses indicate that the Al–Zn substitution induces grain growth, leading to a dense, interconnected grain morphology. Mossbauer analyses indicate that the Al3+ occupies the octahedral site, whereas Zn2+ is substituted at the tetrahedral site. Due to equal magnetic dilution of both tetrahedral and octahedral sites caused by the presence of Zn and Al at the appropriate sites, the saturation magnetization MS of CZFAO is the same as that of pure CFO. All other magnetic parameters (coercivity HC, magnetocrystalline anisotropy constant K1, and Curie temperature TC) decrease with increasing x, where the decline observed is mostly due to superexchange interaction (A-O-B) reduction caused by the substituents’ non-magnetic character. CZFAO materials exhibit higher magnetostriction strain λ and strain sensitivity dλ/dH at relatively low magnetic fields; among all the samples, x = 0.15 demonstrates a maximum strain sensitivity of −2.53 × 10–9 m/A at 23 kA/m magnetic field. The composition-tuned CZFAO materials with desirable properties are suitable for application in torque sensors.

Effect of High-Anisotropic Co2+ Substitution for Ni2+ on the Structural, Magnetic, and Magnetostrictive Properties of NiFe2O4: Implications for Sensor Applications.

This work reports on the effect of substituting a low-anisotropic and low-magnetic cation (Ni2+, 2μB) by a high-anisotropic and high-magnetic cation (Co2+, 3μB) on the crystal structure, phase, microstructure, magnetic properties, and magnetostrictive properties of NiFe2O4 (NFO). Co-substituted NFO (Ni1-xCoxFe2O4, NCFO, 0 ≤ x ≤ 1) nanomaterials were synthesized using glycine-nitrate autocombustion followed by postsynthesis annealing at 1200 °C. The X-ray diffraction measurements coupled with Rietveld refinement analyses indicate the significant effect of Co-substitution for Ni, where the lattice constant (a) exhibits a functional dependence on composition (x). The a-value increases from 8.3268 to 8.3751 Å (±0.0002 Å) with increasing the "x" value from 0 to 1 in NCFO. The a-x functional dependence is derived from the ionic-size difference between Co2+ and Ni2+, which also induces grain agglomeration, as evidenced in electron microscopy imaging. The chemical bonding of NCFO, as probed by Raman spectroscopy, reveals that Co(x)-substitution induced a red shift of the T2g(2) and A1g(1) modes, and it is attributed to the changes in the metal-oxygen bond length in the octahedral and tetrahedral sites in NCFO. X-ray photoelectron spectroscopy confirms the presence of Co2+, Ni2+, and Fe3+ chemical states in addition to the cation distribution upon Co-substitution in NFO. Chemical homogeneity and uniform distribution of Co, Ni, Fe, and O are confirmed by EDS. The magnetic parameters, saturation magnetization (MS), remnant magnetization (Mr), coercivity (HC), and anisotropy constant (K1) increased with increasing Co-content "x" in NCFO. The magnetostriction (λ) also follows a similar behavior and almost linearly varies from -33 ppm (x = 0) to -227 ppm (x = 1), which is primarily due to the high magnetocrystalline anisotropy contribution from Co2+ ions at the octahedral sites. The magnetic and magnetostriction measurements and analyses indicate the potential of NCFO for torque sensor applications. Efforts to optimize materials for sensor applications indicate that, among all of the NCFO materials, Co-substitution with x = 0.5 demonstrates high strain sensitivity (-2.3 × 10-9 m/A), which is nearly 2.5 times higher than that obtained for their intrinsic counterparts, namely, NiFe2O4 (x = 0) and CoFe2O4 (x = 1).

Effect of Ba/Sr ratio on the nonlinear optical properties of Ba1-xSrxTiO3 (x = 0.1–0.9) thin films

Perovskite-structured barium strontium titanate (Ba1-xSrxTiO3, x = 0.1–0.9) films have been epitaxially fabricated by using a pulsed-laser deposition technique. The third-order nonlinear optical properties were studied through a z-scan method, allowing the resolution of the nonlinear refractive and absorptive contributions to the responses. Although all the samples show almost the same value of nonlinear absorption coefficient, the extracted nonlinear refractive index of the sample of x = 0.3 is apparently larger than that of other samples. Dependency of the nonlinear optical properties on the Ba/Sr ratio is discussed in terms of the crystal phase transformation and metal-oxygen bond length of the selected materials.

Oxygen vacancies at the origin of pinned moments in oxide interfaces: The example of tetragonal CuO/SrTiO3

Obtaining an accurate theoretical description of the emergent phenomena in oxide heterostructures is a major challenge. Recently, intriguing paramagnetic spin and pinned orbital moments have been discovered by x-ray magnetic circular dichroism measurements at the Cu ${L}_{2,3}$ edge of a tetragonal $\mathrm{CuO}/{\mathrm{SrTiO}}_{3}$ heterostructure. Using first-principles calculations, we propose a scenario that explains both types of moments, based on the formation of oxygen vacancies in the ${\mathrm{TiO}}_{2}$ interface layer. We show the emergence of a paramagnetic two-dimensional electron gas hosted in the interface CuO layer. It is invisible at the Ti ${L}_{2,3}$ edge since the valence of the Ti atoms remains unchanged. Strong structural distortions breaking both the local and global fourfold rotation ${C}_{4}$ symmetries at the interface lead to the in-plane pinning of the Cu orbital moment close to the vacancy. Our results, and in particular the pinning of the orbital moment, may have implications for other systems, especially monoxide/dioxide interfaces with similar metal-oxygen bond length and weak spin-orbit coupling.

Synthesis of Tantalum Doped NMC811 and Its Impact on Crystal Structure and Electrochemical Performance at Higher Upper Cut-off Voltage

Ni-rich, cathode active materials such as Li[Ni0.8Mn0.1Co0.1]O2 (NMC811) suffer from capacity fade, especially at higher upper cut-off voltages. There is an increasing interest on dopants such as Mo or Ta as a promising dopants for high-Ni materials. In this study four cathode active materials with Li[(Ni0.78Mn0.11Co0.11)1−xMx]O2 (NMC811, x = 0, 0.005, 0.007, 0.013; M = Ta) were synthesized via a batch co-precipitation synthesis followed by calcination. To increase the probability of homogeneous distribution of the dopant, the highly porous hydroxide precursor was impregnated with Ta-ethoxide and afterwards calcined. Focus of this study is the effect of Ta on the crystal structure and its influence on stability at high voltage operation. Using Rietveld method, trends for several unit cell parameters, such as c-parameter, crystallite size, microstrain, transition metal-oxygen bond lengths) were found. Doping NMC811 with 0.5 mol% Ta leads to <3% lower initial capacity (205 mAh g−1 at C/10), >5% higher capacity retention (>85%), and >7% higher accumulated energy output over 100 cycles at 1 C (66.3 kWh kg−1 for 3.0–4.5 V vs Li+/Li). This performance enhancement is attributed to the high transition metal—oxygen bond dissociation energy, which is thought to significantly suppress surface phase transformations, and hence, reduce the vulnerability towards material degradation.

Microwave-triggered low temperature thermal reduction of Zr-modified high entropy oxides with extraordinary thermochemical H2 production performance

Solar-driven two-step thermochemical H2O splitting has emerged as a promising strategy for hydrogen production, but the conversion efficiency of solar-to-fuel has to be improved to make it more economically viable. Here, we present insights on microwave-triggered low temperature (600 °C) oxygen exchange for water splitting based on using Zr-modified high entropy oxides. The as-synthesized nanoparticles were characterized by XRD, FTIR, SEM, TEM, BET and EIS. The characterization results showed that the introduction of Zr4+ enlarged the Metal-Oxygen bond length of the spinel phase of FeMgCoNiOx and produced more oxygen vacancies with the increase of Zr4+ concentration, which greatly improved the thermochemical performance of FeMgCoNiOx. Water was splitted via reaction with FeMgCoNiOx/Zry that was previously reduced by microwave irradiation. By tuning the level of Zr4+ content from 0.0 to 1.0, FeMgCoNiOx/Zr0.6 was found to show the best trade-off, giving rise to a H2 yield of 4.84 mmol/g that was two times higher than that of FeMgCoNiOx (2.35 mmol/g) and outstanding thermodynamic energy efficiency (50 %). This work demonstrates that designing materials with abundant oxygen vacancies is a very efficient strategy to improve their thermochemical H2 evolution activity. Moreover, the energy efficiency will be further improved through tighter control of microwave reduction energetics.

Understanding the correlation between orbital degree of freedom, lattice-striction and magneto-dielectric coupling in ferrimagnetic Mn1.5Cr1.5O4

Dielectric anomaly observed in cubic Mn1.5Cr1.5O4 around ferrimagnetic ordering temperature (T N) suggests a possible magneto-dielectric coupling in the system. This report confirms the presence of a weak but significant magneto-dielectric coupling in the system. The ab initio calculations show a band gap of around 1.2 eV, with Fermi-level closer to the conduction band. The major features of conduction band nearest to the Fermi-level correspond to d xz and d 3z 2 −r 2 orbitals of Mn3+ ion. Temperature-dependent neutron diffraction results show a rapid decay in structural parameters (lattice-striction and transition metal–oxygen bond length) around T N. We confirmed that these changes in structural parameters at T N are not related to structural transition but the consequences of orbital-ordering of Mn3+. The rapid decay in transition metal–oxygen bond length under internal magnetism of the system shows that magnetism could certainly manipulate the electric dipole moment and hence the dielectric constant of the system. Magneto-striction acts as a link between magnetic and dielectric properties.

Optical properties of anatase and rutile TiO2 films deposited by using a pulsed laser.

Transparent titanium dioxide (TiO2) films were deposited on quartz substrates by using a pulsed-laser deposition technique. The effects of deposition temperatures on the crystalline phases and optical properties of the films were investigated. Phase-pure anatase and rutile TiO2 films were obtained at temperatures of 600°C and 800°C, respectively. The deposited TiO2 films were transparent in the visible spectral region, and the linear refractive indices of the films were determined by using the optical transmission spectra. The optical bandgaps were obtained as 3.25 and 3.02 eV for the anatase and rutile TiO2 films, respectively. The third-order nonlinear optical properties of the films were investigated by using a single-beam z-scan method at a wavelength of 532 nm. The values of the nonlinear absorption coefficient and the nonlinear refractive index of the rutile TiO2 film were determined to be -7.44×10-10m/W and -1.36×10-16m2/W, respectively, which are greater than those of the anatase TiO2 film. The metal-oxygen bond lengths based on the bond-orbital theory could be used to explain the results.

Light-Induced Ultrafast Dynamics of Spin Crossovers under High Pressure

Within the multielectron model of magnetic insulator with two different spin terms at each cation and spin crossover under high pressure we have studied dynamics of a sudden excited non equilibrium spin state. We obtain the different relaxation of the magnetization, high spin/low spin occupation numbers, and the metal-oxygen bond length for different values of the external pressure. For each pressure-temperature values stationary state agrees to the mean field phase diagrams. We found the long living oscillations of magnetization for the high spin ground state at small pressure. Close to crossover pressure the smooth relaxation is accompanied with a set of sharp strongly non linear oscillations of magnetization and HS/LS occupation numbers that are accompanied by the Franck–Condon resonances.

Operando X-ray absorption spectroscopy investigations on NaxNi1/3Fe1/3Mn1/3O2 positive electrode materials for sodium and sodium ion batteries

NaxMn1/3Fe1/3Ni1/3O2 (x = 2/3 and 1) layered oxides are synthesized and applied as positive electrode materials for sodium batteries. The crystal structure of the material changes from the O3 single phase to P2/O3 mixed phases as the Na content decreases from 1 to 2/3. The mixed-phases Na2/3Mn1/3Fe1/3Ni1/3O2 shows superior cycling performance compared to the single-phase NaMn1/3Fe1/3Ni1/3O2 due to different redox process and structural change that are demonstrated by operando X-ray absorption spectroscopy (XAS). The Na2/3Mn1/3Fe1/3Ni1/3O2 experiences redox reactions of Ni3+|Ni4+ and Fe3+|Fe4+ with inactive Mn ions during the charge/discharge processes, while the NaMn1/3Fe1/3Ni1/3O2 undergoes deeper redox reactions from Ni2+|Ni3+|Ni4+ and Fe3+|Fe4+ that Mn ions are irreversibly oxidized to Mn4+ in the 1st charge process. In addition, the mixed-phases material has smaller changes in the transition metal oxygen bond lengths during cycling, corresponding to less distortions of TMO6 units in the crystal structure. The better reversibility of the redox reactions and the occurrence of less structural changes are both responsible for the enhanced cycling performance obtained from the mixed-phases material compared to the single-phase material. These results strengthen the understanding of interactions of transition metals in the layered cathode and provide guidelines for designing positive electrode materials for sodium batteries.

Structural and Spectroscopic Investigation of Two Plutonium Mellitates.

The aqueous reaction of mellitic acid (H6mell) with 242PuBr3·nH2O forms two plutonium mellitates, 242Pu2(mell)(H2O)9·H2O (Pu-1α) and 242Pu2(mell)(H2O)8·2H2O (Pu-1β). These compounds are compared to the isomorphous lanthanide mellitates with similar ionic radii via bond length analysis. Both plutonium compounds form three-dimensional metal-organic frameworks, with Pu-1α having two unique metal centers and Pu-1β having one. All plutonium metal centers exhibit nine-coordinate geometries. Our results show metal-oxygen bond lengths for plutonium significantly shorter than those of the previously reported lanthanum and herein reported cerium analogues, consistent with the nine-coordinate ionic radii. Clear Laporte-forbidden 5f → 5f transitions are observed in the ultraviolet-visible-near-infrared spectra and are assigned to trivalent plutonium. However, there is a distinct color difference between the two plutonium compounds.

Structural studies of spray pyrolysis synthesized oxygen deficient anatase TiO2 thin films by using X-ray absorption spectroscopy.

The present work focuses on synthesis and X-ray absorption studies of single phase oxygen deficient anatase TiO2 thin films. These films are prepared in a two-step method viz. the synthesis of near stoichiometric anatase TiO2 films using an open atmospheric spray pyrolysis method followed by vacuum annealing at their corresponding synthesis temperatures (Ts = 450 °C, 500 °C) for different time durations (t = 2, 4, 6, 8 hours). XRD and Raman studies of these films ascertained the formation and retention of the anatase phase post annealing, indicating that there was no phase change due to prolonged annealing. Extended X-ray absorption spectra (EXAFS) and X-ray absorption near edge spectra (XANES) revealed the presence of an oxygen vacancy and its effect on the local co-ordination. The co-ordination number obtained from EXAFS analysis revealed that the number density of the oxygen vacancy is higher in the case of thin films synthesized at 450 °C than in the case of the films synthesized at 500 °C. As the oxygen vacancy leads to changes in local co-ordination, which in turn have a profound effect on the pre-edge features of the X-ray absorption spectra (XAS), theoretically simulated XAS spectra of pure anatase TiO2 and oxygen deficient anatase TiO2 were generated using FEFF and were found to match with the experimentally observed spectra. In addition, the ambiguities in whether a change in the metal-oxygen bond length has any effect on the pre-edge features or not were delineated in the present study by comparing the pre-edge peak positions of the oxygen deficient TiO2 films. Our results matched with those of some of the researchers who have studied the rutile phase TiO2, wherein it was concluded that in the case of the titanium-oxygen system, the mean Ti-O bond length does have an effect on the pre-edge peak position. It was observed that as the Ti-O mean bond length increases, the pre-edge peak positions shift towards lower energy, which is in concurrence with the literature available for other Ti-O systems with similar geometry. The second pre-edge peak intensity, which is a measure of disorder, is higher for TiO2450 °C-2 h and TiO2500 °C-2 h thin films. This is attributed to the annealing effect, which suggests that TiO6 octahedrons are the most disordered for thin films annealed for 2 h and become more ordered upon annealing for longer times.

Performance of density functionals for the structure and energetics of (M–O)0,± (M=Al, Si, Sc–Zn)

ABSTRACTWe report the results of the performance of 20 exchange–correlation functionals of density functional theory (DFT) in the structure (Metal–Oxygen bond length) and energetical properties (bond dissociation energy, adiabatic ionisation energy, and adiabatic electron affinity) of twelve metal monoxides (M–O, M=Al, Si, Sc–Zn). The calculated results show that the selected DFT functionals have the ability to reproduce the M–O bond length with a mean deviation of 0.01–0.05 Å, the energy values are reproduced with a mean deviation of 0.20–1.00 eV. In general, the functionals with significant HF exchange show decent performance in the calculation of bond length and harmonic vibrational frequency. These functionals show poor performance in energetics. Our calculated results show that the M06-L, B3LYP, and TPSSh functionals give good performance in both structure and energetical properties of metal monoxides. These functionals are recommended for the studies of structure and energetics in metal oxide systems. Further, our studies indicate that M06-L can be used for the studies in larger molecular systems. Among the 20 DFT functionals, the recently developed N12 functional gives poor performance in the studies of metal monoxides. Hence this functional is not recommended for the studies of structure and energetics in metal oxide systems.

Time-of-flight neutron total scattering with applied electric fields: Ex situ and in situ studies of ferroelectric materials.

Characterizing the structural response of functional materials (e.g., piezoelectrics and ferroelectrics) to electric fields is key for the creation of structure-property relationships. Here, we present a new sample environment and data reduction routines which allow the measurement of time-of-flight neutron total scattering during the in situ or ex situ application of high voltage (<10 kV) to a sample. Instead of utilizing the entire detector space of the diffractometer, only selected regions of detector pixels with scattering at the desired angle to the sample electric field are interrogated, which allows the generation of orientation-dependent reciprocal space patterns and real-space pair distribution functions (PDFs). We demonstrate the method using the relaxor ferroelectric Na1/2Bi1/2TiO3 and observe lattice expansion parallel and contraction perpendicular to the electric field for both in situ and ex situ experiments, revealing the irreversible nature of the local scale structural changes to this composition. Additionally, changes in the distributions of nearest neighbor metal-oxygen bond lengths are observed, which have been difficult to observe in previously measured analogous orientation-dependent X-ray PDFs. Considerations related to sample positioning and background subtraction are discussed, and future research directions are suggested.

Atomic-level insight into super-efficient electrocatalytic oxygen evolution on iron and vanadium co-doped nickel (oxy)hydroxide

It is of great importance to understand the origin of high oxygen-evolving activity of state-of-the-art multimetal oxides/(oxy)hydroxides at atomic level. Herein we report an evident improvement of oxygen evolution reaction activity via incorporating iron and vanadium into nickel hydroxide lattices. X-ray photoelectron/absorption spectroscopies reveal the synergistic interaction between iron/vanadium dopants and nickel in the host matrix, which subtly modulates local coordination environments and electronic structures of the iron/vanadium/nickel cations. Further, in-situ X-ray absorption spectroscopic analyses manifest contraction of metal–oxygen bond lengths in the activated catalyst, with a short vanadium–oxygen bond distance. Density functional theory calculations indicate that the vanadium site of the iron/vanadium co-doped nickel (oxy)hydroxide gives near-optimal binding energies of oxygen evolution reaction intermediates and has lower overpotential compared with nickel and iron sites. These findings suggest that the doped vanadium with distorted geometric and disturbed electronic structures makes crucial contribution to high activity of the trimetallic catalyst.

Spectroscopic and Computational Characterization of Diethylenetriaminepentaacetic Acid/Transplutonium Chelates: Evidencing Heterogeneity in the Heavy Actinide(III) Series.

The chemistry of trivalent transplutonium ions (Am3+ , Cm3+ , Bk3+ , Cf3+ , Es3+ …) is usually perceived as monotonic and paralleling that of the trivalent lanthanide series. Herein, we present the first extended X-ray absorption fine structure (EXAFS) study performed on a series of aqueous heavy actinide chelates, extending past Cm. The results obtained on diethylenetriaminepentaacetic acid (DTPA) complexes of trivalent Am, Cm, Bk, and Cf show a break to much shorter metal-oxygen nearest-neighbor bond lengths in the case of Cf3+ . Corroborating those results, density functional theory calculations, extended to Es3+ , suggest that the shorter Cf-O and Es-O bonds could arise from the departure of the coordinated water molecule and contraction of the ligand around the metal relative to the other [MIII DTPA(H2 O)]2- (M=Am, Cm, Bk) complexes. Taken together, these experimental and theoretical results demonstrate inhomogeneity within the trivalent transplutonium series that has been insinuated and debated in recent years, and that may also be leveraged for future nuclear waste reprocessing technologies.

Theoretical Study on the Negative Thermal Expansion Perovskite LaCu3Fe4O12: Pressure-Triggered Transition of Magnetism, Charge, and Spin State.

The A-site ordered negative thermal expansion material LaCu3Fe4O12 (LaCFO) was comprehensively investigated by using first-principles calculations. A pressure-triggered crystal structural phase transition from space group Im3̅ (No. 204) to Pn3̅ (No. 201) and magnetic transformation from a G-type antiferromagnetic (G_AFM) ground state to ferrimagnetic (FerriM) coupling were observed in LaCFO via gradual compression of the equilibrium volume. Correspondingly, the Fe-Cu intersite charge transfer from Fe to Cu 3dxy orbital, expressed as 4Fe3+ + 3Cu3+ → 4Fe3.75+ + 3Cu2+, was simulated along with the magnetic phase transformation from the G_AFM configuration to the FerriM state. Intriguingly, the Fe charge disproportionation, formulated as 8Fe3.75+ → 5Fe3+ + 3Fe5+, appeared and was attributed to the strong hybridization between Fe 3d and O 2p orbitals in the FerriM state when the volumes were substantially compressed up to less than or equal to 80%V. Meanwhile, the external hydrostatic pressure also leads to a spin flip from a high-spin Fe3+ antiferromagnetically arranged LaCu3+3Fe3+4O12 Mott insulator at low pressure and goes through a FerriM LaCu2+3Fe3.75+4O12 half-metal to a low-spin FerriM coupled LaCu2+3Fe3+5/2Fe5+3/2O12 metal at high pressure. Therefore, the crossover from high spin to low spin is responsible for the charge disproportionation in LaCFO. Essentially, the charge transfer and spin flip originate from the discontinuous changes of metal-oxygen bond lengths and angles in the compressed atomic structure. Finally, the negative thermal expansion behavior and mechanism of LaCFO were theoretically examined and clearly revealed.