Variation of optical gaps in perovskite-type 3d transition-metal oxides

Strain-Induced Gradient Distribution of Dopants at a Perovskite Interface

Interfaces often trap dopants for segregation and significantly affect the material properties. Because interfacial segregation sometimes is harmful, revealing mechanisms to reduce the extent of interface segregation is important. Here we investigate the Sr segregation behavior in LaTiO3.5/LaTiO3/LaTiO3.5 sandwich heterostructures by transmission electron microscopy and first-principles calculations. This reveals that Sr atoms prefer to segregate in LaTiO3 by replacing La atoms rather than LaTiO3.5. The Sr concentration in LaTiO3 increases with the thickness of the LaTiO3 layer and exhibits an obvious gradient distribution. First-principles calculations suggest that the electrostatic potential drives Sr atoms from LaTiO3.5 to LaTiO3. The low Sr concentration at the interfaces is induced by an interfacial strain. The sandwiched LaTiO3 layer changes from n-type conduction to insulation and to p-type conduction with an increase in Sr concentration. The finding that the strain concentration sometimes reduces the extent of interface segregation provides a new approach for the design and regulation of material interfaces.

The Structural, Electronic and Vibrational Properties of LaCrO 3 $$ {}_3 $$ . A Quantum Mechanical Investigation by Using an All Electron Gaussian Type Basis Set and a Full Range Hybrid Functional

The geometrical, electronic and vibrational properties of LaCrO3 $$ {}_3 $$ have been investigated by using an all electron Gaussian type basis set, the B3LYP functional and the CRYSTAL code, and compared with KVF3 $$ {}_3 $$ : in the two compounds the transition metal is formally in d3 $$ {}^3 $$ configuration. The high spin t2 g 3 $$ {}_{2g}^3 $$ ground state excludes the Jahn Teller deformation and the orbital ordering. The energy gain due to the rotation of the octahedra (from the cubic space group Pm3 ¯ m $$ \overline{3}\mathrm{m} $$ , N. 221, to space groupP 4 m bm $$ P\frac{4}{m} bm $$ , N.127, and toI 4 m cm $$ I\frac{4}{m} cm $$ , N. 140) in the oxide is about 70 times larger than in the fluoride (5.4 vs. 0.08 mEh $$ {}_h $$ ), due to the larger electrostatic forces (a factor four, as the formal charge doubles in going from F- to O2-) and the consequently reduced B-X distances. In KVF3 $$ {}_3 $$ , the p states of fluorine are separated by 6.4 eV from the d states of vanadium, whose band is quite narrow (1 eV). In the oxide, on the contrary, the oxygen p states overlap to a large amount with the d states of chromium, whose band is more than 6 eV large. The FM and AFM energy differences, the spin density maps and profiles, and the Mulliken analysis data are also provided for documenting the differences between the oxide and the fluoride.

Sulfur-doping effects on oxygen vacancy formation in LaBO3 (B = Fe, Co, and Ni) perovskites

Oxygen vacancy (VO) formation in perovskites plays an important role in improving their functional applications. Using density functional theory calculations, we investigated the effect of sulfur (S) doping on VO formation in LaBO3 (B = Fe, Co, and Ni) perovskites, considering the HS, IS, and LS states of Co ions in LaCoO3 to examine the influence of spin states. Our results show that the weaker electronegativity of S2- than that of O2- leads to a decrease in the magnetic moment of B atoms directly adjacent to the substituted S and an increase in the electrical conductivity of insulating systems. Formation energy (Ef) calculations suggest that S doping is beneficial for VO formation. In particular, VOs are more likely to form at oxygen positions adjacent to S ions. Moreover, upon S doping, spin state transition is not a necessary condition to lower the Ef. Instead, the main reason for reducing the Ef of VOs is the decreased relaxation energy of the lattice following VO formation. Therefore, we revealed a common mechanism for S-doping-promoted VO formation, which could be extended to other perovskites.

Electronic, magnetic and optical properties of the charge-disproportionated YNiO3 compound calculated using the GGA+U method

The electronic, magnetic and optical properties of YNiO3 were studied using the GGA+U method. This compound presents charge disproportionation with two nonequivalent Ni ion sites, namely, Ni1 with Ni(3-δ)+ and Ni2 with Ni(3+δ)+. A band-structure calculation was performed for the observed P21/n monoclinic phase; a 2 × 1 × 2 supercell with 80 atoms was used to reproduce the observed magnetic ordering. The density of states (DOS) calculation clearly showed the bonding differences between the Ni1 and the Ni2 sites; the Ni1 site has a more ionic character, whereas the Ni2 site presents a more covalent character. The band-structure results correspond to an insulator phase with a band gap of about 0.5 eV. The magnetic ordering is antiferromagnetic and the magnetic moments are about 1.24 μ B for Ni1 and around 0.00 μ B for Ni2. The calculated magnetic ordering results are in good agreement with neutron diffraction results. The calculated optical conductivity is similar to previous experimental data.

Structural, magnetic and optical properties of disordered double perovskite Gd2CoCrO6 nanoparticles

We have synthesized disordered double perovskite Gd2CoCrO6 (GCCO) nanoparticles with an average particle size of 71 ± 3 nm by adopting a citrate sol-gel method to investigate their structural, magnetic, and optical properties. Rietveld refinement of the X-ray diffraction pattern showed that GCCO is crystallized in a monoclinic structure with space group P21/n, which is further confirmed by Raman spectroscopic analysis. The absence of perfect long-range ordering between Co and Cr ions is confirmed by the mixed valence states of Co and Cr. A Néel transition was observed at a higher temperature of TN = 105 K compared to that of an analogous double perovskite Gd2FeCrO6 due to a greater degree of magnetocrystalline anisotropy of Co than Fe. Magnetization reversal (MR) behavior with a compensation temperature of Tcomp = 30 K was also observed. The hysteresis loop obtained at 5 K exhibited the presence of both ferromagnetic (FM) and antiferromagnetic (AFM) domains. Super-exchange and Dzyaloshinskii-Moriya (DM) interactions between various cations via oxygen ligands are responsible for the observed FM or AFM ordering in the system. Furthermore, UV-visible and photoluminescence spectroscopy demonstrated the semiconducting nature of GCCO with a direct optical bandgap of 2.25 eV. The Mulliken electronegativity approach revealed the potential applicability of GCCO nanoparticles in photocatalytic H2 and O2 evolution from water. Due to a favorable bandgap and potentiality as a photocatalyst, GCCO can be a promising new member of double perovskite materials for photocatalytic and related solar energy applications.

Potential of AMnO3 (A=Ca, Sr, Ba, La) as Active Layer in Inorganic Perovskite Solar Cells

Inorganic perovskite CaMnO3 ${{}_{3}}$ was proposed as a substitution for the TiO2 ${{}_{2}}$ anatase in electron transport layers of solar cells containing the hybrid perovskite CH3 ${{}_{3}}$ NH3 ${{}_{3}}$ PbI3 ${{}_{3}}$ based on increased mobility of electrons and better optical matching. Due to a suitable band gap concerning the absorption of sunlight, we investigate the potential of CaMnO3 ${{}_{3}}$ and similar manganite perovskites, where Ca is replaced by either Sr, Ba or La, as an absorber layer in inorganic perovskite solar cells. In this study, we have used optical measurements on the synthesized AMnO3 ${{}_{3}}$ (A=Ca, Sr, Ba, La) samples to aid density functional theory calculations (DFT) in order to accurately simulate the electronic and optical properties of AMnO3 ${{}_{3}}$ compounds and gauge their potential for the role of absorber layer. Both experimental measurements and theoretical calculations show suitable band gap of 1.1-1.5 eV, depending on the compound, and absorption coefficients of the order of10 5 ${{10}^{5}}$ cm- 1 ${{}^{-1}}$ in the visible part of the spectrum.

Narrow-Bandgap LaMO3 (M = Ni, Co) nanomaterials for efficient interfacial solar steam generation

Photothermal water evaporation provides a pathway towards a promising solution to global freshwater scarcity. Synergistic integration of functions in a material in diverse directions is a key strategy for designing multifunctional materials. Lanthanum-based perovskite complex oxides LaMO₃ (M = Ni and Co) have narrow band gaps with a high absorption coefficient. These functionalities have not been appropriately explored for photothermal energy conversion. Here, we synthesized nanostructured metallic LaNiO₃ and semiconducting LaCoO₃ and used them to design interfacial solar steam generators. Effective light absorption capability over the entire solar spectrum of these materials leads to a photothermal efficiency of the order of 83% for both materials. Using a cone-shaped 3D interfacial steam generator with a LaNiO₃ absorber, we achieved an evaporation rate of 2.3 kg m^-2 h^-1, corresponding to solar vapor generation efficiency of over 95%. To the best of our knowledge, this evaporation rate is higher than any oxide-based interfacial solar steam generator reported so far. Furthermore, we have also shown an effective way of using such evaporators for long-term seawater desalination.

Unraveling the Structural, Dielectric, Magnetic, and Optical Characteristics of Nanostructured La2NiMnO6 Double Perovskites

Double perovskite La2NiMnO6 (LNMO) nanoparticles and nanorods were synthesized via a hydrothermal process, where only aqueous inorganic solvents are used to regulate the microscopic morphology of the products without using any organic template. They crystallized in a monoclinic (P21/n) double perovskite crystal structure. The LNMO nanoparticles exhibited spherical morphology with an average particle size of 260 ± 60 nm, and the LNMO nanorods had diameters of 430 ± 120 nm and length about 2.05 ± 0.65 μm. Dual chemical oxidation states of the Ni and Mn ions were confirmed in the LNMO samples by X-ray photoelectron spectroscopy. Strong frequency dispersion dielectric behavior observed in the LNMO ceramics, is attributed to the space charge polarization and the oxygen vacancy induced dielectric relaxation. A ferroelectric—paraelectric phase transition appearing near 262 K (or 260 K) in the LNMO ceramics prepared from nanoparticles (or nanorods) was identified to be a second-order phase transition. The LNMO samples are ferromagnetic at 5 K but paramagnetic at 300 K. The LNMO nanoparticles had larger saturation magnetization (MS = 6.20 μB/f.u. @ 5 K) than the LNMO nanorods (MS = 5.68 μB/f.u.) due to a lower structural disorder in the LNMO nanorods. The semiconducting nature of the nanostructured LNMO with an optical band gap of 0.99 eV was revealed by the UV–visible absorption spectra. The present results enable the nanostructured LNMO to be a promising candidate for practical spintronic devices.

Rational Design of 3d Transition-Metal Compounds for Thermoelectric Properties by Using Periodic Trends in Electron-Correlation Modulation

The electronic structures in solid-state transition-metal compounds can be represented by two parameters: the charge-transfer energy (Δ), which is the energy difference between the p-band of an anion and an upper Hubbard band contributed by transition-metal d-orbitals, and the onsite Coulomb repulsion energy (U), which represents the energy difference between lower and upper Hubbard bands composed of split d-orbitals in transition metals. These parameters can facilitate the classification of various types of electronic structures. In this study, the dependences of anion species (N^3-, P^3-, As^3-, O^2-, S^2-, Se^2-, Te^2-, F^-, Cl^-, Br^-, and I^-) on Δ and U of 566 different binary and ternary 3d transition-metal compounds were investigated using ionic-model calculations. We were able to identify the systematic chemical trends in the variations in Δ and U values with the anion species of 11 different families of 3d transition-metal compounds in a comprehensive manner. The effective use of Δ-U diagrams given here, to facilitate the discovery and development of functional compounds, was demonstrated on thermoelectric compounds by classifying the thermoelectric properties of 3d transition-metal compounds and by predicting unrealized high-performance thermoelectric compounds.

Structural Characterization and Physical Properties of Double Perovskite La2FeReO6+δ Powders

The structural, optical, dielectric, and magnetic properties of double perovskite La₂FeReO_6+δ (LFRO) powders synthesized by solid-state reaction method under CO reduced atmosphere are reported on in this paper. Reitveld refinements on the XRD data revealed that the LFRO powders crystallized in an orthogonal structure (Pbnm space group) with column-like morphology. The molar ratios of La, Fe, and Re elements were close to 2:1:1. XPS spectra verified the mixed chemical states of Fe and Re ions, and two oxygen species in the LFRO powders. The LFRO ceramics exhibited a relaxor-like dielectric behavior, and the associated activation energy was 0.05 eV. Possible origins of the dielectric relaxation behavior are discussed based on the hopping of electrons among the hetero-valence ions at B-site, oxygen ion hopping through the vacant oxygen sites, and the jumping of electrons trapped in the shallower level created by oxygen vacancy. The LFRO powders display room temperature ferromagnetism with Curie temperature of 746 K. A Griffiths-like phase was observed in the LFRO powders with a Griffiths temperature of 758 K. The direct optical band gap of the LFRO powders was 2.30 eV, deduced from their absorption spectra, as confirmed by their green photoluminescence spectra with a strong peak around 556 nm.

The Role of Different Lanthanoid and Transition Metals in Perovskite Gas Sensors

Beginning with LaFeO₃, a prominent perovskite-structured material used in the field of gas sensing, various perovskite-structured materials were prepared using sol–gel technique. The composition was systematically modified by replacing La with Sm and Gd, or Fe with Cr, Mn, Co, and Ni. The materials synthesized are comparable in grain size and morphology. DC resistance measurements performed on gas sensors reveal Fe-based compounds solely demonstrated effective sensing performance of acetylene and ethylene. Operando diffuse reflectance infrared Fourier transform spectroscopy shows the sensing mechanism is dependent on semiconductor properties of such materials, and that surface reactivity plays a key role in the sensing response. The replacement of A-site with various lanthanoid elements conserves surface reactivity of AFeO₃, while changes at the B-site of LaBO₃ lead to alterations in sensor surface chemistry.

Three dimensional band-filling control of complex oxides triggered by interfacial electron transfer

The d-band-filling of transition metals in complex oxides plays an essential role in determining their structural, electronic and magnetic properties. Traditionally, at the oxide heterointerface, band-filling control has been achieved via electrostatic modification in the structure of field-effect transistors or electron transfer, which is limited to the quasi-two-dimension at the interface. Here we report a three-dimensional (3D) band-filling control by changing the local lattice coordination in a designed oxide heterostructure. At the LaCoO₃/LaTiO₃ heterointerface, due to the Fermi level mismatch, electrons transfer from LaTiO₃ to LaCoO₃. This triggers destabilisation of the CoO₆ octahedrons, i.e. the formation of lattice configurations with a reduced Co valence. The associated oxygen migration results in the 3D topotactic phase transition of LaCoO₃. Tuned by the thickness of LaTiO₃, different crystalline phases and band-fillings of Co occur, leading to the emergence of different magnetic ground states.

Revisiting the valence stability and preparation of perovskite structure type oxides ABO3 with the use of Madelung electrostatic potential energy and lattice site potential

Valence stability of aliovalent ions is mostly correlated with lattice site potential in ionic crystals. Madelung electrostatic potential is obtained by adding all the lattice site potentials for all the ions present in a crystal structure. Therefore, valence stability and the stability of a crystal structure can be better understood with consideration of both the lattice site potential and Madelung electrostatic potential. This was first demonstrated more than four decades ago by one of the present authors. We revisit this situation by using re-calculated lattice site potential and Madelung electrostatic potential for perovskite structure type ABO₃ compounds using a new computer program VESTA. We show that the formation of a perovskite structure type compound with the general formula ABO₃ (where A and B are cations and O is an oxide ion) becomes energetically favorable when it has a higher Madelung electrostatic potential than the combined Madelung electrostatic potential of parent binary compounds AO and B₂O₃ or BO₂. It is further shown that strong lattice site potential results in stability of high valence or high valence ions can be stabilized in a lattice site with strong lattice-site potential. It further follows that certain ions experience maximum lattice site potential at the B ion lattice site of the perovskite structure when compared to other structures such as fluorite BO₂, rutile BO₂ and corundum B₂O₃. Therefore, (i) the stability of an ion with a high (and uncommon) valence state at the B site being higher than that at the A site, (ii) occurrence of point defects at A or O sites with weak lattice site potentials, respectively and (iii) instability of perovskite A⁴⁺B²⁺O₃, and A⁵⁺B¹⁺O₃ compounds, respectively can be rationalized by lattice site potential and Madelung electrostatic potential analysis.

The stability of perovskite structure type oxides and higth valence state of cations, respectively may be better understood by considering lattice site potential and Madelung electrostatic potential energy.

Epitaxial Stabilization of Single-Crystal Multiferroic YCrO3 Thin Films

We report on the growth of stoichiometric, single-crystal YCrO₃ epitaxial thin films on (001) SrTiO₃ substrates using pulsed laser deposition. X-ray diffraction and atomic force microscopy reveal that the films grew in a layer-by-layer fashion with excellent crystallinity and atomically smooth surfaces. Magnetization measurements demonstrate that the material is ferromagnetic below 144 K. The temperature dependence of dielectric permittivity shows a characteristic relaxor-ferroelectric behavior at T_C = 375-408 K. A dielectric anomaly at the magnetic transition temperature indicates a close correlation between magnetic and electric order parameters in these multiferroic YCrO₃ films. These findings provide guidance to synthesize rare-earth, chromite-based multifunctional heterostructures and build a foundation for future studies on the understanding of magnetoelectric effects in similar material systems.

Determination of Formation Energies and Phase Diagrams of Transition Metal Oxides with DFT+U

Knowledge about the formation energies of compounds is essential to derive phase diagrams of multicomponent phases with respect to elemental reservoirs. The determination of formation energies using common (semi-)local exchange-correlation approximations of the density functional theory (DFT) exhibits well-known systematic errors if applied to oxide compounds containing transition metal elements. In this work, we generalize, reevaluate, and discuss a set of approaches proposed and widely applied in the literature to correct for errors arising from the over-binding of the O₂ molecule and from correlation effects of electrons in localized transition-metal orbitals. The DFT+U method is exemplarily applied to iron oxide compounds, and a procedure is presented to obtain the U values, which lead to formation energies and electronic band gaps comparable to the experimental values. Using such corrected formation energies, we derive the phase diagrams for LaFeO₃, Li₅FeO₄, and NaFeO₂, which are promising materials for energy conversion and storage devices. A scheme is presented to transform the variables of the phase diagrams from the chemical potentials of elemental phases to those of precursor compounds of a solid-state reaction, which represents the experimental synthesis process more appropriately. The discussed workflow of the methods can directly be applied to other transition metal oxides.

The spin–orbit–phonon coupling and crystalline elasticity of LaCoO3 perovskite

Based on an integrated study of magnetic susceptibility, specific heat, and thermal expansion of single-crystal LaCoO₃ free from cobalt and oxygen vacancies, two narrow spin gaps are identified before and after the phonon softening of gap size ΔE ∼ 0.5 meV in a CoO₆-octahedral crystal electric field (CEF) and the thermally activated spin gap Q ∼ 25 meV, respectively. Significant excitation of Co³⁺ spins from a low-spin (LS) to a high-spin (HS) state is confirmed by the thermal activation behavior of spin susceptibility χ^S of energy gap Q ∼ 25 meV, which follows a two-level Boltzmann distribution to saturate at a level of 50% LS/50% HS statistically above ∼200 K, without the inclusion of a postulated intermediate spin (IS) state. A threefold increase in the thermal expansion; coefficient (α) across the same temperature range as that of thermally activated HS population growth is identified, which implies the non-trivial spin–orbit–phonon coupling caused the bond length of Co^3+(LS↔HS)–O fluctuation and the local lattice distortion. The unusually narrow gap of ΔE ∼ 0.5 meV for the CoO₆ octahedral CEF between e_g–t_2g indicates a more isotropic negative charge distribution within the octahedral CEF environment, which is verified by the Electron Energy Loss Spectroscopy (EELS) study to show nontrivial La–O covalency.

Considering the before and after phonon softening, the gap in a CoO₆-octahedral crystal electric fields (CEF) and the thermally activated spin gap, were observed of ∼0.5 meV and Q ∼ 25 meV in defect-free LaCoO₃ single crystal, respectively.

Strain-dependent structure and Raman behaviours in the heavy-ion irradiated manganite at extreme low dose

The microstrains in heavy-ion irradiated manganite LaMnO₃ can be managed in linear response of irradiation dose, and the corresponding internal pressure up to 8 GPa can be induced by varying doses. The response of structure under stress is studied by means of Density Functional Theory and Lattice Dynamic Calculation. All obtained Raman scattering lines are discussed in details to shed light onto structural changes during ion implantation. There appears new resonance peak at around 550 cm^-1, which splits from broad features in the spectra, and attributes to the anti-symmetric vibrations of O₆ cages. The blue shift of this peak scales to ~2.4 cm^-1 per 1 GPa of stress. Another strong feature showing considerable blue shift is seen in the vicinity of 640 cm^-1 and corresponds to one of rhombohedral distortion related soft modes. A weak mode, not frequently reported, is seen at around 420 cm^-1 and corresponds to translation-like motions of fixed O₆ cages.

Optical Study of the Electronic Structure and Lattice Dynamics of NdBaMn2O6 Single Crystals

We investigated the electronic structure and lattice dynamics of double perovskite NdBaMn2O6 single crystals through spectroscopic ellipsometry and Raman scattering spectroscopy. The optical absorption band centered at approximately 0.88 eV was assigned to on-site d-d transitions in Mn, whereas the optical feature at approximately 4.10 eV was assigned to charge-transfer transitions between the 2p state of O and 3d state of Mn. Analysis of the temperature dependence of the d-d transition indicated anomalies at 290 and 235 K. The activated phonon mode, which appeared at approximately 440 cm-1 alongside with the enhancement of the 270 cm-1 phonon mode, coupled strongly to the metal-insulator transition at 290 K, which was associated with a charge/orbital ordering. Moreover, the MnO6 octahedral breathing mode at 610 cm-1 exhibited softening at a temperature lower than 235 K (temperature of the antiferromagnetic phase transition), which revealed the strong coupling between the lattice and magnetic degrees of freedom. The spin-phonon coupling constant obtained was λ = 2.5 cm-1. These findings highlight the importance of charge-orbital-spin interactions in establishing NdBaMn2O6 phases with novel properties.

Strain induced disordered phonon modes in Cr doped PrFeO3

Room temperature optical absorption spectroscopy (OAS) and Raman spectroscopy measurements have been carried out in order to understand the effect of structural disorder on the electronic and phononic states. For this purpose, polycrystalline samples of Cr doped PrFeO₃ have been prepared via wet chemical route. OAS analysis suggests the systematic scaling of electronic disorder with Cr doping; whereas, shifting in Raman line shapes and an increase in Raman line width has been observed with Cr doping. X-ray diffraction analysis clearly suggests the increase in structural disorders in the form of crystallographic strain with Cr doping, which is consistent with the broadening in Raman line shapes. The major contribution to Raman line width has been understood in terms of temperature independent terms i.e. structural disorder induced by doping. The generation of a new phonon mode at ~510 cm^-1 has been observed and understood as a disorder phonon mode due to strain induced structural disorder. Moreover, a systematic correlation between crystallographic strain, Raman line width, disordered parameter (σ) and Urbach energy has been observed, which implies that structural disorder affects phononic as well as electronic states of the system. Such comparative study allows us to find the correlation between densities of tail states, structural disorders and anharmonic effects probed by Raman spectroscopy.

BEEF-vdW+U method applied to perovskites: thermodynamic, structural, electronic, and magnetic properties

The recently developed BEEF-vdW exchange-correlation method provides a reasonably reliable description of both long-range van der Waals interactions and short-range covalent bonding between molecules and surfaces. However, this method still suffers from the excessive electron delocalization that is connected with the self-interaction error and, consequently, the calculated chemical and physical properties such as formation energy and band gap deviate markedly from the experimental values, especially when strongly correlated systems are under investigation. In this contribution, BEEF-vdW+U calculations have been performed to study the thermodynamic, structural, electronic, and magnetic properties of La-based perovskites. An effective interaction parameter [Formula: see text] and an energy adjustment [Formula: see text] are determined simultaneously by a mixing GGA and GGA+U method, where the enthalpy or Gibbs free energy of formation of oxides containing a transition metal in different oxidation states are fitted to available experimental data. The [Formula: see text] is found to have its origin in the fact that the GGA+U method gives rise to the offsets in the total energy that include not only the desired physical correction but also an arbitrary contribution. Calculated results indicate that the BEEF-vdW method provides a more accurate description of the bonding in the O₂ molecule than the PBE method and has generally smaller [Formula: see text] values for the 3d-block transition metals, thereby giving rise to band gaps and magnetic moments that are in better agreement with the experimentally measured values.

Band Gap Tuning of Solution-Processed Ferroelectric Perovskite BiFe1-x Co x O3 Thin Films

Ferroelectric perovskite oxides are emerging as a promising photoactive layer for photovoltaic applications because of their very high stability and their alternative ferroelectricity-related mechanism for solar energy conversion that could lead to extraordinarily high efficiencies. One of the biggest challenges so far is to reduce their band gap toward the visible region while simultaneously retaining ferroelectricity. To address these two issues, herein an elemental composition engineering of BiFeO₃ is performed by substituting Fe by Co cations, as a means to tune the characteristics of the transition metal-oxygen bond. We demonstrate by solution processing the formation of epitaxial, pure phase, and stable BiFe_1-x Co _x O₃ thin films for x ≤ 0.3 and film thickness up to 100 nm. Importantly, the band gap can be tuned from 2.7 to 2.3 eV upon cobalt substitution while simultaneously enhancing ferroelectricity. As a proof of concept, nonoptimized vertical devices have been fabricated and, reassuringly, the electrical photoresponse in the visible region of the Co-substituted phase is improved with respect to the unsubstituted oxide.

The influence of oxygen vacancy on the electronic and optical properties of ABO3−δ (A = La, Sr, B = Fe, Co) perovskites

ABO_3−δ (A = La, Sr, B = Fe, Co) perovskites are useful in a wide range of applications, including their recent exploration for application in high-temperature optical oxygen sensing for energy conversion devices such as solid oxide fuel cells.

Ferromagnetic ligand holes in cobalt perovskite electrocatalysts as an essential factor for high activity towards oxygen evolution

The definition of the interplay between chemical composition, electro-magnetic configuration and catalytic activity requires a rational study of the orbital physics behind active materials.

Structural dynamics of LaVO3 on the nanosecond time scale

Due to the strong dependence of electronic properties on the local bonding environment, a full characterization of the structural dynamics in ultrafast experiments is critical. Here, we report the dynamics and structural refinement at nanosecond time scales of a perovskite thin film by combining optical excitation with time-resolved X-ray diffraction. This is achieved by monitoring the temporal response of both integer and half-integer diffraction peaks of LaVO₃ in response to an above-band-gap 800 nm pump pulse. We find that the lattice expands by 0.1% out of plane, and the relaxation is characterized by a biexponential decay with 2 and 12 ns time scales. We analyze the relative intensity change in half-integer peaks and show that the distortions to the substructure are small: the oxygen octahedral rotation angles decrease by ∼0.3° and La displacements decrease by ∼0.2 pm, which directly corresponds to an ∼0.8° increase in the V-O-V bond-angles, an in-plane V-O bond length reduction of ∼0.3 pm, and an unchanged out-of-plane bond length. This demonstration of tracking the atomic positions in a pump-probe experiment provides experimentally accessible values for structural and electronic tunability in this class of materials and will stimulate future experiments.

Modulation of Manganite Nanofilm Properties Mediated by Strong Influence of Strontium Titanate Excitons

Hole-doped perovskite manganites have attracted much attention because of their unique optical, electronic, and magnetic properties induced by the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, a comprehensive investigation of the optical, electronic, and magnetic properties of La_0.7Sr_0.3MnO₃ thin films on SrTiO₃ (LSMO/STO) and other substrates is conducted using a combination of temperature-dependent transport, spectroscopic ellipsometry, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. A significant difference in the optical property of LSMO/STO that occurs even in thick (87.2 nm) LSMO/STO from that of LSMO on other substrates is discovered. Several excitonic features are observed in thin film nanostructure LSMO/STO at ∼4 eV, which could be attributed to the formation of anomalous charged excitonic complexes. On the basis of the spectral weight transfer analysis, anomalous excitonic effects from STO strengthen the electronic correlation in LSMO films. This results in the occurrence of optical spectral changes related to the intrinsic Mott-Hubbard properties in manganites. We find that while lattice strain from the substrate influences the optical properties of the LSMO thin films, the coexistence of strong electron-electron (e-e) and electron-hole (e-h) interactions which leads to the resonant excitonic effects from the substrate plays a much more significant role. Our result shows that the onset of anomalous excitonic dynamics in manganite oxides may potentially generate new approaches in manipulating exciton-based optoelectronic applications.

Structural transformation, Griffiths phase and metal-insulator transition in polycrystalline Nd2-x Sr x NiMnO6 (x = 0, 0.2, 0.4, 0.5 and 1) compound

Polycrystalline double perovskite Nd_2-x Sr _x NiMnO₆ (x  =  0, 0.2, 0.4, 0.5 and 1) samples were synthesized using the solid state reaction method. There occurs a structural transformation from monoclinic (P2₁/n, for x  =  0 to x  =  0.5) to cubic (Fm [Formula: see text] m, for x  =  1) with increasing Sr doping. Raman spectroscopy reveals the increase in static disorder with doping. The Curie temperature (T _C) shows a small increase from x  =  0 to 0.5 (T _C ~ 200 K), but for x  =  1, T _C increases drastically upto ~264 K. The deviation of 1/χ(T) from Curie-Weiss behaviour for doped samples with exponent less than one, indicates a development of the Griffiths phase with doping. The systematic reduction in magnetic moment at 5 K suggests an increase in anti-site disorders with doping. Mn 3s x-ray photoemission spectra show an increase in exchange splitting, indicating a decrease in the valency of Mn. The x-ray absorption spectra at Ni and Mn 2p edges show that the formal valence remains 2+  (Ni) and 4+  (Mn) for all the samples, with changes in spectral weights. Ni 2p x-ray photoemission spectra show characteristic feature similar to Ni³⁺ systems, only for x  =  1 sample. Our GGA-based calculations for the ordered supercell, predict half metallic character for doping (x  >  0) samples due to delocalization of Ni e_g orbitals. The calculations with anti-site disorders yield drastic reduction in Ni moments, with the disordered anti-ferromagnetic phase having lowest energy at maximum doping. Temperature dependent resistivity measurements exhibit a clear metallic region for x  =  0.2 sample, while for higher dopings (x  >  0.2), the metallicity gets suppressed due to increase in anti-site disorders in these samples.

Tailoring Materials for Mottronics: Excess Oxygen Doping of a Prototypical Mott Insulator

The Mott transistor is a paradigm for a new class of electronic devices-often referred to by the term Mottronics-which are based on charge correlations between the electrons. Since correlation-induced insulating phases of most oxide compounds are usually very robust, new methods have to be developed to push such materials right to the boundary to the metallic phase in order to enable the metal-insulator transition to be switched by electric gating. Here, it is demonstrated that thin films of the prototypical Mott insulator LaTiO₃ grown by pulsed laser deposition under oxygen atmosphere are readily tuned by excess oxygen doping across the line of the band-filling controlled Mott transition in the electronic phase diagram. The detected insulator to metal transition is characterized by a strong change in resistivity of several orders of magnitude. The use of suitable substrates and capping layers to inhibit oxygen diffusion facilitates full control of the oxygen content and renders the films stable against exposure to ambient conditions. These achievements represent a significant advancement in control and tuning of the electronic properties of LaTiO_3+x thin films making it a promising channel material in future Mottronic devices.

Nonzero Berry phase in quantum oscillations from giant Rashba-type spin splitting in LaTiO3/SrTiO3 heterostructures

The manipulation of the spin degrees of freedom in a solid has been of fundamental and technological interest recently for developing high-speed, low-power computational devices. There has been much work focused on developing highly spin-polarized materials and understanding their behavior when incorporated into so-called spintronic devices. These devices usually require spin splitting with magnetic fields. However, there is another promising strategy to achieve spin splitting using spatial symmetry breaking without the use of a magnetic field, known as Rashba-type splitting. Here we report evidence for a giant Rashba-type splitting at the interface of LaTiO₃ and SrTiO₃. Analysis of the magnetotransport reveals anisotropic magnetoresistance, weak anti-localization and quantum oscillation behavior consistent with a large Rashba-type splitting. It is surprising to find a large Rashba-type splitting in 3d transition metal oxide-based systems such as the LaTiO₃/SrTiO₃ interface, but it is promising for the development of a new kind of oxide-based spintronics.

Rashba-type splitting is an effective way to manipulate the spin degrees of freedom in a solid without external magnetic field. Here, the authors demonstrate a strong Rashba-type splitting at the interface of LaTiO₃ and SrTiO₃ which is promising for the development of oxide-based spintronics.

Weakly spin-dependent band structures of antiferromagnetic perovskite LaMO3 (M = Cr, Mn, Fe)

We investigate the spin-dependent electronic states of antiferromagnetic (AFM) lanthanum chromite (LaCrO₃), lanthanum manganite (LaMnO₃), and lanthanum ferrite (LaFeO₃) using spin-polarized first-principles density functional theory with Hubbard U correction. The band structures are calculated for 15 types of their different AFM structures. It is verified for these structures that there is a very simple rule to identify which wave number [Formula: see text] exhibits spin splitting or degeneracy in the band structure. This rule uses the symmetry operations that map the up-spin atoms onto the down-spin atoms. The resulting spin splitting is very small for the most stable spin configuration of the most stable experimental structure. We discuss a plausible benefit of this characteristic, i.e. the direction-independence of the spin current, in electrode applications.

A first principles study of p-type defects in LaCrO3

Recently, Sr-doped LaCrO₃ has been experimentally introduced as a new p-type transparent conducting oxide. It is demonstrated that substituting Sr for La results in inducing p-type conductivity in LaCrO₃. Performing first principles calculations we study the electronic structure and formation energy of various point defects in LaCrO₃. Our results for the formation energies show that in addition to Sr, two more divalent defects, Ca and Ba, substituting for La in LaCrO₃, behave as shallow acceptors in line with previous experimental reports. We further demonstrate that under oxygen-poor growth conditions, these shallow acceptors will be compensated by intrinsic donor-like defects (an oxygen vacancy and Cr on an oxygen site), but in the oxygen-rich growth regime the shallow acceptors have the lowest formation energies between all considered defects and will lead to p-type conductivity.

Correlating the Influence of Two Magnetic Ions at the A-Site with the Electronic, Magnetic, and Catalytic Properties in Gd1-x Dy x CrO3

Considering the absence of reports dealing with the perovskite-structured orthochromites containing two A-site magnetic rare-earth ions, GdCrO₃ and progressively Dy³⁺-substituted samples of the series Gd_1-x Dy _x CrO₃ have been synthesized employing the epoxide-mediated sol-gel procedure. The samples were characterized extensively using high-resolution powder X-ray diffraction, thermal analysis, Fourier transform infrared, Raman, and UV-visible spectroscopies, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements. Monophasic samples possessing an orthorhombic perovskite structure emerged by calcining the xerogels formed by the reaction of rare-earth nitrates, chromium(III)chloride, and propylene oxide at 800 °C for 2 h. Uniform presence of wormlike morphology was observed in both the field emission SEM (FE-SEM) and TEM images of the samples. Zero-field and field-cooled magnetic measurements using a SQUID magnetometer down to 4 K showed that the Neel temperature of Gd_0.5Dy_0.5CrO₃ was 155 K, more or less midway between the values observed for GdCrO₃(169 K) and DyCrO₃ (146 K). For the Gd_0.5Dy_0.5CrO₃ sample, a spin reorientation was observed at ∼38 K when measured under an applied field. Because the optical band gap, determined by Kubelka-Munk function, of these chromites was around 3 eV, their application as a catalyst for the photodegradation of the aqueous rhodamine-6G dye solution was demonstrated, in which the percentage of the total dye that was degraded varied with the average ionic radius of A-site ions. A similar systematic trend was observed even for the catalytic oxidation of the XO dye in the presence of H₂O₂, with DyCrO₃ influencing the reaction to a greater extent followed by Gd_0.5Dy_0.5CrO₃ and GdCrO₃. Both the photocatalytic and catalytic reactions followed pseudo-first-order kinetics.

High-Quality LaVO3 Films as Solar Energy Conversion Material

Mott insulating oxides and their heterostructures have recently been identified as potential photovoltaic materials with favorable absorption properties and an intrinsic built-in electric field that can efficiently separate excited electron-hole pairs. At the same time, they are predicted to overcome the Shockley-Queisser limit due to strong electron-electron interaction present. Despite these premises a high concentration of defects commonly observed in Mott insulating films acting as recombination centers can derogate the photovoltaic conversion efficiency. With use of the self-regulated growth kinetics in hybrid molecular beam epitaxy, this obstacle can be overcome. High-quality, stoichiometric LaVO₃ films were grown with defect densities of in-gap states up to 2 orders of magnitude lower compared to the films in the literature, and a factor of 3 lower than LaVO₃ bulk single crystals. Photoconductivity measurements revealed a significant photoresponsivity increase as high as tenfold of stoichiometric LaVO₃ films compared to their nonstoichiometric counterparts. This work marks a critical step toward the realization of high-performance Mott insulator solar cells beyond conventional semiconductors.

First-principles study on the electronic, optical and thermodynamic properties of ABO3 (A = La,Sr, B = Fe,Co) perovskites

The electronic, optical and thermodynamic properties of ABO₃ (A = La,Sr, B = Fe,Co) perovskites are investigated using first-principles calculations.

Effect of cation off-stoichiometry on optical absorption in epitaxial LaFeO3 films

The effect of A- and B-site cation deficiency on the optical absorption spectrum is presented for a series of LaFeO_3−δ epitaxial films providing insights into the relationship between defect chemistry and electronic structure in this semiconducting perovskite oxide.

Modelling oxygen defects in orthorhombic LaMnO3 and its low index surfaces

The findings of this work represent the first comprehensive study of the formation of oxygen defects in bulk orthorhombic LaMnO₃ and at its low index surfaces.

Ferroelectric BiFeO3 as an Oxide Dye in Highly Tunable Mesoporous All-Oxide Photovoltaic Heterojunctions

As potential photovoltaic materials, transition-metal oxides such as BiFeO₃ (BFO) are capable of absorbing a substantial portion of solar light and incorporating ferroic orders into solar cells with enhanced performance. But the photovoltaic application of BFO has been hindered by low energy-conversion efficiency due to poor carrier transport and collection. In this work, a new approach of utilizing BFO as a light-absorbing sensitizer is developed to interface with charge-transporting TiO₂ nanoparticles. This mesoporous all-oxide architecture, similar to that of dye-sensitized solar cells, can effectively facilitate the extraction of photocarriers. Under the standard AM1.5 (100 mW cm^-2 ) irradiation, the optimized cell shows an open-circuit voltage of 0.67 V, which can be enhanced to 1.0 V by tailoring the bias history. A fill factor of 55% is achieved, which is much higher than those in previous reports on BFO-based photovoltaic devices. The results provide here a new viable approach toward developing highly tunable and stable photovoltaic devices based on ferroelectric transition-metal oxides.

Spontaneous Polarization and Bulk Photovoltaic Effect Driven by Polar Discontinuity in LaFeO_{3}/SrTiO_{3} Heterojunctions

Structurally coherent and chemically abrupt interfaces formed between polar and nonpolar perovskite oxides provide an ideal platform for examining the purely electronic reconstruction known as the polar catastrophe and the emergence of mobile or bound charges at the interface. The appearance of mobile charges induced by the polar catastrophe is already established in the LaAlO_{3}/SrTiO_{3} heterojunctions. Although not experimentally verified, the polar catastrophe can also lead to the emergence of spontaneous polarization. We report that thin films of originally nonpolar LaFeO_{3} grown on SrTiO_{3} are converted to polar as a consequence of the polar catastrophe. The induced spontaneous polarization evokes photovoltaic properties distinct from conventional p-n junctions, such as a switching of the photocurrent direction by changing the interfacial atomic sequence. The control of the bulk polarization by engineering the interface demonstrated here will expand the possibilities for designing and realizing new polar materials with photovoltaic functions.

Empirical correction for PM7 band gaps of transition-metal oxides

A post-calculation correction is established for PM7 band gaps of transition-metal oxides. The correction is based on the charge on the metal cation of interest, as obtained from MOPAC PM7 calculations. Application of the correction reduces the average error in the PM7 band gap from ~3 eV to ~1 eV. The residual error after correction is shown to be uncorrelated to the Hartree-Fock method upon which PM7 is based. Graphical Abstract Comparison between calculated band gaps and experimental band gaps for binary oxides. The orange crosses are for corrected PM7 band gaps. Blue squares are uncorrected values. The orange crosses fall closer to the diagonal dashed line, showing an overall improvement of the accuracy of calculated values.