Perovskite Transition Metal Oxides Research Articles

Oxygen diffusion barriers for epitaxial thin-film heterostructures with highly conducting SrMoO3 electrodes

Transition metal perovskite oxide SrMoO3 with a Mo4+ 4d2 electronic configuration exhibits a room-temperature resistivity of 5.1 μΩcm in a single-crystal form and, therefore, is considered a prominent conducting electrode material for all-oxide microelectronic devices. Stabilization of the unfavorable Mo4+ valence state in SrMoO3 thin films necessitates reductive growth conditions that are often incompatible with a highly oxidative environment necessary to grow epitaxial heterostructures with fully oxygenated functional layers (e.g., tunable dielectric BaxSr1−xTiO3). Interestingly, only a few unit cells of the perovskite titanate capping layers SrTiO3, BaTiO3, and Ba0.5Sr0.5TiO3 act as an efficient oxygen barrier and minimize SrMoO3 oxidation into electrically insulating SrMoO4 in the broad range of the thin-film growth parameters. The Mo valence state in SrMoO3, determined by x-ray photoelectron spectroscopy, is used to analyze oxygen diffusion through the capping layers. The lowest level of oxygen diffusion is observed in Ba0.5Sr0.5TiO3. A Ba0.5Sr0.5TiO3 film with a thickness of only 6 unit cells preserves the Mo4+ oxidation state in the SrMoO3 underlayer up to the oxygen partial pressure of 8 mTorr at the temperature of 630 °C. Results, therefore, indicate that SrMoO3 films covered with atomically thin Ba0.5Sr0.5TiO3 remain conducting in an oxygen environment and can be integrated into all-oxide thin-film heterostructures with other functional materials.

Spray-pyrolysis deposited La1−xSrxCoO3 thin films for potential non-volatile memory applications

In this work thin films of the La1-xSrxCoO3 (0.05 < x < 0.26) compound were grown, employing the so-called spray pyrolysis process. The as-grown thin films exhibit polycrystalline microstructure, with uniform grain size distribution, and observable porosity. Regarding their electrical transport properties, the produced thin films show semiconducting-like behavior, regardless the Sr doping level, which is most likely due to both the oxygen deficiencies and the grainy nature of the films. Furthermore, room temperature current-voltage (I-V) measurements reveal stable resistance switching behavior, which is well explained in terms of space-charge limited conduction mechanism. The presented experimental results provide essential evidence regarding the engagement of low cost, industrial-scale methods of growing perovskite transition metal oxide thin films, for potential applications in random access memory devices.

Relationship between charge redistribution and ferromagnetism at the heterointerface between the perovskite oxides LaNiO3 and LaMnO3

To investigate the relationship between the charge redistribution and ferromagnetism at the heterointerface between perovskite transition-metal oxides LaNiO$_3$ (LNO) and LaMnO$_3$ (LMO), we performed x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) measurements. In the LNO/LMO heterostructures with asymmetric charge redistribution, the electrons donated from Mn to Ni ions are confined within one monolayer (ML) of LNO at the interface, whereas holes are distributed over 3-4 ML on the LMO side. A detailed analysis of the Ni-$L_{2,3}$ and Mn-$L_{2,3}$ XMCD spectra reveals that Ni magnetization is induced only by the Ni$^{2+}$ ions in the 1 ML LNO adjacent to the interface, while the magnetization of Mn ions is increased in the 3-4 ML LMO of the interfacial region. The characteristic length scale of the emergent (increased) interfacial ferromagnetism of the LNO (LMO) layers is in good agreement with that of the charge distribution across the interface, indicating a close relationship between the charge redistribution due to the interfacial charge transfer and the ferromagnetism of the LNO/LMO interface. Furthermore, the XMCD spectra clearly demonstrate that the vectors of induced magnetization of both ions are aligned ferromagnetically, suggesting that the delicate balance between the exchange interactions occurring inside each layer and across the interface may induce the canted ferromagnetism of Ni$^{2+}$ ions, resulting in weak magnetization in the 1 ML LNO adjacent to the interface.

Cooperative elastic fluctuations provide tuning of the metal–insulator transition

Metal-to-insulator transitions1 driven by strong electronic correlations occur frequently in condensed matter systems, and are associated with remarkable collective phenomena in solids, including superconductivity and magnetism. Tuning and control of the transition holds the promise of low-power, ultrafast electronics2, but the relative roles of doping, chemistry, elastic strain and other applied fields have made systematic understanding of such transitions difficult. Here we show that existing data3-5 on the tuning of metal-to-insulator transitions in perovskite transition-metal oxides through ionic size effects provides evidence of large systematic effects on the phase transition owing to dynamical fluctuations of the elastic strain, which have usually been neglected6. We illustrate this using a simple yet quantitative statistical mechanical calculation in a model that incorporates cooperative lattice distortions coupled to the electronic degrees of freedom. We reproduce the observed dependence of the transition temperature on the cation radius in the well studied manganite7 and nickelate8 materials. Because elastic couplings are generally strong, we anticipate that these conclusions will generalize to all metal-to-insulator transitions that couple to a change in lattice symmetry.

A Perovskite Electronic Structure Descriptor for Electrochemical CO2 Reduction and the Competing H2 Evolution Reaction

The electrochemical reduction of CO2 (CO2RR) to high-value products is an attractive means to simultaneously address the negative effects of increasing CO2 emissions and the ever-present societal demand for chemicals and fuels. However, the rational development of novel catalyst chemistries for this reaction is needed to tune the activity and selectivity of the CO2RR, especially for increasingly complex chemistries, such as oxides, sulfides, and phosphides. In this work, we explore a diverse chemical range of transition-metal perovskite oxides (ABO3) by determining their surface binding energies toward COads and Hads using density functional theory (DFT) and by evaluating their CO2RR activity and selectivity. We find that tuning perovskite chemistry results in the ability to electrochemically reduce CO2 to CH4. We propose the O 2p-band center as a rational design parameter that faithfully captures the energetics of Hads and COads binding that influence the hydrogen evolution reaction (HER) and CO2RR activity. A higher O 2p-band center results in higher HER activity, while an intermediate O 2p-band center improves favorability for CH4 evolution. In the process, we identify a group of perovskites (i.e., LaCoO3, GdCoO3, NdCoO3) as materials that have not, to date, been reported previously for CH4 evolution activity at relatively low overpotentials (−0.6 to −0.8 VRHE).

Origin of band gaps in 3d perovskite oxides

With their broad range of properties, ABO3 transition metal perovskite oxides have long served as a platform for device applications and as a testing bed for different condensed matter theories. Their insulating character and structural distortions are often ascribed to dynamical electronic correlations within a universal, symmetry-conserving paradigm. This view restricts predictive theory to complex computational schemes, going beyond density functional theory (DFT). Here, we show that, if one allows symmetry-breaking energy-lowering crystal symmetry reductions and electronic instabilities within DFT, one successfully and systematically recovers the trends in the observed band gaps, magnetic moments, type of magnetic and crystallographic ground state, bond disproportionation and ligand hole effects, Mott vs. charge transfer insulator behaviors, and the amplitude of structural deformation modes including Jahn-Teller in low temperature spin-ordered and high temperature disordered paramagnetic phases. We then provide a classification of the four mechanisms of gap formation and establish DFT as a reliable base platform to study the ground state properties in complex oxides.

Manipulation of the Electronic Transport Properties of Charge-Transfer Oxide Thin Films of NdNiO3 Using Static and Electric-Field-Controllable Dynamic Lattice Strain

Strongly correlated perovskite transition-metal oxides have received considerable attention due to their intriguing physical properties, including a metal-insulator transition that can be used in devices. Key questions remain, though, including whether in-plane tensile strain helps or hinders the transition. This study uses both static and dynamic lattice strain to manipulate the transport properties of NdNiO${}_{3}$ films systematically. The electric-field-tunable ferroelastic/piezoelectric strain approach can be used not only to obtain deeper insight into the intrinsic properties of nickelate films, but also as a simple, energy-efficient means to construct multistate resistive memory.

Origin of Improved Photoelectrochemical Water Splitting in Mixed Perovskite Oxides

AbstractOwing to the versatility in their chemical and physical properties, transition metal perovskite oxides have emerged as a new category of highly efficient photocatalysts for photoelectrochemical (PEC) water splitting. Here, to understand the underlying mechanism for the enhanced PEC water splitting in mixed perovskites, ideal epitaxial thin films of the BiFeO3–SrTiO3 system are explored. The electronic structure and carrier dynamics are determined from both experiment and density‐functional theory calculations. The intrinsic phenomena are measured in this ideal system, contrasting to commonly studied polycrystalline solid solutions where extrinsic structural features obscure the intrinsic phenomena. It is determined that when SrTiO3 is added to BiFeO3 the conduction band minimum position is raised and an exponential tail of trap states from hybridized Ti 3d and Fe 3d orbitals emerges near the conduction band edge. The presence of these trap states strongly suppresses the fast electron–hole recombination and improves the photocurrent density in the visible‐light region, up to 16× at 0 VRHE compared to the pure end member compositions. This work provides a new design approach for optimizing the PEC performance in mixed perovksite oxides.

A review of sorbents for high-temperature hydrogen sulfide removal from hot coal gas

Integrated gasification combined cycle (IGCC) and solid oxide fuel cell (SOFC) systems are considered as the most promising clean coal technologies. Syngas derived from coal gasification is the major fuel sources for IGCC and SOFC power systems; however, large amounts of sulfur compounds, mainly in the form of hydrogen sulfide, are produced during gasification. Hydrogen sulfide has to be removed prior to syngas utilization to protect downstream equipment from high-temperature corrosion. Therefore, hydrogen sulfide removal from raw syngas plays a key role in successful application of IGCC and SOFC systems. Hot coal gas desulfurization using solid sorbents is a more efficient technique for hydrogen sulfide removal, compared with conventional cold coal gas desulfurization with amine solution. This article reviews solid sorbents for high-temperature desulfurization, e.g., transition metal oxides, rare-earth oxides, spinel oxides, perovskite oxides, nanoelemental metals and mesoporous desulfurizers. Composite oxides that combine the properties of diverse metal oxides are promising candidates for hot coal gas desulfurization. The extensive background knowledge and the state of the art on high-temperature sorbents for hydrogen sulfide removal in this review provides inspiration and guidance to develop new cost-effective sorbents.

Transition-Metal Oxide (111) Bilayers

Correlated electron systems on a honeycomb lattice have emerged as a fertile playground to explore exotic electronic phenomena. Theoretical and experimental work has appeared to realize novel behavior, including quantum Hall effects and valleytronics, mainly focusing on van der Waals compounds, such as graphene, chalcogenides, and halides. In this article, we review our theoretical study on perovskite transition-metal oxides (TMOs) as an alternative system to realize such exotic phenomena. We demonstrate that novel quantum Hall effects and related phenomena associated with the honeycomb structure could be artificially designed by such TMOs by growing their heterostructures along the [111] crystallographic axis. One of the important predictions is that such TMO heterostructures could support two-dimensional topological insulating states. The strong correlation effects inherent to TM d electrons further enrich the behavior.

Nanocarbon‐Based Electrocatalysts for Rechargeable Aqueous Li/Zn‐Air Batteries

AbstractRechargeable aqueous Li/Zn‐air batteries have become the most attractive energy storage devices because of their extremely high theoretical energy density, which enables electric vehicles to drive long range. However, current achievements still suffer from poor cycle life, low practical energy density, low round‐trip efficiency, and high manufacturing costs. One of the key challenges is the sluggish kinetics of oxygen electrochemistry during discharge and charge cycles. Thus, significant breakthroughs in design and synthesis of efficient electrocatalysts for the oxygen redox reaction are highly demanded. Nanocarbons, especially heteroatoms doped nanocarbons, are potential oxygen reduction reaction (ORR) catalysts due to the reasonable balance between catalytic activity, durability, and cost. Importantly, by introduction of transition metal nanoparticles, spinel oxides, layered‐double hydroxides, perovskite oxides, and other metal oxides, the constructed nanocarbon‐based materials could deliver promising bifunctional ORR and oxygen evolution reaction (OER) catalytic properties, which could be employed as air‐cathode materials to improve energy storage performances, even to get commercialized. Here, an overview of the recent progress of nanocarbon‐based electrocatalysts as air‐cathode materials for rechargeable aqueous Li/Zn‐air batteries is provided, aiming to highlight the benefits and issues of nanocarbon‐based electrocatalysts as well as to outline the most promising results and applications so far.

Substrate-dependent structural and CO sensing properties of LaCoO3 epitaxial films

LaCoO3 thin films were grown on different (0 0 1) oriented LaAlO3, SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 by the polymer assisted deposition method, respectively. All the LaCoO3 thin films are in epitaxial growth on these substrates, with tetragonal distortion of CoO6 octahedrons. Due to different in-plane lattice mismatch, the LaCoO3 film on LaAlO3 has the largest tetragonal distortion of CoO6 octahedrons while the film grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 has the smallest tetragonal distortion. The relative contents of the surface absorbed oxygen species are found to increase for the LaCoO3 epitaxial films grown on (0 0 1) oriented (LaAlO3)0.3(Sr2AlTaO6)0.7, SrTiO3 and LaAlO3 substrates, sequentially. The film sensors exhibit good CO sensing properties at 150 °C, and the LaCoO3 film on LaAlO3 shows the highest response but the film on (LaAlO3)0.3(Sr2AlTaO6)0.7 shows the lowest. It reveals that the larger degree of Jahn-Teller-like tetragonal distortion of CoO6 octahedrons may greatly improve the surface absorbing and catalytic abilities, corresponding to more excellent CO sensing performance. The present study suggests that the formation of epitaxial films is an efficient methodology for controlling the octahedral distortion and thereby improving the gas sensing performance of perovskite transition metal oxides.

Converged GW quasiparticle energies for transition metal oxide perovskites

The ab initio calculation of quasiparticle (QP) energies is a technically and computationally challenging problem. In condensed matter physics the most widely used approach to determine QP energies is the GW approximation. Although the GW method has been widely applied to many typical semiconductors and insulators, its application to more complex compounds such as transition metal oxide perovskites has been comparatively rare, and its proper use is not well established from a technical point of view. In this work, we have applied the single-shot G0W0 method to a representative set of transition metal oxide perovskites including 3d (SrTiO3, LaScO3, SrMnO3, LaTiO3, LaVO3, LaCrO3, LaMnO3, and LaFeO3), 4d (SrZrO3, SrTcO3, and Ca2RuO4) and 5d (SrHfO3, KTaO3 and NaOsO3) compounds with different electronic configurations, magnetic orderings, structural characteristics and bandgaps ranging from 0.1 to 6.1 eV. We discuss the proper procedure to obtain well converged QP energies and accurate bandgaps within single-shot G0W0 by comparing the conventional approach based on an incremental variation of a specific set of parameters (number of bands, energy cutoff for the plane-wave expansion and number of k-points and the basis-set extrapolation scheme [Phys. Rev. B 90, 075125 (2014)]. In addition, we have inspected the difference between the adoption of norm-conserving and ultrasoft potentials in GW calculations. A minimal statistical analysis indicates that the correlation of the GW data with the DFT gap is more robust than the correlation with the experimental gaps; moreover we identify the static dielectric constant as alternative useful parameter for the approximation of GW gap in high-throughput automatic procedures. Finally, we compute the QP band structure and spectra within the random phase approximation and compare the results with available experimental data.

Structural stability and half-metallic ferromagnetism in PbMoO3: The role of electronic correlation

We theoretically investigate the electronic and magnetic properties of the recently reported cubic and orthorhombically (Ortho.) distorted phases of PbMoO3, a 4d transition-metal perovskite oxide with almost half-filled t2g states. Our spin-polarized bare generalized gradient approximation results exhibit that the cubic phase is of low energy structure than the Ortho. one. However, on-site Coulomb repulsion in the range of 3.3 eV ≤ Ueff ≤ 4.5 eV inclusion on Mo 4d orbitals reveals that at each value of Ueff, the Ortho. phase is more stable than the cubic one and with the increase in Ueff, both phases show more stability. We find a non-magnetic n-type conductivity with a high charge carrier density of ∼1022 cm−3 in both phases. Interestingly, a non-magnetic to magnetic phase transition occurs at Ueff = 3.8 eV and 3.5 eV for the cubic and Ortho. phases, respectively. Moreover, a half-metallic ferromagnetic state is obtained at Ueff = 4.1 eV and 4.3 eV for the cubic and Ortho. phases, respectively. Calculations also indicate a strong orbital hybridization between Pb 6p and Mo 4d, with a significant contribution of O 2p states. All findings are confirmed by the Yukawa screened Perdew-Burke-Ernzerhof 0 (YS-PBE0) hybrid functional. This work provokes further experimental investigations of magnetic properties of PbMoO3.

The Many Faces of Mixed Ion Perovskites: Unraveling and Understanding the Crystallization Process

Understanding the crystallization process of organic–inorganic halide perovskites is of paramount importance for fabrication of reproducible and efficient perovskite solar cells. We report for the first time on the discovery and interplay of ubiquitous hexagonal polytypes (6H and 4H) during the crystallization process of mixed ion perovskite, namely (FAPbI3)x(MAPbBr3)1–x. These polytypes, the first reported 3D hexagonal lead-halide-based perovskites, orchestrate a perovskite crystallization sequence revealed as 2H (delta phase)-4H-6H-3R(3C), commonly found among inorganic transition metal oxide perovskites under extreme conditions. We show that the chemical pressure arising from the incorporation of >3% Cs+ cations into the lattice successfully inhibits the formation of these environmentally sensitive polytypes, elucidating the origin of the widely reported improved device stability and reproducibility of Cs+-containing mixed ion perovskites.

(Invited) Cationic, Anionic & Electronic Disorder on Perovskite Oxide Surface for (Electro)Catalysis

Transition-metal perovskite oxides are being investigated as catalysts and electrocatalysts for reactions such as aqueous oxygen evolution and elevated-temperature oxygen incorporation. These oxides are characterized by their tendency to exhibit cationic, anionic and electronic disorder, such as Schottky defect pairs, oxygen vacancies, and small polarons, respectively. When present in significant concentrations, these defects alter the phase stability of the near-surface region, reactivity, and carrier transport. We have investigated a range of Ti, Co, and Fe-based materials using high-resolution transmission electron microscopy, in-situ X-ray photoemission spectroscopy and X-ray absorption spectroscopy. In this talk, I will discuss the role of these lattice disorder in catalysis and electrocatalysis involving oxygen molecules.

Electronic doping of transition metal oxide perovskites

CaFeO3 is a prototypical negative charge transfer oxide that undergoes electronic metal-insulator transition concomitant with a dilation and contraction of nearly rigid octahedra. Altering the charge neutrality of the bulk system destroys the electronic transition, while the structure is significantly modified at high charge content. Using density functional theory simulations, we predict an alternative avenue to modulate the structure and the electronic transition in CaFeO3. Charge distribution can be modulated using strain-rotation coupling and thin film engineering strategies, proposing themselves as a promising avenue for fine tuning electronic features in transition metal-oxide perovskites.

Erratum: Phase transitions via selective elemental vacancy engineering in complex oxide thin films.

Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, O vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understanding the role of defects in complex oxides have been largely limited to controlling O vacancies. In this study, we report on the selective formation of different types of elemental vacancies and their individual roles in determining the atomic and electronic structures of perovskite SrTiO3 (STO) homoepitaxial thin films fabricated by pulsed laser epitaxy. Structural and electronic transitions have been achieved via selective control of the Sr and O vacancy concentrations, respectively, indicating a decoupling between the two phase transitions. In particular, O vacancies were responsible for metal-insulator transitions, but did not influence the Sr vacancy induced cubic-to-tetragonal structural transition in epitaxial STO thin film. The independent control of multiple phase transitions in complex oxides by exploiting selective vacancy engineering opens up an unprecedented opportunity toward understanding and customizing complex oxide thin films.

Phase transitions via selective elemental vacancy engineering in complex oxide thin films.

Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, O vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understanding the role of defects in complex oxides have been largely limited to controlling O vacancies. In this study, we report on the selective formation of different types of elemental vacancies and their individual roles in determining the atomic and electronic structures of perovskite SrTiO3 (STO) homoepitaxial thin films fabricated by pulsed laser epitaxy. Structural and electronic transitions have been achieved via selective control of the Sr and O vacancy concentrations, respectively, indicating a decoupling between the two phase transitions. In particular, O vacancies were responsible for metal-insulator transitions, but did not influence the Sr vacancy induced cubic-to-tetragonal structural transition in epitaxial STO thin film. The independent control of multiple phase transitions in complex oxides by exploiting selective vacancy engineering opens up an unprecedented opportunity toward understanding and customizing complex oxide thin films.

Spatial distribution of transferred charges across the heterointerface between perovskite transition metal oxides LaNiO3 and LaMnO3

To investigate the interfacial charge-transfer phenomena between perovskite transition metal oxides LaNiO3 (LNO) and LaMnO3 (LMO), we have performed in situ x-ray absorption spectroscopy (XAS) measurements on LNO/LMO multilayers. The Ni-L2,3 and Mn-L2,3 XAS spectra clearly show the occurrence of electron transfer from Mn to Ni ions in the interface region. Detailed analysis of the thickness dependence of these XAS spectra has revealed that the spatial distribution of the transferred charges across the interface is significantly different between the two constituent layers. The observed spatial distribution is presumably described by the charge spreading model that treats the transfer integral between neighboring transition metal ions and the Coulomb interaction, rather than the Thomas–Fermi screening model.