Search

Rare earth nickelates including LaNiO3 are promising catalysts for water electrolysis to produce oxygen gas. Recent studies report that Fe substitution for Ni can significantly enhance the oxygen evolution reaction (OER) activity of LaNiO3. However, the role of Fe in increasing the activity remains ambiguous, with potential origins that are both structural and electronic in nature. On the basis of a series of epitaxial LaNi1-xFexO3 thin films synthesized by molecular beam epitaxy, we report that Fe substitution tunes the Ni oxidation state in LaNi1-xFexO3 and a volcano-like OER trend is observed, with x = 0.375 being the most active. Spectroscopy and ab initio modeling reveal that high-valent Fe3+δ cationic species strongly increase the transition-metal (TM) 3d bandwidth via Ni-O-Fe bridges and enhance TM 3d-O 2p hybridization, boosting the OER activity. These studies deepen our understanding of structural and electronic contributions that give rise to enhanced OER activity in perovskite oxides.

Two-dimensional materials exhibit properties promising for novel applications. Topologically protected states at their edges can be harnessed for use in quantum devices. We use ab initio simulations to examine properties of edges in 1T'-WTe2 monolayers, known to exhibit topological order, and their interactions with Cu atoms. Comparison of (010)-oriented edges that have the same composition but different terminations shows that, as the number of Cu atoms increases, their thermodynamically preferred arrangement depends on the details of the edge structure. Cu atoms aggregate into a cluster at the most stable edge; while the cluster is nonmagnetic, it spin-polarizes the W atoms along the edge, which removes the topological protection. At the metastable edge, Cu atoms form a chain incorporated into the WTe2 lattice; the topological state is preserved in spite of the dramatic edge restructuring. This suggests that exploiting interactions of metal species with metastable edge terminations can provide a path toward noninvasive interfaces.

Chromium iodide monolayers, which have different magnetic properties in comparison to the bulk chromium iodide, have been shown to form skyrmionic states in applied electromagnetic fields or in Janus-layer devices. In this work, we demonstrate that spin-canted solutions can be induced into monolayer chromium iodide by select substitution of iodide atoms with isovalent impurities. Several concentrations and spatial configurations of halide substitutional defects are selected to probe the coupling between the local defect-induced geometric distortions and orientation of chromium magnetic moments. This work provides atomic-level insight into how atomically precise chemical doping can be used to create and control complex magnetic patterns in chromium iodide layers and lays out the foundation for investigating the field- and geometric-dependent magnetic properties in similar two-dimensional materials.

We report the formation of neutral nitrogen molecules in the cages of [Ca12Al14O32]2+ (C12A7) framework compensated by extra-framework anions. NH3 treatment of C12A7 electride (C12A7:e-) at 800 °C leads to the formation of N2 and NH2- species in the C12A7 cages. N2 and NHx species in the cages are identified using the Raman spectroscopy of 14NH3 and 15NH3-treated C12A7:e-. The concentration of H and N in the C12A7 cages after NH3 treatment is ∼1021 cm-3. We propose a two-step mechanism, supported by density functional theory (DFT) modeling, of N2 incorporation into the C12A7 cages, i.e., incorporation of NH2- formed from decomposition of NH3 at C12A7:e- surface followed by the NH2- species reacting to form N2 molecules. Encapsulation of neutral molecules, as opposed to negatively charged species reported in C12A7 previously, offers new opportunities for trapping and storing gaseous substances in nanoporous materials.

Internal electric fields that underpin functioning of multi-component materials systems and devices are coupled to structural and compositional inhomogeneities associated with interfaces in these systems. Hard-x-ray photoelectron spectroscopy is a valuable source of information on band-edge profiles, governed by the distribution of internal fields, deep inside semiconductor thin films and heterojunctions. However, extracting this information requires robust and physically meaningful decomposition of spectra into contributions from individual atomic planes. We present an approach that utilizes the physical requirements of a monotonic dependence of the built-in electrostatic potential on depth and continuity of the potential function and its derivatives. These constraints enable efficient extraction of band-edge profiles and allow one to capture details of the electronic structure, including determination of the signs and magnitudes of the band bending as well as the valence band offsets. The utility of this approach to generate quantitative insight into the electronic structure of complex materials is illustrated for epitaxial $$\hbox {SrTiO}_3$$SrTiO3 on intrinsic Si(001).

Efficient ammonia synthesis and decomposition processes under mild conditions are important to meet the expanding demand in major applications of ammonia as the energy carrier and to provide feedst...

Neural network potentials (NNPs) can greatly accelerate atomistic simulations relative to ab initio methods, allowing one to sample a broader range of structural outcomes and transformation pathways. In this work, we demonstrate an active sampling algorithm that trains an NNP that is able to produce microstructural evolutions with accuracy comparable to those obtained by density functional theory, exemplified during structure optimizations for a model Cu–Ni multilayer system. We then use the NNP, in conjunction with a perturbation scheme, to stochastically sample structural and energetic changes caused by shear-induced deformation, demonstrating the range of possible intermixing and vacancy migration pathways that can be obtained as a result of the speedups provided by the NNP. The code to implement our active learning strategy and NNP-driven stochastic shear simulations is openly available at https://github.com/pnnl/Active-Sampling-for-Atomistic-Potentials .

We demonstrate that the interfacial dipole associated with bonding across the $\mathrm{SrTi}{\mathrm{O}}_{3}/\mathrm{Si}$ heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into $\mathrm{SrTi}{\mathrm{O}}_{3}$. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions.

Rare earth nickelates including LaNiO

We investigate the structure, thermodynamic stability, and electronic properties of (001), (110), and (111) surfaces of mayenite (12CaO·7Al2O3 or C12A7) in its oxygen-deficient phase known as mayenite-derived electride Ca12Al14O32 using simulations based on the density functional theory. Seven types of surface terminations were identified, depending on the orientation of the lattice nanosize cages with respect to the surface plane. We find that the surface termination has a strong effect on the distribution of electron anions in the near-surface region, as demonstrated by the range of the ionic charges and binding energies of adsorbed Au atoms and dimers. Notably, in all cases, Au species acquire a negative charge and preferentially adsorb as isolated ions rather than aggregate into clusters. These results show that C12A7 electride can provide a simple, yet versatile substrate to achieve high adsorption capacity and steric selectivity for single atom catalysts.

Two-dimensional materials exhibit properties promising for novel applications. Topologically protected states at their edges can be harnessed for use in quantum devices. We use

Since the advent of the first computers, chemists have been at the forefront of using computers to understand and solve complex chemical problems. As the hardware and software have evolved, so have the theoretical and computational chemistry methods and algorithms. Parallel computers clearly changed the common computing paradigm in the late 1970s and 80s, and the field has again seen a paradigm shift with the advent of graphical processing units. This review explores the challenges and some of the solutions in transforming software from the terascale to the petascale and now to the upcoming exascale computers. While discussing the field in general, NWChem and its redesign, NWChemEx, will be highlighted as one of the early codesign projects to take advantage of massively parallel computers and emerging software standards to enable large scientific challenges to be tackled.

Forced mixing to a single phase or supersaturated solid solution (SSS) and its prerequisite microstructure evolution in immiscible systems has been a focus of research for both fundamental science and a variety of applications. Controlling the formation of SSS by shear deformation assisted processing could enable a material design beyond conventional equilibrium microstructure in immiscible systems. Here, a highly immiscible (mixing enthalpy of ~ 20 kJ·mol-1) Cu-50 at. % Cr binary alloy was employed to investigate the microstructure evolution and localized tendencies of SSS during severe shear deformation. The present results demonstrate distinctive defect mediated microstructure refinement process in each phase and how it leads to localized SSS as a function of shear strain. Preferential dynamic recrystallization occurs in the softer Cu phase due to strain localization, leading to substantial grain refinement. The refinement of Cr phase in the top-most layer, however, is enabled by the progressive evolution of grain lamination, splitting, and fragmentation as a function of shear strain. The eventual SSS is found to be strongly dependent on the local environments that affect the dislocation activity, including the level of microstructure refinement, the interfacial orientation relationship, hardness difference, and supposed stability of oxidation. Ab initio simulations confirm that it is more favorable to oxidize Cr than Cu at incoherent Cu/Cr interfaces which then limits the mass transport on an incoherent boundary. Our results shed light on the underpinning mechanism for non-equilibrium mass transport in immiscible systems upon severe deformation that can be applicable to a variety of processing techniques aimed at producing immiscible alloys with superior mechanical properties.

The starting alloy microstructure can be tailored to achieve varying degrees of grain refinement and enhance mechanical properties through severe plastic shear deformation during solid-phase processing. Crystal plasticity-based grain misorientation modeling, coupled with systematic pin-on-disk tribometry-based subsurface shear deformation experiments on as-cast Al-xSi alloys (x = 0, 1, 4 at %), was conducted. The post-deformation microstructural analysis, through a combined computational and experimental approach, conclusively shows that the initial volume fraction of the hard Si phase enhances the evolution of local lattice misorientation, leading to efficient grain refinement during severe plastic shear deformation. The shear-deformation–induced nanostructure resulted in more than double the nanoindentation hardness in the processed alloy.

Extreme shear deformation is used for several material processing methods and is unavoidable in many engineering applications in which two surfaces are in relative motion against each other while in physical contact. The mechanistic understanding of the microstructural evolution of multi-phase metallic alloys under extreme shear deformation is still in its infancy. Here, we highlight the influence of shear deformation on the microstructural hierarchy and mechanical properties of a binary as-cast Al-4 at.% Si alloy. Shear-deformation-induced grain refinement, multiscale fragmentation of the eutectic Si-lamellae, and metastable solute saturated phases with distinctive defect structures led to a two-fold increase in the flow stresses determined by micropillar compression testing. These results highlight that shear deformation can achieve non-equilibrium microstructures with enhanced mechanical properties in Al–Si alloys. The experimental and computational insights obtained here are especially crucial for developing predictive models for microstructural evolution of metals under extreme shear deformation. Extreme deformation of alloys is important for processing and applications. Here, extreme shear deformation of an Al–Si alloy induces grain refinement, multi-scale fragmentation of lamellae and generation of various defects, enhancing the mechanical properties of micropillars.

Understanding and controlling heat transfer across interfaces has become an important issue for the performance of micro- and nanoscale electronics, as well as achieving a high figure of merit for thermoelectrics. Intrinsic and extrinsic defects can have a significant impact on thermal transport in bulk materials and across interfaces, but the mechanism is not well understood. In this work, nonequilibrium molecular dynamics simulations are used to determine the impact of interfacial atomic vacancies on thermal transport across a Cu/Si junction. In contrast to the reduction in thermal transport typically seen with bulk defects, we find that by introducing atomic vacancies at a concentration of 6.3% near the interface in either or both materials, the interfacial thermal conductance can be increased by up to 76%. By controlling the initial positions of the vacancies and keeping track of their movements and population, we find that interfacial thermal transport is dependent on temperature, vacancy concentration, and distribution, and a positive correlation between the conductance and point defect activities (extent of vacancy migration, rate of Frenkel defect creation and annihilation) is observed. Further calculations based on the phonon density of states and normal mode decomposition reveal that the increase in interfacial thermal conductance originates primarily from high-frequency phonons, supported by enhanced inelastic phonon transport which contributes to more than 60% of the increase. Our findings suggest a practical way to manipulate inelastic phonon conversion through the presence of defects, which provides an alternative perspective on improving thermal transport between materials with a large lattice mismatch.

Creating new functionality in materials containing transition metals is predicated on the ability to control the associated charge states. For a given transition metal, there is an upper limit on valence that is not exceeded under normal conditions. Here, it is demonstrated that this limit of 3+ for Ni and Fe can be exceeded via synthesis of (SrNiO

Two-dimensional stacking fault defects embedded in a bulk crystal can provide a homogeneous trapping potential for carriers and excitons. Here we utilize state-of-the-art structural imaging coupled with density-functional and effective-mass theory to build a microscopic model of the stacking-fault exciton. The diamagnetic shift and exciton dipole moment at different magnetic fields are calculated and compared with the experimental photoluminescence of excitons bound to a single stacking fault in GaAs. The model is used to further provide insight into the properties of excitons bound to the double-well potential formed by stacking fault pairs. This microscopic exciton model can be used as an input into models which include exciton-exciton interactions to determine the excitonic phases accessible in this system.

Solid-phase processing (SPP) allows one to create complex microstructures, not achievable via thermal processing alone. The resulting structures exhibit a rich palette of defects, both thermal and non-thermal, including defect substructures, such as dislocation networks. It is essential to understand the mechanisms of deformation and defect structure formation to guide SPP towards achieving desired microstructures and material properties. In this study, large-scale molecular dynamics simulations are used to investigate the effects of inhomogeneous strain distribution, that mimics deformation conditions of tribological tests, on the evolution of defects under severe shear deformation in polycrystalline Al. Analysis of defect nucleation and reaction pathways reveals that strong geometric constraints suppress the nucleation and slide of low energy dislocation 1/2⟨110⟩{111} but promote the nucleation and slide of high energy dislocations, such as 1 1 ¯ 0 (001) and 1/2 1 1 ¯ 2 ¯ (1 1 ¯ 1). A rough contact surface, characteristic to tribological tests, imposes an inhomogeneous stress field leading to inhomogeneous defect substructures due to location-dependent activation of slip systems. The results suggest that high-energy dislocations can dominate the evolution of grain structures in highly constrained environments, which should be considered in modeling plastic deformation and grain refinement during SPP.

Interfaces and their misfit defects determine the material properties of a wide range of applications, such as electronic devices, photocatalysts, and structural materials. However, current atomic-level understanding of interfacial structures is limited. Here we reveal that a special interfacial structure, a mismatched but oriented interface formed by two differently structured facets, gives rise to Ag3PO4 polypods at room temperature in aqueous solution. Transmission and scanning electron microscopy results suggest that interfaces of {1 0 0} and {1 1 0}({1 0 0}/(1 1 0}), {1 0 0}/{1 1 1}, {1 1 0}/{1 1 1}, and {1 0 0}/{1 0 0} have certain orientation relationships, corresponding to the structural energy minima and coincident site lattices of interfacial atoms, as demonstrated by molecular dynamics simulations. Density functional theory calculations indicate that the formation of pyrophosphates and/or phosphates rotation across the interface, as well as deformation of Ag-O bonds, compensate for the lattice mismatch at the interfaces. Our work opens a new avenue for a much wider range of interfacial structures, allow for a higher diversity of structures, and shines light on tailoring crystal structures, morphologies, and their resulting properties.

The starting alloy microstructure can be tailored to achieve varying degrees of grain refinement and enhance mechanical properties through severe plastic shear deformation during solid-phase processing. Crystal plasticity-based grain misorientation modeling, coupled with systematic pin-on-disk tribometry-based subsurface shear deformation experiments on as-cast Al-xSi alloys (x = 0, 1, 4 at%), was conducted. The post-deformation microstructural analysis, through a combined computational and experimental approach, conclusively shows that the initial volume fraction of the hard Si phase enhances the evolution of local lattice misorientation, leading to efficient grain refinement during severe plastic shear deformation. The shear-deformation–induced nanostructure resulted in more than double the nanoindentation hardness in the processed alloy.

The epitaxial growth of multifunctional oxides on semiconductors has opened a pathway to introduce new functionalities to semiconductor device technologies. In particular, the integration of gate materials that enable non-volatile or hysteretic functionality in field-effect transistors could lead to device technologies that consume less power, or allow for novel modalities in computing. Here we present electrical characterization of ultrathin single crystalline SrZrxTi1-xO3 (x = 0.7) films epitaxially grown on a high mobility semiconductor, Ge. Epitaxial films of SrZrxTi1-xO3 exhibit relaxor behavior, characterized by a hysteretic polarization that can modulate the surface potential of Ge. We find that gate layers as thin as 5 nm corresponding to an equivalent-oxide-thickness of just 1.0 nm exhibit a ~ 2 V hysteretic window in the capacitance-voltage characteristics. The development of hysteretic metal-oxide-semiconductor capacitors with nanoscale gate thicknesses opens new vistas for nanoelectronic devices.

We investigate the formation mechanisms of vacancy-ordered phase and collective mass transport in epitaxial SrCrO3−δ films using ab initio simulations within the density functional theory formalism. We show that as the concentration of oxygen vacancies (VO) increases, they form 1D chains that feature Cr-centered tetrahedra. Aggregation of these 1D VO chains results in the formation of (111)-oriented oxygen-deficient planes and an extended vacancy-ordered phase observed in recent experiments. We discuss atomic-scale mechanisms enabling the quasi-2D VO aggregates to expand along and translate across (111) planes. The corresponding lowest activation energy pathways necessarily involve rotation of Cr-centered tetrahedra, which emerges as a universal feature of fast ionic conduction in complex oxides. These findings explain reversible oxidation and reduction in SrCrO3−δ at low temperatures and provide insights into transient behavior necessary to harness ionic conductive oxides for high-performance and low-tem...

We explore how the structure and composition of the terminal layer of $n\text{\ensuremath{-}}\mathrm{SrTi}{\mathrm{O}}_{3}(001)$ determine key surface electronic properties. We have measured and calculated from first principles the absolute work functions and electron affinities of bulk $\mathrm{SrN}{\mathrm{b}}_{0.01}\mathrm{T}{\mathrm{i}}_{0.99}{\mathrm{O}}_{3}(001)$ terminated along the $\mathrm{Ti}{\mathrm{O}}_{2}$ and SrO planes. The match between theory and experiment is quite satisfactory for the $\mathrm{Ti}{\mathrm{O}}_{2}$ termination if an ideal, bulk-truncated surface structure is assumed. In contrast, the ideal SrO termination leads to a calculated work function considerably lower than the experimental value. We show that this discrepancy can be associated with defects on the SrO surface that act as electron scavengers. These defects deplete the concentration of itinerant electrons in the subsurface region and increase the negative charge density on the surface, thus increasing the work function. Several different surface defect configurations were modeled; the ones that yield the best agreement with experiment involve Sr vacancies in the terminal layer along with ${\mathrm{O}}^{2\ensuremath{-}}$, $\mathrm{O}{\mathrm{H}}^{\ensuremath{-}}/{\mathrm{H}}^{\ensuremath{-}}$ pairs or ${{\mathrm{O}}_{2}}^{2\ensuremath{-}}$ occupying anion sites adjacent to the Sr vacancies.

We report charge transfer and built-in electric fields across the epitaxial SrNb_{x}Ti_{1-x}O_{3-δ}/Si(001) interface. Electrical transport measurements indicate the formation of a hole gas in the Si and the presence of built-in fields. Hard x-ray photoelectron measurements reveal pronounced asymmetries in core-level spectra that arise from these built-in fields. Theoretical analysis of core-level spectra enables built-in fields and the resulting band bending to be spatially mapped across the heterojunction. The demonstration of tunable charge transfer, built-in fields, and the spatial mapping of the latter, lays the groundwork for the development of electrically coupled, functional heterojunctions.

We have investigated the electronic and optical properties of epitaxial $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Fe}{\mathrm{O}}_{3}$ for $0\ensuremath{\le}x\ensuremath{\le}1$ prepared by molecular-beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing $x$, indicating downward movement of the Fermi level toward the valence-band maximum. Combining valence-band photoemission and O $K$-edge x-ray absorption data, we map the evolution of the occupied and unoccupied bands and observe a narrowing of the gap, along with a transfer of state density from just below to just above the Fermi level as a result of hole doping. In-plane transport measurements confirm that the material becomes a $p$-type semiconductor at lower doping levels and exhibits a conversion from semiconducting to metallic behavior at $x=1$. Low-energy optical transitions revealed by spectroscopic ellipsometry are explained based on insight from theoretical densities of states and first-principles calculations of optical absorption spectra.

Metastable orientation relationships (ORs) between Cu and Cr are stabilized via epitaxial thin film deposition, with the initial layer of Cu(001) or Cr(001) grown epitaxially on MgO(001). The Bain OR is observed by x-ray diffraction and scanning/transmission electron microscopy for Cu(001) / Cr(001). In contrast, three Cr/Cu ORs are found for Cr deposition on Cu(001): the Pitsch OR, and two previously unreported ORs related to the Bain and Pitsch ORs, respectively. Ab initio calculations predict the energetics of these metastable ORs, and reveal that the deformation resistance of Cr makes the Bain OR energetically unfavorable when Cr films are deposited on Cu(001). These results show that kinetic constraints imposed by controlling the substrate surface and deposition conditions can lead to metastable interfacial structures.

Emergent phenomena at complex oxide interfaces could provide the basis for a wide variety of next-generation devices, including photovoltaics and spintronics. To date, detailed characterization and computational modeling of interfacial defects, cation intermixing, and film stoichiometry have helped to explain many of the novel behaviors observed at a single heterojunction. Unfortunately, many of the techniques employed to characterize a single heterojunction are less effective for a superlattice made up of a repeating series of interfaces that induce collective interfacial phenomena throughout a film. These repeating interfaces present an untapped opportunity to introduce an additional degree of complexity, such as confined electric fields, that cannot be realized in a single heterojunction. In this work, we explore the properties of SrTiO3–LaCrO3 superlattices to understand the role of defects, including variations in cation stoichiometry of individual layers of the superlattice, intermixing across inter...

The electronic and optical properties of α-(Fe1-xVx)2O3 at low (x = 0.04) and high (x = 0.5) doping levels are investigated using a combination of periodic and embedded cluster approaches, and time-dependent density functional theory. At low V concentrations the onset of the optical absorption is ∼0.5 eV (i.e., nearly 1.6 eV lower than that in pure α-Fe2O3) and corresponds to the electron transitions from V 3d to Fe 3d* orbitals. At high V concentrations, optical absorption energies and intensities are sensitive to specific arrangements of Fe and V atoms and their spin configuration that determine Fe-V hybridization. The onset of the lowest inter-vanadium absorption band in the case of Fe2O3/V2O3 hetero-structures is as low as ∼0.3 eV and the corresponding peak is at ∼0.7 eV. In contrast, in the case of solid solutions this peak has lower intensity and is shifted to higher energy (∼1.2 eV). Analysis of the orbital character of electronic excitation suggests that Fe2O3/V2O3 hetero-structures absorb light much more effectively than random alloys, thus promoting efficient photo-induced carrier generation. These predictions can be tested in α-(Fe1-xVx)2O3 thin films synthesized with well-controlled spatial distribution of Fe and V species.

Structure, thermodynamic stability, and electronic properties of CaH2 surfaces in (001), (110), and (111) crystallographic orientations are investigated using ab initio modeling. We show that stoichiometric surfaces terminated with a hydrogen atomic plane are the most energetically favorable and discuss properties of hydrogen vacancies (VH) at these surfaces. The average calculated work function of the most stable pristine surfaces (∼5.2 eV) is in agreement with experimental data for powder samples. Neutral hydrogen vacancies host localized electrons and induce defect states in the band gap, thereby shifting the effective work function to much lower values of ∼2.7 eV. Surface VH are predicted to aggregate into dimers and form electron-rich centers (e−)Ca2+(e−) stable to over 800 K. These results suggest that hydrogen-deficient surfaces of CaH2 can host a large concentration of localized electrons and, thus, give rise to new catalytic functionalities involving electron transfer between the surface, catalysts supported on it, and reacting species.

Double perovskites of the form R2BB′O6 (where R is a rare earth cation and B and B′ are chemically distinct transition metal cations with half-filled and empty eg orbitals, respectively) are of significant interest for their magnetoelectric properties. La2MnNiO6 is particularly attractive because of its large expected ferromagnetic moment per formula unit (5 μB f.u.–1) and its semiconducting character. If the ideal structure nucleates, superexchange coupling can take place via the B—O—B′ bonds that form, and the moment per formula unit can attain its maximum theoretical value. However, we show that even in the case of layer-by-layer deposition via molecular beam epitaxy, the system can follow multiple reaction pathways that lead to deviations from the double perovskite structure. In particular, we observe a spatially extended phase in which B-site cation disorder occurs, resulting in Mn—O—Mn and Ni—O—Ni antiferromagnetic domains, as well as the formation of quasi-epitaxial, antiferromagnetic NiO nanoscale...

Creating new functionality in materials containing transition metals is predicated on the ability to control the associated charge states. For a given transition metal, there is an upper limit on valence that is not exceeded under normal conditions. Here, it is demonstrated that this limit of 3+ for Ni and Fe can be exceeded via synthesis of (SrNiO3 )m /(LaFeO3 )n superlattices by tuning n and m. The Goldschmidt tolerance constraints are lifted, and SrNi4+ O3 with holes on adjacent O anions is stabilized as a perovskite at the single-unit-cell level (m = 1). Holding m = 1, spectroscopy reveals that the n = 1 superlattice contains Ni3+ and Fe4+ , whereas Ni4+ and Fe3+ are observed in the n = 5 superlattice. It is revealed that the B-site cation valences can be tuned by controlling the magnitude of the FeO6 octahedral rotations, which, in turn, determine the energy balance between Ni3+ /Fe4+ and Ni4+ /Fe3+ , thus controlling emergent electrical properties such as the band alignment and resulting hole confinement. This approach can be extended to other systems for synthesizing novel, metastable layered structures with new functionalities.

This chapter reviews a class of methods that allow for high accuracy and lift the constraints imposed by the periodic boundary conditions. Known under the generic name of the embedded cluster approach, this group of methods stems from the molecular perspective on matter, i.e., all materials are finite and can be represented using a finite collections of atoms, subjected to the boundary conditions that reproduce the rest of the system that is not necessarily periodic. We then give a few examples of using these methods in materials modeling and offer an outlook for the future.

Antiphase boundaries (APBs) are observed in as-synthesized and processed LiCoO