Inorganic materials exhibiting second-harmonic generation (SHG) are used to generate coherent radiation at wavelengths where solid-state laser sources are not available; that is, the deep UV (DUV) ...below 200 nm. Here, we describe the structure and optical property requirements that should be assessed to conclusively demonstrate the discovery of a functional DUV material for nonlinear optical (NLO) applications.
Electronic structure calculations based on density functional theory have uncovered a novel mechanism for inducing ferroelectric polarizations in cation ordered perovskites. We outline a materials ...selection strategy for designing this behavior. The guidelines are based on the octahedral rotations found in the two constituent oxides and the way the perovskite building blocks are interwoven to form the superlattice.
During the last century, inorganic oxide compounds laid foundations for materials synthesis, characterization, and technology translation by adding new functions into devices previously dominated by ...main-group element semiconductor compounds. Today, compounds with multiple anions beyond the single-oxide ion, such as oxyhalides and oxyhydrides, offer a new materials platform from which superior functionality may arise. Here we review the recent progress, status, and future prospects and challenges facing the development and deployment of mixed-anion compounds, focusing mainly on oxide-derived materials. We devote attention to the crucial roles that multiple anions play during synthesis, characterization, and in the physical properties of these materials. We discuss the opportunities enabled by recent advances in synthetic approaches for design of both local and overall structure, state-of-the-art characterization techniques to distinguish unique structural and chemical states, and chemical/physical properties emerging from the synergy of multiple anions for catalysis, energy conversion, and electronic materials.
The burgeoning field of anion engineering in oxide‐based compounds aims to tune physical properties by incorporating additional anions of different size, electronegativity, and charge. For example, ...oxychalcogenides, oxynitrides, oxypnictides, and oxyhalides may display new or enhanced responses not readily predicted from or even absent in the simpler homoanionic (oxide) compounds because of their proximity to the ionocovalent‐bonding boundary provided by contrasting polarizabilities of the anions. In addition, multiple anions allow heteroanionic materials to span a more complex atomic structure design palette and interaction space than the homoanionic oxide‐only analogs. Here, established atomic and electronic principles for the rational design of properties in heteroanionic materials are contextualized. Also described are synergistic quantum mechanical methods and laboratory experiments guided by these principles to achieve superior properties. Lastly, open challenges in both the synthesis and the understanding and prediction of the electronic, optical, and magnetic properties afforded by anion‐engineering principles in heteroanionic materials are reviewed.
Heteroanionic materials are solid‐state inorganic compounds with multiple unique elements forming distinct anionic sublattices. A scheme is described to disentangle the structural and chemical degrees‐of‐freedom stimulated by the addition of a secondary anion and to understand and design novel heteroanionic materials with targeted properties. Technologically interesting oxyfluorides are reviewed within this framework.
The confluence of state‐of‐the‐art electronic‐structure computations and modern synthetic materials growth techniques is proving indispensable in the search for and discovery of new functionalities ...in oxide thin films and heterostructures. Here, we review the recent contributions of electronic‐structure calculations to predicting, understanding, and discovering new materials physics in thin‐film perovskite oxides. We show that such calculations can accurately predict both structure and properties in advance of film synthesis, thereby guiding the search for materials combinations with specific targeted functionalities. In addition, because they can isolate and decouple the effects of various parameters which unavoidably occur simultaneously in an experiment–such as epitaxial strain, interfacial chemistry and defect profiles–they are able to provide new fundamental knowledge about the underlying physics. We conclude by outlining the limitations of current computational techniques, as well as some important open questions that we hope will motivate further methodological developments in the field.
Electronic structure calculations based on density functional theory are proving to be powerful tools for disentangling complex and often interwoven interactions in complex oxide thin films. We survey recent theoretical contributions in understanding how local atomic distortions arise in epitaxial perovskite films, and in turn influence the electronic and magnetic properties. These ab initio derived atomic structure/electronic property relationships are essential to engineering multifunctionality into synthetic materials integrated into thin film device architectures.
The existence of ~30 noncentrosymmetric metals (NCSM) suggests a contraindication between crystal structures without inversion symmetry and metallic behaviour. Those containing oxygen are especially ...scarce. Here we propose and demonstrate a design framework to remedy this property disparity and accelerate NCSM oxide discovery. The primary ingredient relies on the removal of inversion symmetry through displacements of atoms whose electronic degrees of freedom are decoupled from the states at the Fermi level. Density functional theory calculations validate this crystal-chemistry strategy, and we predict a new polar ruthenate exhibiting robust metallicity. We demonstrate that the electronic structure is unaffected by the inclusion of spin-orbit interactions, and that cation-ordered SrCaRu2O6 exhibits a large thermopower anisotropy (|ΔS⊥|~6.3 μV K(-1) at 300 K) derived from its polar structure. Our findings provide chemical and structural selection guidelines to aid in the search of NCSM with enhanced thermopower anisotropy.
Phase transitions in ABX 3 perovskites are often accompanied by rigid rotations of the corner-connected BX 6 octahedral network. Although the mechanisms for the preferred rotation patterns of ...perovskite oxides are fairly well recognized, the same cannot be said of halide variants (i.e., X = Cl, Br, or I), several of which undergo an unusual displacive transition to a tetragonal phase exhibiting in-phase rotations about one axis (a 0 a 0 c + in Glazer notation). To discern the chemical factors stabilizing this unique phase, we investigated a series of 12 perovskite bromides and iodides using density functional theory calculations and compared them with similar oxides. We find that in-phase tilting provides a better arrangement of the larger bromide and iodide anions, which minimizes the electrostatic interactions, improves the bond valence of the A-site cations, and enhances the covalency between the A-site metal and Br– or I– ions. The opposite effect is present in the oxides, with out-of-phase tilting maximizing these factors.
The equilibrium structure and functional properties exhibited by brownmillerite oxides, a family of perovskite-derived structures with alternating layers of BO6 octahedra and BO4 tetrahedra, viz., ...ordered arrangements of oxygen vacancies, is dependent on a variety of competing crystal-chemistry factors. We use electronic structure calculations to disentangle the complex interactions in two ferrates, Sr2Fe2O5 and Ca2Fe2O5, relating the stability of the equilibrium (strain-free) and thin film structures to both previously identified and herein newly proposed descriptors. We show that cation size and intralayer separation of the tetrahedral chains provide key contributions to the preferred ground state. We show the bulk ground-state structure is retained in the ferrates over a range of strain values; however, a change in the orientation of the tetrahedral chains, i.e., a perpendicular orientation of the vacancies relative to the substrate, is stabilized in the compressive region. The structure stability under strain is largely governed by maximizing the intraplane separation of the dipoles generated from rotations of the FeO4 tetrahedra. Lastly, we find that the electronic band gap is strongly influenced by strain, manifesting as an unanticipated asymmetric-vacancy alignment dependent response. This atomistic understanding establishes a practical route for the design of functional electronic materials in thin film geometries.
Deep ultraviolet (absorption edge <200 nm, band gap >6.2 eV) nonlinear optical (NLO) materials are of current interest owing to their technological applications and materials design challenges. ...Technologically, the materials are used in laser systems, atto-second pulse generation, semiconductor manufacturing, and photolithography. Designing and synthesizing a deep UV NLO material requires crystallographic non-centrosymmetry, a wide UV transparency range, a large second-harmonic generating coefficient (d ij > 0.39 pm/V), moderate birefringence (Δn ∼ 0.07), chemical stability and resistance to laser damage, and ease in the growth of large high-quality single crystals. This review examines the known deep UV NLO materials with respect to their crystal structure, band gap, SHG efficiency, laser damage threshold, and birefringence. Finally, future directions with respect to new deep UV NLO materials are discussed.
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