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Abstract Many of the studies on the entropy‐stabilized oxide (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O have been heavily application‐based. Previous works have studied effects of cation stoichiometry on the entropy‐driven reaction to form a single phase, but a fundamental exploration of the effects of anion stoichiometry and/or redox chemistry on electrical properties is lacking. Using near‐edge X‐ray absorption fine structure (NEXAFS) and electrical measurements, we show that oxidizing thin film samples of (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O affects primarily the valence of Co, leaving the other cations in this high‐entropy system unchanged. This oxidation increases electrical conduction in these thin films, which occurs via small polaron hopping mediated by the Co valence shift from 2+ to a mixed 2+/3+ state. In parallel, we show that bulk samples sintered in an oxygen‐rich atmosphere have a lower activation energy for electrical conduction than those equilibrated in a nitrogen (reducing) atmosphere. Combining feasible defect compensation scenarios with electrical impedance measurements and NEXAFS data, we propose a self‐consistent interpretation of Co redox‐mediated small polaron conduction as the dominant method of charge transfer in this system. 

Abstract Ternary metal‐oxide material systems commonly crystallize in the perovskite crystal structure, which is utilized in numerous electronic applications. In contrast to oxides, there are no known nitride perovskites, likely due to the competition with oxidation, which makes the formation of pure nitride materials difficult and synthesis of oxynitride materials more common. While deposition of oxynitride perovskite thin films is important for many electronic applications, it is difficult to control oxygen and nitrogen stoichiometry. Lanthanum tungsten oxynitride (LaWN_3−δO_δ) thin films with varying La:W ratio are synthesized by combinatorial sputtering and characterized for their chemical composition, crystal structure, and microstructure. A three‐step synthesis method, which involves co‐sputtering, capping layer deposition, and rapid thermal annealing, is established for producing crystalline thin films while minimizing the oxygen content. Elemental depth profiling results show that the cation‐stoichiometric films contain approximately one oxygen for every five nitrogen (δ = 0.5). Synchrotron‐based diffraction indicates a tetragonal perovskite crystal structure. These results are discussed in terms of the perovskite tolerance factors, octahedral tilting, and bond valence. Overall, this synthesis and characterization is expected to pave the way toward future thin film property measurements of lanthanum tungsten oxynitrides and eventual synthesis of oxygen‐free nitride perovskites. 

The bismuth-based perovskites are an interesting class of materials that exhibit a variety of coupled ferroic properties. Through epitaxial growth in the (001) pseudo-cubic [(001)pc] orientation, various phases with variable ferroelectric polarization can be stabilized. Using density-functional theory calculations, we predict the phase stability of the bismuth-based perovskite oxides as a function of pseudo-cubic in-plane (IP) lattice constant, mimicking (001)pc epitaxial films. We find that the BiMnO3, BiCoO3, and BiNiO3 systems each exhibit only one stable phase over a wide range of IP lattice constants. In the BiFeO3 (BFO) and BiCrO3 (BCO) systems, by comparison, we find several phases that are energetically favorable, depending on the value of the IP strain. The BFO phases predicted to be stable, in order of increasing compressive IP strain, are monoclinic Cc, triclinic P1, monoclinic Cm, and tetragonal P4mm. In the BCO system, we find two orthorhombic Pbnm phases, respectively, under no IP strain and under compressive IP strain, and one monoclinic Cc phase to be stable under tensile IP strain. Our results serve to guide experimental efforts in terms of selecting growth substrates with the goal of achieving desired epitaxial-stabilized perovskite phases and to support future investigations of the tunability of BXO properties with epitaxial strain. 

Piezoelectric materials are commonplace in modern devices, and the prevalence of these materials is poised to increase in the years to come. The majority of known piezoelectrics are oxide materials, due in part to the related themes of a legacy of ceramists building off of mineralogical crystallography and the relative simplicity of fabricating oxide specimens. However, diversification beyond oxides offers exciting opportunities to identify and develop new materials perhaps better suited for certain applications. Aluminum nitride (and recently, its Sc-modified derivative) is the only commercially integrated piezoelectric nitride in use today, although this is likely to change in the near future with increased use of high-throughput techniques for materials discovery and development. This review covers modern methods—both computational and experimental—that have been developed to explore chemical space for new materials with targeted characteristics. Here, the authors focus on the application of computational and high-throughput experimental approaches to discovering and optimizing piezoelectric nitride materials. While the focus of this review is on the search for and development of new piezoelectric nitrides, most of the research approaches discussed in this article are both chemistry- and application-agnostic.