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Active radiation environments and high-rate backgrounds pose a significant challenge towards improving the detection and characterization of Special Nuclear Material (SNM) using Associated Particle Imaging (API). Ongoing research seeks to reduce the sensitivity of a charged particle, or associated particle, detector to x-ray backgrounds through the creation and characterization of an alpha-sensitive, Ce-doped surface region of Yttrium Aluminium Perovskite (YAP). A wet-chemical layer of trivalent cerium (III) acetylacetonate hydrate was thermally diffused with a reducing atmosphere into single YAP crystals for a temperature range of 800 °C to 1500 °C. The diffusion concentration profile of the cerium compound was assayed with Rutherford Backscattering Spectrometry (RBS), where the Ce diffusion activation energy and the pre-exponential factor within YAP were found to be 1.02 ± 0.10 eV and 9.7 ± 2.0 × 10 - 15 m 2 s - 1 , respectively. Resulting transient polycrystalline structural features at the surface of the YAP crystals were investigated with X-Ray Diffraction (XRD). YAP, Yttrium Aluminium Garnet (YAG), and Yttrium Aluminium Monoclinic (YAM) phases were identified, as well as contributions from Y2O3, CeO2, Ce2O3, and the metastable hexagonal polymorph of YAlO3 (YAH). The luminescent responses of the complex surface layers to x-ray and α excitement were examined as functions of annealing temperature. Radioluminescence emissions typical of the Ce3+ 4f → 5d transition occupying both the Y3+ and Al3+ sites in YAP and YAG were observed. The brightness of the samples under alpha radiation increased as a function of diffused Ce concentration until the point of optical darkening.

In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and Sr+2 at concentrations known to induce superconductivity in the simple parent compounds, Nd2CuO4 and La2CuO4. Electron doped (La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped (La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be single crystal, epitaxially strained, and highly uniform. Transport measurements demonstrate that all as-grown films are insulating regardless of doping. Annealing studies show that resistivity can be tuned by modifying oxygen stoichiometry and inducing metallicity but without superconductivity. These results in turn are connected to extended x-ray absorption fine structure (EXAFS) results indicating that the lack of superconductivity in the high entropy cuprates likely originates from a large distortion within the Cu-O plane ({\sigma}2>0.015 {\AA}2) due to A-site cation size variance, which drives localization of charge carriers. These findings describe new opportunities for controlling charge- and orbital-mediated functional responses in Ruddlesden-Popper crystal structures, driven by balancing of cation size and charge variances that may be exploited for functionally important behaviors such as superconductivity, antiferromagnetism, and metal-insulator transitions, while opening less understood phase spaces hosting doped Mott insulators, strange metals, quantum criticality, pseudogaps, and ordered charge density waves.

The influence of hole doping on magnetic properties is mapped for the compositionally complex high-entropy oxide ${AB\mathrm{O}}_{3}$ perovskite ${\mathrm{La}}_{1\text{\ensuremath{-}}x}{\mathrm{Sr}}_{x}({\mathrm{Cr}}_{0.2}{\mathrm{Mn}}_{0.2}{\mathrm{Fe}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Ni}}_{0.2}){\mathrm{O}}_{3}$ $(0\ensuremath{\le}x\ensuremath{\le}0.5)$. It is found that aliovalent A-site substitution is a viable means to manipulate the magnetically active B-site sublattice. A series of single-crystal films are synthesized and show a general trend toward stronger ferromagnetic response and a shift in magnetic anisotropy as the Sr concentration increases. Magnetometry demonstrates complex and nonuniform responses similar to rigid and uncoupled composites at intermediate dopings. This behavior points to the presence of locally inhomogeneous magnetic phase competition, where ferromagnetic and antiferromagnetic magnetic contributions create a frustrated matrix containing uncompensated spins at the boundaries between these regions. The observations are discussed in the context of known responses to hole doping in the less complex ternary $\mathrm{La}{T}_{M}{\mathrm{O}}_{3}$ $({T}_{M}=\mathrm{Cr},\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni})$ oxides, and they are found to be different from a simple sum of the doped parents. The results are summarized in a preliminary magnetic phase diagram.

In this study, elastic constants of the equiatomic body-centered cubic NbHfZrTi high-entropy alloy (HEA) were experimentally evaluated using resonant ultrasound spectroscopy and compared directly with calculations based on density functional theory. Elastic properties, including Young's modulus, shear modulus, bulk modulus, Poisson's ratio, and Debye temperature, of polycrystalline aggregates were obtained from measurements and calculations, and excellent agreement was found between the experimental and theoretical data. We also made efforts to employ the rule-of-mixtures (ROM) to predict the elastic moduli of the current alloy, as well as other HEAs reported in the literature, and found that the lower-bound prediction provided a reasonable estimate of the elastic moduli of single-phase HEAs.

Relaxor ferrolectrics are important in technological applications due to a strong electromechanical response, energy storage capacity, electrocaloric effect, and pyroelectric energy conversion properties. Current efforts to discover and design new materials in this class generally rely on substitutional doping of known ferroelectrics, as slight changes to local compositional order can significantly affect the Curie temperature, morphotropic phase boundary, and electromechanical responses. In this work, we demonstrate that moving to the strong limit of compositional complexity in an ABO3 perovskite allows stabilization of novel relaxor responses that do not rely on a single narrow phase transition region. Entropy-assisted synthesis approaches are used to create single crystal Ba(Ti0.2Sn0.2Zr0.2Hf0.2Nb0.2)O3 [Ba(5B)O] films. The high levels of configurational disorder present in this system is found to influence dielectric relaxation, phase transitions, nano-polar domain formation, and Curie temperature. Temperature-dependent dielectric, Raman spectroscopy and second-harmonic generation measurements reveal multiple phase transitions, a high Curie temperature of 570 K, and the relaxor ferroelectric nature of Ba(5B)O films. The first principles theory calculations are used to predict possible combinations of cations to quantify the relative feasibility of formation of highly disordered single-phase perovskite systems. The ability to stabilize single-phase perovskites with such a large number of different cations on the B-sites offers new possibilities for designing high-performance materials for piezoelectric, pyroelectric and tunable dielectric applications.

Disorder can have a dominating influence on correlated and quantum materials leading to novel behaviors which have no clean limit counterparts. In magnetic systems, spin and exchange disorder can provide access to quantum criticality, frustration, and spin dynamics, but broad tunability of these responses and a deeper understanding of strong limit disorder is lacking. In this work, we demonstrate that high entropy oxides present an unexplored route to designing quantum materials in which the presence of strong local compositional disorder hosted on a positionally ordered lattice can be used to generate highly tunable emergent magnetic behavior--from macroscopically ordered states to frustration-driven dynamic spin interactions. Single crystal La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 films are used as a structurally uniform model system hosting a magnetic sublattice with massive microstate disorder in the form of site-to-site spin and exchange type inhomogeneity. A classical Heisenberg model is found to be sufficient to describe how compositionally disordered systems can paradoxically host long-range magnetic uniformity and demonstrates that balancing the populating elements based on their discrete quantum parameters can be used to give continuous control over ordering types and critical temperatures. Theory-guided experiments show that composite exchange values derived from the complex mix of microstate interactions can be used to design the required compositional parameters for a desired response. These predicted materials are synthesized and found to possess an incipient quantum critical point when magnetic ordering types are designed to be in direct competition; this leads to highly controllable exchange bias sensitivity in the monolithic single crystal films previously accessible only in intentionally designed bilayer heterojunctions.

Magnetic insulators are important materials for a range of next-generation memory and spintronic applications. Structural constraints in this class of devices generally require a clean heterointerface that allows effective magnetic coupling between the insulating layer and the conducting layer. However, there are relatively few examples of magnetic insulators that can be synthesized with surface qualities that would allow these smooth interfaces and precisely tuned interfacial magnetic exchange coupling, which might be applicable at room temperature. In this work, we demonstrate an example of how the configurational complexity in the magnetic insulator layer can be used to realize these properties. The entropy-assisted synthesis is used to create single-crystal (Mg

Single-phase solid-solution refractory high-entropy alloys (HEAs) show remarkable mechanical properties, such as their high yield strength and substantial softening resistance at elevated temperatures. Hence, the in-depth study of the deformation behavior for body-centered cubic (BCC) refractory HEAs is a critical issue to explore the uncovered/unique deformation mechanisms. We have investigated the elastic and plastic deformation behaviors of a single BCC NbTaTiV refractory HEA at elevated temperatures using integrated experimental efforts and theoretical calculations. The in situ neutron diffraction results reveal a temperature-dependent elastic anisotropic deformation behavior. The single-crystal elastic moduli and macroscopic Young's, shear, and bulk moduli were determined from the in situ neutron diffraction, showing great agreement with first-principles calculations, machine learning, and resonant ultrasound spectroscopy results. Furthermore, the edge dislocation-dominant plastic deformation behaviors, which are different from conventional BCC alloys, were quantitatively described by the Williamson-Hall plot profile modeling and high-angle annular dark-field scanning transmission electron microscopy.

The 2D layered Ruddlesden-Popper crystal structure can host a broad range of functionally important behaviors. Here we establish extraordinary configurational disorder in a two dimensional layered Ruddlesden-Popper (RP) structure using entropy stabilization assisted synthesis. A protype A2CuO4 RP cuprate oxide with five components (La, Pr, Nd, Sm, Eu) on the A-site sublattice is designed and fabricated into epitaxial single crystal films using pulsed laser deposition. By comparing (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 crystals grown under identical conditions but different substrates, it is found that heteroepitaxial strain plays an important role in crystal phase formation. When grown on a near lattice matched substrate, the high entropy oxide film features a T'-type RP structure with uniform A-site cation mixing and square-planar CuO4 units, however, growing under strong compressive strain results in a single crystal non-RP cubic phase consistent with a CuX2O4 spinel structure. These observations are made with a range of combined characterizations using X-ray diffraction, atomic-resolution scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray absorption spectroscopy measurements. Designing configurational complexity and moving between 2D layered RP and 3D cubic crystal structures in this class of cuprate materials opens many opportunities for new design strategies related to magnetoresistance, unconventional superconductivity, ferroelectricity, catalysis, and ion transport.

Configurational disorder can have a dominating role in the formation of macroscopic functional responses in strongly correlated materials. Here, we use entropy-stabilization synthesis to create single-crystal epitaxial $\mathrm{AB}{\mathrm{O}}_{3}$ perovskite thin films with equal atomic concentration of 3d transition-metal cations on the B-site sublattice. X-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy of $\mathrm{La}(\mathrm{C}{\mathrm{r}}_{0.2}\mathrm{M}{\mathrm{n}}_{0.2}\mathrm{F}{\mathrm{e}}_{0.2}\mathrm{C}{\mathrm{o}}_{0.2}\mathrm{N}{\mathrm{i}}_{0.2}){\mathrm{O}}_{3}$ (L5BO) films demonstrate excellent crystallinity, smooth film surfaces, and uniform mixing of the 3d transition-metal cations throughout the B-site sublattice. The magnetic properties are strongly dependent on substrate-induced lattice anisotropy and suggest the presence of long-range magnetic order in these exceptionally disordered materials. The ability to populate multiple elements onto a single sublattice in complex crystal structures opens new possibilities to design functionality in correlated systems and enable novel fundamental studies seeking to understand how diverse local bonding environments can work to generate macroscopic responses, such as those driven by electron-phonon channels and complex exchange interaction pathways.

Twelve multicomponent spinels, comprised of (Mg, Cr, Mn, Co, Fe, Ni, Cu, and/or Zn)${(\mathrm{Cr},\phantom{\rule{0.16em}{0ex}}\mathrm{Fe},\phantom{\rule{0.16em}{0ex}}\mathrm{or}\phantom{\rule{0.16em}{0ex}}\mathrm{Al})}_{2}{\mathrm{O}}_{4}$, were prepared using solid state synthesis methods, resulting in nine homogenous, single phase samples with a $Fm\ensuremath{-}3m$ structure, and three samples with multiple phases. Using dc magnetometry in conjunction with x-ray diffraction, scanning electron microscopy with energy dispersive x-ray spectroscopy, and x-ray absorption spectroscopy, the effects of multicomponent material design on the structural, magnetic, and chemical properties are explored. The ferritic spinel high-entropy oxide (HEO) samples show high-temperature ferrimagnetic transitions and both ferritic and chromium-based HEO spinel samples show evidence of low-temperature antiferromagnetic ordering. Blocking temperatures are evident in some samples and magnetic transition temperatures are reported. Constituent valence states and temperature dependent valence is described for the example case of $(\mathrm{M}{\mathrm{g}}_{0.2}\mathrm{F}{\mathrm{e}}_{0.2}\mathrm{C}{\mathrm{o}}_{0.2}\mathrm{N}{\mathrm{i}}_{0.2}\mathrm{C}{\mathrm{u}}_{0.2})\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$, including the unexpected presence of $\mathrm{C}{\mathrm{r}}^{4+}$, indicating a 2--4 type spinel configuration. Valence trends for two ferritic HEO spinels are also discussed. Some of these compositions are synthesized for the first time and this work provides an investigation into the magnetic properties of the novel class of cubic spinel multicomponent oxides showing interesting behavior that warrants further investigation.

A new class of ceramics, called entropy stabilized oxides, High Entropy Oxides (HEOs), multicomponent oxides, compositionally complex oxides, or polycation oxides, has generated considerable research interest since the first report in 2015. This multicomponent approach has created new opportunities for materials design and discovery. This Perspective will highlight some current research developments and possible applications while also providing an overview of the many successfully synthesized HEO systems to date. The polycation approach to composition development will be discussed along with a few case studies, challenges, and future possibilities afforded by this novel class of materials.

Examples of single-crystal epitaxial thin films of a high entropy perovskite oxide are synthesized. Pulsed laser deposition is used to grow the configurationally disordered $AB{\mathrm{O}}_{3}$ perovskite $\mathrm{Ba}(\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{S}{\mathrm{n}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.2}\mathrm{H}{\mathrm{f}}_{0.2}\mathrm{N}{\mathrm{b}}_{0.2}){\mathrm{O}}_{3}$ epitaxially on $\mathrm{SrTi}{\mathrm{O}}_{3}$ and MgO substrates. X-ray diffraction and scanning transmission electron microscopy demonstrate that the films are single phase with excellent crystallinity and atomically abrupt interfaces to the underlying substrates. Atomically resolved electron-energy-loss spectroscopy mapping shows a uniform and random distribution of all $B$-site cations. The ability to stabilize perovskites with this level of configurational disorder offers new possibilities for designing materials from a much broader combinatorial cation pallet while providing a fresh avenue for fundamental studies in strongly correlated quantum materials where local disorder can play a critical role in determining macroscopic properties.