arXiv subjects

Saeed S. I. Almishal

Publications and source records attributed to Saeed S. I. Almishal.

18 recordsLinked to original sources

arXiv2026

Epitaxial Oxide Thin Films with High Entropy and Anomalous Crystallinity

Pulsed laser deposition (PLD) expands the compositional and structural possibilities of high-entropy oxide (HEO) thin films through far-from-equilibrium growth and kinetic arrest of metastable configurations. This review connects growth kinetics and epitaxial constraints to the roles of individual cations in shaping the resulting films. We focus primarily on rock salt MgCoNiCuZnO and its derivatives, with selected fluorite, spinel, and perovskite examples illustrating broader applicability. In MgCoNiCuZnO, Co valence and Cu coordination illustrate how chemistry and growth conditions jointly enable lattice tuning and coherent nanostructure formation, while aliovalent and size-misfit additions expand the accessible defect and compositional landscapes. These coupled degrees of freedom underpin anomalous crystallinity, the coexistence of extensive chemical disorder with exceptional crystalline quality and epitaxial order, including full coherence in selected systems, particularly rock salts grown on MgO. This combination opens opportunities to discover and engineer functional configurations in high-quality single-layer films and heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Jahn-Teller Distortion-Driven Lattice Parameter Tunability in Coherent High-Entropy Oxide Thin Films

Here, we isolate how Cu concentration and Jahn-Teller distortions control the out-of-plane lattice parameter of pseudomorphic MgCoNiCuZnO-derived high-entropy oxide films on MgO. We prepare nine compositions spanning 0-20% Cu, verify target composition and phase by X-ray fluorescence and diffraction, and grow films by pulsed laser deposition across multiple substrate temperatures. At 500 °C, the lattice parameter increases nearly linearly with Cu content; at 300°C, it decreases nonlinearly, producing a 2.6% difference between temperature regimes for equimolar MgCoNiCuZnO at our growth conditions. We attribute this change in lattice parameter to temperature-dependent coupling between Cu-driven Jahn-Teller distortions and Co valence change. While prior work identifies Co valence as the primary driver of the MgCoNiCuZnO lattice response with temperature; our results establish Cu Jahn-Teller distortion as an equally important structural control parameter.

cond-mat.mtrl-sci↗

arXiv2026

Ionic Diffusion Properties of Rare-Earth High-Entropy Oxides from a Machine-Learned Interatomic Potential

Rare-earth high-entropy oxides (RE-HEOs) have emerged as a promising class of functional ceramics for solid-state electrochemical applications due to their chemical complexity, structural tunability, and potential for fast oxygen-ion transport. In this work, we investigate oxygen diffusion in ceria-based RE-HEOs of the form Ce$_x$(YLaPrSm)$_{1-x}$O$_{2-δ}$ using classical molecular dynamics simulations driven by the Crystal Hamiltonian Graph Neural Network (CHGNet) machine-learned interatomic potential. To improve predictive accuracy for lanthanide-containing systems, we benchmark three CHGNet variants, including a fine-tuned r$^2$SCAN-trained model, against targeted density functional theory (DFT) data that explicitly include f-valence electrons. Simulations across temperature, Ce content, oxygen vacancy concentration, and both fluorite and bixbyite structures reveal that oxygen transport in RE-HEOs is governed by the interplay of two factors: the concentration of mobile vacancies and the local cation environment through which they hop. At fixed composition, ionic conductivity exhibits a non-monotonic dependence on vacancy concentration, with optimal diffusion occurring at moderate vacancy levels and reduced mobility at higher concentrations. Increasing Ce content lowers migration activation energies and enhances diffusivity through low-barrier diffusion networks built from Ce-Ce and Ce-Y edges. Analysis of individual oxygen hopping events provides atomistic insight into how local chemical environments and short-range cation ordering govern transport in high-entropy oxides. Overall, this work demonstrates that machine-learned interatomic potentials can resolve composition-structure-transport relationships in chemically complex oxides, and identifies active pathways through which compositional tuning can enhance oxygen-ion conductivity in RE-HEO materials.

cond-mat.mtrl-sci↗

arXiv2026

Hydrothermally-Assisted Sintering of Calcium Hydroxide Sputtering Targets: A Route to Quantum-Grade CaO Thin Films

In this report we demonstrate dense polycrystalline calcium hydroxide ceramics fabricated by hydrothermally-assisted sintering - often referred to as cold-sintering - to produce high-purity calcium hydroxide targets for calcium oxide thin film deposition. Calcium hydroxide ceramics exhibit up to 98% theoretical density without thermal dehydration, when sintered at temperatures between 100 °C - 300° C with 400 MPa applied uniaxial pressure for 1 hour. The brucite phase is preserved in calcium hydroxide targets at all temperatures. Small equivalent fractions of calcium carbonate are present in both the calcium hydroxide precursor powder and final targets suggesting minimal additional production during formation and densification. Microstructure evolution during densification is documented by scanning electron microscopy, indicating both mass transport and plastic deformation densification mechanisms. The hydrothermal-assisted sintering process is scaled up to produce 2-inch diameter calcium hydroxide targets suitable for sputter deposition. We also report epitaxial calcium oxide film deposition from these targets on r-plane sapphire substrates. (002) oriented epitaxial films are achieved with a time-stable 1.2 nm per minute deposition rate. We note that energetic bombardment during growth can be substantial at these rates even when 1 mol% oxygen is added to the sputtering process necessitating the low deposition rate conditions.

cond-mat.mtrl-sci↗

arXiv2026

Anomalous Crystallinity and Magnetism in Chemically Disordered Coherent Heterostructures

High-entropy oxide (HEO) thin films uniquely superimpose exceptional chemical disorder with exceptional crystalline quality and coherence - an intersection we term anomalous crystallinity that arises from coupled structural, chemical, and valence degrees of freedom unique to the entropy-stabilized condition. Here, we demonstrate unexpected and predictive control of this state using formulation, epitaxial constraints, and kinetic arrest of metastable macrostates. Specifically, aliovalent cation substitutions, tightly controlled substrate temperatures, and conditions favoring significant adatom kinetic energy, can program the out-of-plane lattice parameter of coherent rock salt HEOs while preserving in-plane epitaxial pinning to MgO. Lattice strains exceeding 5% can be stabilized in multilayer heterostructures using this approach, where 3+ cations compensated by cation vacancies predominate the defect chemistry landscape. We highlight the exemplar (Sc,Mg,Co,Ni,Cu,Zn)O/(Cr,Mg,Co,Ni,Cu,Zn)O (JSc/JCr) system where Sc and Cr substitution into the rock salt structure produces pseudomorphic heterostructures between individual antiferromagnets exhibiting exceptional strain and abrupt interfaces across which the Co valence switches from mostly 2+ to an even 2+/3+ mixture. These unprecedented valence interfaces are accompanied by a 2x exchange bias boost compared to single-layer constituents, that could be attributed to enhanced uncompensated spins in the layers themselves or around the buried JSc/JCr interface. These results establish pseudomorphic valence interfaces with anomalous crystallinity as a source of new magnetic macrostates that host emergent magnetic and spintronic functionality.

cond-mat.mtrl-sci↗

arXiv2025

Compositional and Oxygen-Vacancy Effects on Phase Stability and Electronic Properties in Ceria-Based Lanthanide High-Entropy Oxides

Cerium-based lanthanide high-entropy oxides (LN-HEOs) are promising candidates for solid-state electrolyte (mass transport) applications due to their ability to accommodate high concentrations of oxygen vacancies while retaining a fluorite-derived structure. However, synthesis often yields undesired ordered oxygen-deficient phases, such as bixbyite, depending on composition and processing conditions. We utilize first-principles density functional theory (DFT) calculations to systematically investigate phase stability in the model system Ce$_x$(YLaPrSm)$_{1-x}$O$_{2-δ}$, with the aim of elucidating the thermodynamic factors governing fluorite-bixbyite competition and identifying structure-property relationships to oxygen transport. By independently varying cerium concentration and oxygen vacancy content, we predict that the transition from disordered fluorite to ordered bixbyite is driven primarily by compositional and vacancy-ordering effects, rather than through changes in cation valence. Free-energy analysis reveals that at high vacancy concentrations, bixbyite is enthalpically favored due to ordered oxygen vacancies, while fluorite is stabilized at lower vacancy concentrations and higher cerium content through configurational entropy of the anion sublattice. These DFT results clarify the competing energetic contributions that control phase stability and structure-valence relationships in LN-HEOs and establishes a mechanistic framework for designing vacancy-tolerant oxide electrolytes with tunable phase behavior.

cond-mat.mtrl-sci↗

arXiv2025

Resolving Structural Transitions in Lanthanide High-Entropy Oxides

We report a temperature-composition phase diagram for the chemically disordered and CeO2-LA2O3 high entropy oxides (HEOs), where LA denotes equimolar Y, La, Sm, and Pr, delineating stability regions for bixbyite, disordered fluorite, and intermediate vacancy-ordered fluorite phases. The diagram is constructed from a characterization package applied to bulk ceramics including X-ray diffraction (XRD), transmission electron microscopy (TEM) electron diffraction, Raman spectroscopy, energy-dispersive spectroscopy, X-ray absorption near-edge structure spectroscopy, and ultraviolet-visible spectroscopy, to quantify crystal structure at multiple length-scales, local coordination environments, and electronic structures across the formulation space. This comprehensive measurement suite is critical to identify boundaries between the closely related phases. For example, Raman scattering reveals local structural and defect environments unique to bixbyite local order that persist to ~50% Ce under equilibrium synthesis conditions but are invisible to XRD and TEM. We also report a companion thin film study to demonstrate that quenched kinetic energy from a physical deposition process can metastabilize the high symmetry, and thus high entropy, fluorite phase with only 20% Ce. This is noteworthy because electroneutrality constraints demand an exceptionally vacated oxygen sublattice; we estimate 16.7%, approaching that of delta-Bi2O3. Together, our equilibrium ceramics and far-from-equilibrium thin films show that when synthesis is coupled with rigorously chosen, multi-length-scale characterization, now one can identify the phase stability thermodynamic drivers and simultaneously derive practical guidelines for experimentally realizing targeted phases and structures - and thereby deliberately engineer properties in CeO2-LA2O3 HEOs, whose broad defect chemistries demand such an approach.

cond-mat.mtrl-sci↗

arXiv2025

Chemically-Disordered Transparent Conductive Perovskites with High Crystalline Fidelity

This manuscript presents a working model linking chemical disorder and transport properties in correlated-electron perovskites with high-entropy formulations and a framework to actively design them. We demonstrate this new learning in epitaxial Sr$x$(Ti,Cr,Nb,Mo,W)O$3$ thin films that exhibit exceptional crystalline fidelity despite a diverse chemical formulation where most B-site species are highly misfit with respect to valence and radius. X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy confirm a unique combination of chemical disorder and structural perfection in thick epitaxial layers. This combination produces significant electron correlation, low electrical resistivity, and an optical transparency window that surpasses that of constituent end-members, with a flattened frequency- and temperature-dependent response. We address the computational challenges of modeling such systems and investigate short-range ordering using cluster expansion. These results showcase that unusual d-metal combinations access an expanded property design space that is predictable using end-member characteristics -- though unavailable to them -- thus offering performance advances in optical, spintronic, and quantum devices.

cond-mat.mtrl-sci↗

arXiv2025

Maximizing solubility in rock salt high-entropy oxides

To explore and quantitatively map the cation-size mismatch solubility limits in high-entropy oxides (HEOs), we report on Ca$^{2+}$ substitution in prototypical MgCoNiCuZnO, because, while isovalent, Ca$^{2+}$ is 38% larger than its partners' average ionic radii. Using the thermodynamics-grounded bond-length distribution descriptor, we identify Ca$^{2+}$-Cu$^{2+}$ interactions as the primary prospective lattice destabilizer. Bulk synthesis confirms only 4% Ca solubility with Cu at 950$^o$C, modestly rising to 5% after Cu removal at 1150$^o$C. We then employ far-from-equilibrium pulsed-laser deposition to investigate metastable solubility: epitaxial films incorporate 10% Ca with Cu and a full 20% Ca without, doubling and quadrupling the respective bulk limits. This Ca uptake additionally enables deterministic lattice-parameter control via Ca concentration. Overall, our results demonstrate both the extended solubility possible in HEO systems, particularly when accessing metastable states through quenching from high-energy plasma, and that the specific constellation of solid-solvent cations can be rationally engineered to minimize bond-length distributions when largely misfit cations are added, thus expanding the accessible compositional space.

cond-mat.mtrl-sci↗

arXiv2025

Disorder by Design: Unveiling Local Structure and Functional Insights in High Entropy Oxides

High entropy oxides (HEOs) are a rapidly growing class of compositionally complex ceramics in which configurational disorder is engineered to unlock novel functionality. While average crystallographic symmetry is often retained, local structural and chemical disorder, including cation size and valence mismatch, oxygen sublattice distortions, and site-specific bonding, strongly governs ionic transport, redox behavior, magnetic ordering, and dielectric response. This review outlines how these modes of disorder manifest across key oxide families such as rock salt, spinel, fluorite, and perovskite. We highlight recent advances in spectroscopy, total scattering, and high-resolution microscopy enable multi-scale insight into short- and intermediate-range order. By integrating experimental observations with theoretical modeling of entropy and local energetics, we establish a framework linking structural heterogeneity to emergent properties. These insights not only deepen our fundamental understanding of disorder-property relationships but also offer a path toward rational design of tunable materials for catalysis, energy storage, electronics, and much more.

cond-mat.mtrl-sci↗

arXiv2025

Lanthanide L-Edge Spectroscopy of High-Entropy Oxides: Insights into Valence and Phase Stability

High-entropy oxides (HEOs) are a promising class of multicomponent ceramics with tunable structural and electronic properties. In this study, we investigate the local electronic structure of rare-earth HEOs in the (Ce, Sm, Pr, La, Y)O2 system using X-ray absorption spectroscopy (XAS). By systematically increasing the Ce concentration, we observe a phase transition from bixbyite to fluorite, tracked by X-ray diffraction (XRD) and corroborated by L-edge XANES analysis of La, Sm, Ce, and Pr. The oxidation states of La and Sm remain trivalent, while Ce exhibits a minor Ce 3+ fraction and Pr shows a consistent mixed-valence state. Density functional theory (DFT) calculations with Bader charge analysis support these findings and reveal that the phase transition is driven by compositional effects rather than cation redox. Our combined experimental and computational approach provides new insights into structure-valence correlations in RE-HEOs and their implications for ionic transport and phase stability.

cond-mat.mtrl-sci↗

arXiv2025

Thermodynamics-Inspired High-Entropy Oxide Synthesis

High-entropy oxide (HEO) thermodynamics transcend temperature-centric approaches, spanning a multidimensional landscape where oxygen chemical potential plays a decisive role. Here, we experimentally demonstrate how controlling the oxygen chemical potential coerces multivalent cations into divalent states in rock salt HEOs. We construct a preferred valence phase diagram based on thermodynamic stability and equilibrium analysis, alongside a high throughput enthalpic stability map derived from atomistic calculations leveraging machine learning interatomic potentials. We identify and synthesize seven equimolar single-phase rock salt compositions that accommodate multivalent Mn, Fe, or both, as confirmed by X-ray diffraction and fluorescence. X-ray absorption fine structure spectra reveal predominantly divalent cations. Ultimately, we introduce oxygen chemical potential overlap as a key complementary descriptor predicting HEO stability and synthesizability. Although we focus on rock salt HEOs, our methods are chemically and structurally agnostic, providing a broadly adaptable framework for navigating HEOs thermodynamics and enabling a broader compositional range with contemporary property interest.

cond-mat.mtrl-sci↗

arXiv2025

Discovering High-Entropy Oxides with a Machine-Learning Interatomic Potential

High-entropy materials shift the traditional materials discovery paradigm to one that leverages disorder, enabling access to unique chemistries unreachable through enthalpy alone. We present a self-consistent approach integrating computation and experiment to understand and explore single-phase rock salt high-entropy oxides. By leveraging a machine-learning interatomic potential, we rapidly and accurately map high-entropy composition space using our two descriptors: bond length distribution and mixing enthalpy. The single-phase stabilities for all experimentally stabilized rock salt compositions are correctly resolved, with dozens more compositions awaiting discovery.

cond-mat.mtrl-sci↗

arXiv2024

Performance of Exchange-Correlation Approximations to Density-Functional Theory for Rare-earth Oxides

Rare-earth oxides (REOs) are an important class of materials owing to their unique properties, including high ionic conductivities, large dielectric constants, and elevated melting temperatures, making them relevant to several technological applications such as catalysis, ionic conduction, and sensing. The ability to predict these properties at moderate computational cost is essential to guiding materials discovery and optimizing materials performance. Although density-functional theory (DFT) is the favored approach for predicting electronic and atomic structures, its accuracy is limited in describing strong electron correlation and localization inherent to REOs. The newly developed strongly constrained and appropriately normed (SCAN) meta-generalized-gradient approximations (meta-GGAs) promise improved accuracy in modeling these strongly correlated systems. We assess the performance of these meta-GGAs on binary REOs by comparing the numerical accuracy of thirteen exchange-correlation approximations in predicting structural, magnetic, and electronic properties. Hubbard U corrections for self-interaction errors and spin-orbit coupling are systematically considered. Our comprehensive assessment offers insights into the physical properties and functional performance of REOs predicted by first-principles and provides valuable guidance for selecting optimal DFT functionals for exploring these materials.

cond-mat.mtrl-sci↗

arXiv2024

Local structure maturation in high entropy oxide (Mg,Co,Ni,Cu,Zn)1-x(Cr,Mn)xO thin films

High entropy oxides (HEO)s have garnered much interest due to their available high degree of tunability. Here, we study the local structure of (MgNiCuCoZn)0.167(MnCr)0.083O, a composition based on the parent HEO (MgNiCuCoZn)0.2O.We synthesized a series of thin films via pulsed laser deposition at incremental oxygen partial pressures. X-ray diffraction shows lattice parameter to decrease with increased pO2 pressures until the onset of phase separation. X-ray absorption fine structure shows that specific atomic species in the composition dictate the global structure of the material as Cr, Co, and Mn shift to energetically favorable coordination with increasing pressure. Transmission electron microscopy analysis on a lower-pressure sample exhibits a rock salt structure, but the higher-pressure sample reveals reflections reminiscent of the spinel structure. In all, these findings give a more complete picture on how (MgNiCuCoZn)0.167(MnCr)0.083O forms with varying initial conditions and advances fundamental knowledge of cation behavior in high entropy oxides.

cond-mat.mtrl-sci↗

arXiv2024

Untangling individual cation roles in rock salt high-entropy oxides

We unravel the distinct roles each cation plays in phase evolution, stability, and properties within Mg1/5Co1/5Ni1/5Cu1/5Zn1/5O high-entropy oxide (HEO) by integrating experimental findings, thermodynamic analyses, and first-principles predictions. Our approach is through sequentially removing one cation at a time from the five-component high-entropy oxide to create five four-component derivatives. Bulk synthesis experiments indicate that Mg, Ni, and Co act as rock salt phase stabilizers whereas only Mg and Ni enthalpically enhance single-phase rock salt stability in thin film growth; synthesis conditions dictate whether Co is a rock salt phase stabilizer or destabilizer. By examining the competing phases and oxidation state preferences using pseudo-binary phase diagrams and first-principles calculations, we resolve the stability differences between bulk and thin film for all compositions. We systematically explore HEO macroscopic property sensitivity to cation selection employing both predicted and measured optical spectra. This study establishes a framework for understanding high-entropy oxide synthesizability and properties on a per-cation basis that is broadly applicable to tailoring functional property design in other high-entropy materials.

cond-mat.mtrl-sci↗

arXiv2024

Dielectric Properties of Disordered A6B2O17 (A = Zr; B = Nb, Ta) Phases

We report on the structure and dielectric properties of ternary A6B2O17 (A = Zr; B = Nb, Ta) thin films and ceramics. Thin films are produced via sputter deposition from dense, phase-homogenous bulk ceramic targets, which are synthesized through a reactive sintering process at 1500 °C. Crystal structure, microstructure, chemistry and dielectric properties are characterized by X-ray diffraction and reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and capacitance analysis respectively. We observe relative permittivities approaching 60 and loss tangents < 10^-2 across the 10^3-10^5 Hz frequency range in the Zr6Nb2O17 and Zr6Ta2O17 phases. These observations create an opportunity space for this novel class of disordered oxide electroceramics.

cond-mat.mtrl-sci↗

arXiv2024

Order evolution from a high-entropy matrix: understanding and predicting paths to low temperature equilibrium

Interest in high-entropy inorganic compounds originates from their ability to stabilize cations and anions in local environments that rarely occur at standard temperature and pressure. This leads to new crystalline phases in many-cation formulations with structures and properties that depart from conventional trends. The highest-entropy homogeneous and random solid-solution is a parent structure from which a continuum of lower-entropy offspring can originate by adopting chemical and/or structural order. This report demonstrates how synthesis conditions, thermal history, and elastic and chemical boundary conditions conspire to regulate this process in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O, during which coherent CuO nano-tweeds and spinel nano-cuboids evolve. We do so by combining structured synthesis routes, atomic-resolution microscopy and spectroscopy, density functional theory, and a phase field modeling framework that accurately predicts the emergent structure and local chemistry. This establishes a framework to appreciate, understand, and predict the macrostate spectrum available to a high-entropy system that is critical to rationalize property engineering opportunities.

cond-mat.mtrl-sci↗