The structural mechanism of the magnetic phase transitions in ${\mathrm{FeTiO}}_3$ ilmenite, including the paramagnetic (PM) to antiferromagnetic (AFM) transition induced by reducing temperature in zero magnetic field and the AFM to ferromagnetic (FM) transition induced by applying strong magnetic field at a fixed temperature below the Néel temperature, $T_{\mathrm{N}}$, are studied by total synchrotron x-ray scattering. It is found that in both cases the concerted effect of Coulomb repulsion, spin-orbit coupling, and exchange interactions is what determines the response of the crystal lattice to variations in external perturbations such as temperature and magnetic field. In particular, due to strong spin-orbit coupling and Coulomb repulsion, Fe and Ti atoms move in sync along the $c$ axis of the crystal lattice such that their separation changes by less than 1%. At the same time, due to strong intralayer ferromagnetic interactions, the Fe-O-Fe bond angle becomes closer to ${90}^{\circ }$. Notably, the direction of the motion occuring during the temperature-induced PM to AFM transition is opposite to that occuring during the magnetic field-induced AFM to FM transition. Altogether, the seemingly simple ${\mathrm{FeTiO}}_3$ ilmenite behaves like a complex physical system where charge, spin, orbital, and lattice degrees of freedom are strongly coupled. Our findings are likely to be relevant to other members of the ilmenite family and spin-orbit coupled magnetic insulators in general.

The site-selective substitution of diamagnetic $d^{10}/d^0$ cations offers an efficient way to fine-tune magnetic characteristics by controlling orbital hybridization and superexchange pathways. Herein, we explore the tunability of magnetic properties in coupled spin-ladder compounds $\mathrm{B}{\mathrm{a}}_2\mathrm{CuT}{\mathrm{e}}_{1\text{−}x}{\mathrm{W}}_x{\mathrm{O}}_6$ ($x=0.0-0.30$). The pristine compound exhibits spin-singlet correlations at elevated temperatures, as evidenced by a magnetic susceptibility peak at $T_{\max }\sim 70$ K, Schottky-like specific heat, unconventional magnetic Raman scattering, and a quasilinear decrease in the electron spin resonance linewidth. Notably, the substitution of ${\mathrm{W}}^{6+}$ for $\mathrm{T}{\mathrm{e}}^{6+}$ brings about several marked changes: an increase in the Curie-Weiss temperature, a suppression in the Néel ordering temperature, enhanced magnetic susceptibility at low temperatures, and a weakening of spin-ladder correlations. These modifications collectively suggest that the newly activated exchange interactions through ${\mathrm{W}}^{6+}$ substitution primarily alter intraladder spin topology while amplifying quantum critical fluctuations. This finding highlights the potential for controlling magnetic correlations in spin-ladder compounds through targeted chemical substitution.

The poor understanding of the optical transitions and luminescent mechanisms critically hindered the development of near-infrared (NIR) ${\mathrm{Fe}}^{3+}$-activated phosphors, and efficient luminescence from Fe(Oh) has rarely been reported. In our study, we delve into these challenges and realize their correlation with the quenching mechanism of ${\mathrm{Fe}}^{3+}$ luminescence. First-principles calculations are utilized to analyze energy levels and electron-phonon coupling parameters, further elucidating potential deactivation pathways and factors influencing the occurrence of photoluminescence. A heuristic rule based on ligand-field strength, determined by the absorption wavelength of ${\mathrm{Cr}}^{3+}$ occupying the same octahedral site in oxides, is proposed to facilitate the prediction of both the potential and wavelength of ${\mathrm{Fe}}^{3+}$ emission. Our study offers consistent and reliable interpretations for the difficulties and challenges of iron-doped crystals, and provides valuable insights on the design and optimization of ${\mathrm{Fe}}^{3+}$-based phosphors.

${\mathrm{CsSnI}}_3$ is widely studied as an environmentally friendly Pb-free perovskite material for optoelectronic device applications. To further improve material and device performance, it is important to understand the surface structures of ${\mathrm{CsSnI}}_3$. We generate surface structures with various stoichiometries, perform density functional theory calculations to create phase diagrams of the ${\mathrm{CsSnI}}_3$ (001), (110), and (100) surfaces, and determine the most stable surfaces under a wide range of Cs, Sn, and I chemical potentials. Under I-rich conditions, surfaces with Cs vacancies are stable, which lead to partially occupied surface states above the valence band maximum. Under I-poor conditions, we find the stoichiometric (100) surface to be stable under a wide region of the phase diagram, which does not have any surface states and can contribute to long charge-carrier lifetimes. Consequently, the I-poor (Sn-rich) conditions will be more beneficial to improve the device performance.

We present x-ray scattering studies resolving structural twinning and phase separation in the charge density wave (CDW) state of the kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. The three-dimensional CDW state in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ is reported to form a complex superposition of Star of David (SoD) or Tri-Hexagonal (TrH) patterns of distortion within its kagome planes, but the out-of-plane stacking is marked by metastability. To resolve the impact of this metastability, we present reciprocal space mapping and real-space images of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ collected across multiple length scales using temperature-dependent high-dynamic range mapping (HDRM) and dark-field x-ray microscopy (DFXM). The experimental data provide evidence for a rich microstructure that forms in the CDW state. Data evidence metastability in the formation of $2\times 2\times 4$ and $2\times 2\times 2$ CDW supercells dependent on thermal history and mechanical deformation. We further directly resolve the real space phase segregation of both supercells, as well as a real-space, structural twinning driven by the broken rotational symmetry of the CDW state. Our combined results provide insights into the role of microstructure and twinning in experiments probing the electronic properties of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ where rotational symmetry is broken by the three-dimensional charge density wave order but locally preserved for any single kagome layer.

Materials with on-demand control of thermal conductivity are the prerequisites to build thermal conductivity switches, where the thermal conductivity can be turned on and off. However, the ideal switch, while required to develop novel approaches to solid-state refrigeration, energy harvesting, and even phononic circuits, is still missing. It should consist of an active material only, be environment friendly, and operate near room temperature with a reversible, fast, and large switching ratio. Here, we first predict by ab initio electronic structure calculations that ferroelectric domains in barium titanate exhibit anisotropic thermal conductivities. We confirm this prediction by combining frequency-domain thermoreflectance and scanning thermal microscopy measurements on a single crystal of barium titanate. We then use this gained knowledge to propose a lead-free thermal conductivity switch without inactive material, operating reversibly with an electric field. At room temperature, we find a switching ratio of $1.6\pm 0.3$, exceeding the performances of state-of-the-art materials suggested for thermal conductivity switches.

The recently synthesized ternary quasi-2D material ${\mathrm{TaRhTe}}_4$ is a bulk Weyl semimetal with an intrinsically layered structure, which poses the question of how the topology of its electronic structure depends on layer separations. Experimentally, these separations may be changed for instance by intercalation of the bulk, or by exfoliation, to reach monolayer or few-layer structures. Here, we show that in the monolayer limit a quantum spin Hall insulator (QSHI) state emerges, employing density functional calculations as well as a minimal four-orbital tight-binding model that we develop. Even for weak spin-orbit couplings the QSHI is present, which has an interesting edge state that features Rashba-split bands with quadratic band minima. Further, we find that a weak topological insulator (WTI) manifests in the bilayer system due to sizable intralayer hopping, contrary to the common lore that only weak interlayer interactions between stacked QSHIs lead to WTIs. Stacked bilayers give rise to a phase diagram as a function of the interlayer separation that comprises a Weyl semimetal, WTI, and normal insulator (NI) phases. These insights on the evolution of topology with dimensions can be transferred to the family of layered ternary transition metal tellurides.

${\mathrm{Fe}}_2\mathrm{P}$-based magnets are known for their significant magnetic properties, making them useful in various technological applications. The aim of this study was to investigate the effects of Si and Co substitution on the physical and magnetic properties of the ${\mathrm{Fe}}_2\mathrm{P}$ compounds. In order to have a systematic understanding we have performed combined ab initio calculations and a set of experiments. Particular emphasis was placed on the study of preferential substitution sites, lattice constants, magnetic moments, and the Curie temperature ($T_C$), which was further explored by considering the exchange interaction energies $J_{ij}$. Satisfactory agreement was observed between theoretical calculations and the predicted phase transition from the hexagonal to the body-centered-orthorhombic (BCO) crystal structure as a function of temperature. Theoretical calculations reveal that the $2c$ position is the preferred site for Si, while Co is expected to occupy the $3f$ sites. Theoretical analysis of the magnetic moments shows an increase up to $3.64{\mu }_B/\mathrm{f}.\mathrm{u}.$ for $x$ = 0.5 Si, which agrees with the experimental values of $3.16{\mu }_B/\mathrm{f}.\mathrm{u}.$ Co substitution in ${\mathrm{Fe}}_{2-y}{\mathrm{Co}}_y{\mathrm{P}}_{0.84}{\mathrm{Si}}_{0.16}$ and ${\mathrm{Fe}}_{2-y}{\mathrm{Co}}_y{\mathrm{P}}_{0.59}{\mathrm{Si}}_{0.41}$ resulted in a decrease in magnetic moments and consequently in other magnetic properties. Focusing on the Curie temperature, three different trends were found depending on the Si concentration. A dependence of the $3f\text{−}3f$ intralayer exchange interaction energies on Si was proposed as the reason for the trends and deduced as the reason for an increase in $T_C$ at low, no change in $T_C$ at medium, and a decrease in $T_C$ at high Si concentrations.

We investigate the crystal and electronic structures as well as magnetic properties of ilmenite-type $\mathrm{NiV}{\mathrm{O}}_3$, which has attracted research interest as an $S$ = 1 honeycomb lattice magnet. Ilmenite-type $\mathrm{NiV}{\mathrm{O}}_3$ samples were synthesized under high-pressure and high-temperature conditions. Synchrotron x-ray diffraction and Rietveld refinement results demonstrated that $\mathrm{NiV}{\mathrm{O}}_3$ underwent a continuous structural phase transition from the triclinic ($P\overline{1}$) phase to the rhombohedral ($R\overline{3}$) phase at 450 K. This transition is concomitant with the decomposition of the V-V dimers formed by the tetravalent V ions. X-ray absorption spectroscopy measurements confirmed that the Ni and V ions were divalent and tetravalent, respectively. Magnetic and specific heat measurements revealed that $\mathrm{NiV}{\mathrm{O}}_3$ underwent an antiferromagnetic transition at 140 K, and a zigzag-type magnetic order with magnetic propagation vector $k=(0,1/2,0)$ was observed by neutron diffraction measurements.

High thermoelectric properties are associated with the phonon-glass electron-crystal paradigm. Conventional wisdom suggests that the optimal bandgap of semiconductor to achieve the largest power factor should be between 6 and $10{\kappa }_{B}T$. To address challenges related to the bipolar effect and temperature limitations, we present findings on Zintl-type ${\mathrm{TlAgI}}_2$, which demonstrates an exceptionally low lattice thermal conductivity of 0.30 W ${\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K. The achieved figure of merit ($ZT$) for ${\mathrm{TlAgI}}_2$, featuring a 2.40 eV bandgap, reaches a value of 1.53 for $p$-type semiconductor. This remarkable $ZT$ is attributed to the existence of extended antibonding states [Ag-I] in the valence band. Furthermore, the bonding hierarchy, influencing phonon anharmonicity, and coordination bonds, facilitating electron transfer between the ligand and the central metal ion, significantly contribute to the electronic transport. This finding serves as a promising avenue for the development of high $ZT$ materials with wide bandgaps at elevated temperatures.

Spinel oxide ($\mathrm{NiC}{\mathrm{o}}_2{\mathrm{O}}_4$) is an attractive catalyst for oxygen evolution reaction (OER) due to its rich redox reactions and unique electronic structure. However, the electrocatalytic OER performance of $\mathrm{NiC}{\mathrm{o}}_2{\mathrm{O}}_4$ has always been limited by the low specific surface area and poor intrinsic conductivity of $\mathrm{NiC}{\mathrm{o}}_2{\mathrm{O}}_4$. Cationic doping is an effective method in modulating the electrocatalytic activity at the atomic level to improve the conductivity and activity. Herein, a series of Fe-doped $\mathrm{NiC}{\mathrm{o}}_2{\mathrm{O}}_4$ electrocatalysts were successfully prepared using a simple solvothermal method. Impressively, Fe-doped $\mathrm{NiC}{\mathrm{o}}_2{\mathrm{O}}_4$ delivers an attractive small overpotential of 341 mV at $10\mathrm{mA}\mathrm{c}{\mathrm{m}}^{-2}$ and a Tafel slope of $74\mathrm{mV}\mathrm{de}{\mathrm{c}}^{-1}$ compared to $\mathrm{NiC}{\mathrm{o}}_2{\mathrm{O}}_4$. The x-ray photoelectron spectroscopy and the density functional theory calculations reveal that Fe dopants can regulate the electronic structure of Ni sites by donating electrons to Co atoms, which leads to an increased $\mathrm{N}{\mathrm{i}}^{3+}$ ratio and a reduced adsorption strength of oxygen intermediates at Ni sites, thus facilitating the conversion of *OH to *O. This work provides an effective approach to enhancing the electrocatalytic activities of non-noble-metal-based catalysts.

Multiple recent studies have identified the metallic antiferromagnet ${\mathrm{Mn}}_2\mathrm{Au}$ to be a candidate for spintronic applications due to apparent in-plane anisotropy, preserved magnetic properties above room temperature, and current-induced Néel vector switching. Crystal growth is complicated by the fact that ${\mathrm{Mn}}_2\mathrm{Au}$ melts incongruently. We present a bismuth flux method to grow millimeter-scale bulk single crystals of ${\mathrm{Mn}}_2\mathrm{Au}$ in order to examine the intrinsic anisotropic electrical and magnetic properties. Flux quenching experiments reveal that the ${\mathrm{Mn}}_2\mathrm{Au}$ crystals precipitate below $550{}^{\circ }\mathrm{C}$, about $100{}^{\circ }\mathrm{C}$ below the decomposition temperature of ${\mathrm{Mn}}_2\mathrm{Au}$. Bulk ${\mathrm{Mn}}_2\mathrm{Au}$ crystals have a room-temperature resistivity of 16–19 $\mu \mathrm{\Omega }\mathrm{cm}$ and a residual resistivity ratio of 41. ${\mathrm{Mn}}_2\mathrm{Au}$ crystals have a dimensionless susceptibility on the order of ${10}^{-4}$ (SI units), comparable to calculated and experimental reports on powder samples. Single-crystal neutron diffraction confirms the in-plane magnetic structure. The tetragonal symmetry of ${\mathrm{Mn}}_2\mathrm{Au}$ constrains the $ab$-plane magnetic susceptibility to be constant, meaning that ${\chi }_{100}={\chi }_{110}$ in the low-field limit, below any spin-flop transition. We find that three measured magnetic susceptibilities ${\chi }_{100}, {\chi }_{110}$, and ${\chi }_{001}$ are the same order of magnitude and agree with the calculated prediction, meaning the low-field susceptibility of ${\mathrm{Mn}}_2\mathrm{Au}$ is quite isotropic, despite clear differences in $ab$-plane and $ac$-plane magnetocrystalline anisotropy. ${\mathrm{Mn}}_2\mathrm{Au}$ is calculated to have an extremely high in-plane spin-flop field above 30 T, which is much larger than that of another in-plane antiferromagnet, ${\mathrm{Fe}}_2\mathrm{As}$ (less than 1 T). The subtle anisotropy of intrinsic susceptibilities may lead to dominating effects from shape, crystalline texture, strain, and defects in devices that attempt spin readout in ${\mathrm{Mn}}_2\mathrm{Au}$.

Yttria-stabilized zirconia (YSZ) coatings have been developed for high temperature energy applications including gas turbines. The objective of this work is to understand how aliovalent Fe substitution affects the optical absorption spectrum of the host YSZ and ${\mathrm{ZrO}}_2$ systems in the ultraviolet–visible–near infrared wavelength range (from 245 to 2500 nm) using both experimental and computational techniques. In the Fe-substituted ${\mathrm{ZrO}}_2$ system, phase-pure ($>99$% purity) samples were synthesized in the monoclinic crystal structure, whereas Fe substitution in YSZ resulted in a two-phase mixture of coexisting tetragonal and monoclinic phases. Optical property characterization performed at room temperature revealed two broad absorption bands in both systems: one centered around 1000 nm and the other centered around 500 nm. Tauc plot analysis of the optical absorption data showed that as the Fe concentration increases, the optical band gaps of both materials systems decrease. Many-body perturbation theory methods, based on $G_0{W}_0$ and the Bethe-Salpeter equation, were used to computationally model the optical absorption spectrum as a function of Fe substitution in the tetragonal and monoclinic crystal structures of YSZ and ${\mathrm{ZrO}}_2$. Supercells were constructed and several Fe- and/or Y-atom configurations were explored in Zr sites. Charge compensating O vacancies were introduced to maintain electrical neutrality. The computations reveal that the observed optical excitations centered around 1000 nm likely have an excitonic character due to defect states, whose origin is traced to the electronic transitions between $\mathrm{Fe}\text{−}3d$ and $\mathrm{Fe}\text{−}3d$ orbitals. Intriguingly, both the tetragonal and monoclinic crystal structures appear to support local polyhedral distortions that promote excitations in the 1000 nm wavelength region. The excitation centered around 500 nm is attributed to the optical band gap of these materials. The outcomes of this work shed light on the radiative properties of Fe-substituted YSZ with implications in thermal barrier coating composition design.

We develop an accurate nanoelectronic modeling approach for realistic three-dimensional topological insulator nanostructures and investigate their low-energy surface-state spectrum. Starting from the commonly considered four-band $\mathrm{k}·\mathrm{p}$ bulk model Hamiltonian for the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ family of topological insulators, we derive new parameter sets for ${\mathrm{Bi}}_2{\mathrm{Se}}_3, {\mathrm{Bi}}_2{\mathrm{Te}}_3,$ and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. We consider a fitting strategy applied to ab initio band structures around the $\mathrm{\Gamma }$ point that ensures a quantitatively accurate description of the low-energy bulk and surface states while avoiding the appearance of unphysical low-energy states at higher momenta, something that is not guaranteed by the commonly considered perturbative approach. We analyze the effects that arise in the low-energy spectrum of topological surface states due to band anisotropy and electron-hole asymmetry, yielding Dirac surface states that naturally localize on different side facets. In the thin-film limit, when surface states hybridize through the bulk, we resort to a thin-film model and derive thickness-dependent model parameters from ab initio calculations that show good agreement with experimentally resolved band structures, unlike the bulk model that neglects relevant many-body effects in this regime. Our versatile modeling approach offers a reliable starting point for accurate simulations of realistic topological material-based nanoelectronic devices.

FeRh is well known in its bulk form for a temperature-driven antiferromagnetic (AFM) to ferromagnetic (FM) transition near room temperature. It has aroused renewed interest in its thin-film form, with particular focus on its biaxial AFM magnetic anisotropy which could serve for data encoding, and the possibility to investigate laser-assisted phase transitions, with varying contributions from electrons, phonons, and magnons. In order to estimate the typical temperature increase occurring in these experiments, we performed modulated thermoreflectance microscopy to determine the thermal conductivity $\kappa$ of FeRh. As often occurs upon alloying, and despite the good crystallinity of the layer, $\kappa$ was found to be lower than the thermal conductivities of its constituting elements. More unexpectedly, given the electrically more conducting nature of the FM phase, it turned out to be three times lower in the FM phase compared to the AFM phase. This trend was confirmed by examining the temporal decay of incoherent phonons generated by a pulsed laser in both phases. To elucidate these results, first- and second-principles simulations were performed to estimate the phonon, magnon, and electron contributions to the thermal conductivity. They were found to be of the same order of magnitude, and to give a quantitative rendering of the experimentally observed ${\kappa }_{\mathrm{AFM}}$. In the FM phase, however, simulations overestimate the low experimental values, implying very different (shorter) electron and magnon lifetimes.

Recently reported remarkably high radiation tolerance of $\gamma /\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ double-polymorphic structure brings this ultrawide-band-gap semiconductor to the frontiers of power electronics applications that are able to operate in challenging environments. Understanding the mechanism of radiation tolerance is crucial for further material modification and tailoring of the desired properties. In this study, we employ machine-learning-enhanced atomistic simulations to assess the stability of both the gallium ($\mathrm{Ga}$) and oxygen ($\mathrm{O}$) sublattices under various levels of damage. Our study uncovers the remarkable resilience and stability of the  -sublattice, attributing this property to the strong tendency of recovery of the $\mathrm{O}$ defects, especially within the more strongly disordered regions. Interestingly, we observe the opposite behavior of the $\mathrm{Ga}$ defects that display enhanced stability in the same regions of increased disorder. Moreover, we observe that highly defective $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ is able to transform into $\gamma \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ upon annealing due to preserved lattice organization of the $\mathrm{O}$ sublattice. This result clearly manifests that the ultrahigh stability of the $\mathrm{O}$ sublattice provides the backbone for the exceptional radiation tolerance of the $\gamma /\beta$ double-polymorphic structure. These computational insights closely align with experimental observations, opening avenues for further exploration of polymorphism in ${\mathrm{Ga}}_2{\mathrm{O}}_3$ and potentially in analogous polymorphic families spanning a broad range of diverse materials of complex polymorphic nature.

CdTe and its alloy CdSeTe are widely used in optoelectronic devices, such as radiation detectors and solar cells, due to their superior electrical properties. However, the formation of defects and defect complexes in these materials can significantly affect their performance. As a result, understanding the defect formation and recombination processes in CdTe and CdSeTe alloy is of great importance. In recent years, density functional theory (DFT) calculations have emerged as a powerful tool for investigating the properties of defects in semiconductors. In this paper, we use $\mathrm{DFT}+\mathrm{U}$ calculations to comprehensively study the properties of intrinsic defects as well as extrinsic defects induced by commonly used dopants, such as Cu and group V elements, in CdTe and CdSeTe alloy. This work provides insights into the effects of these defects on the electrical and optical properties of the material.

Chalcopyrites are ternary crystalline compounds which have found use in a diverse array of applications, from solar cells to thermoelectric devices. These are known to be ternary-compound analogs to diamond or sphalerite. “Defect chalcopyrites,” which are chalcopyrite structures stabilized with ordered vacancies and substitutions, are well attested and can serve as a method of tuning material properties through control of stoichiometry. In particular, defect chalcopyrites related to the thermoelectric compound ${\mathrm{CuInTe}}_2$ comprise a large range of compositions in the pseudobinary system ${\left({\mathrm{Cu}}_2\mathrm{Te}\right)}_x{\left({\mathrm{In}}_2{\mathrm{Te}}_3\right)}_{1-x}$, where $x<0.5$. In contrast, the converse case of “augmented chalcopyrites,” namely, $x>0.5$, is much less known or studied. We report the discovery of a range of ${\mathrm{Cu}}_2\mathrm{Te}$-rich compositions in this binary system where stable phases can potentially be found. Here, the stoichometry of augmented chalcopyrites is likely to be modulated by the concentration of defect clusters ${[{\mathrm{Cu}}_{\text{In}}^{2-}·2{\mathrm{Cu}}_i^{+}]}^0$ in chalcopyrite ${\mathrm{CuInTe}}_2$.