It is widely believed that the origin of a significant cause of voltage and capacity fading observed in lithium (Li)-ion batteries is related to structural modifications occurring in the cathode material during the Li-ion insertion/deinsertion process. The Li-ion insertion/deinsertion mechanism and the resulting structural changes are known to exert a severe strain on the lattice and consequently lead to performance degradation. Here, intending to shed more light on the effect of such strain on the structural properties of the cathode material, we have systematically investigated the pressure dependence of structural and transport properties of a ${\mathrm{Li}}_{\mathrm{x}}{\mathrm{CoO}}_2$ single crystal grown using 5% excess Li in the precursors. Ambient pressure synchrotron diffraction on these crystals reveals that the excess Li during the growth has facilitated the stabilization of a layered rhombohedral phase (space group $R\overline{3}m$) as well as a disordered rock-salt phase (space group $Fm\overline{3}m$). The volume fraction of the rhombohedral and cubic phase is 60:40, respectively, which remains unchanged up to 10.6 GPa. No structural phase transition has been observed up to 10.6 GPa. Further, pressure-dependent structural studies on delithiated mixed-phase material using synchrotrons revealed that the rhombohedral phase of ${\mathrm{Li}}_{0.5}{\mathrm{CoO}}_2$ remains thermodynamically stable at high pressures and prevails as the major phase. An increase in the measured resistance with decreasing temperature revealed the semi-metallic nature of the sample. Further, the application of hydrostatic pressure up to 2.8 GPa shows the enhancement of semi-metallic nature. The experimental results can be explained qualitatively via density functional theory (DFT) and thermodynamics modeling. We find that the calculated density of states was reduced and the activation energy was increased by applied pressure. Our investigations indicate significant phase stability of the mixed-phase crystals under externally applied high pressure and thus suggest the possible use of such mixed-phase materials as a cathode in lithium-ion batteries.

Predicting spectra and related properties such as the dielectric function of crystalline materials based on machine learning has a huge, hitherto unexplored, technological potential. For this reason, we create an ab initio database of 9915 dielectric tensors of semiconductors and insulators calculated in the independent-particle approximation (IPA). In addition, we present the OptiMate family of machine learning models, a series of graph attention neural networks (GAT) trained to predict the dielectric function and refractive index. OptiMate yields accurate prediction of spectra of semiconductors using only their crystal structure. Smooth, artifact-free curves are obtained without these properties being enforced by penalties.

Hydrogen boride (HB) sheets, a two-dimensional material composed of hexagonal boron (B) with bridging hydrogen (H), have recently been synthesized and shown significant potential in electronic devices and catalytic applications. Recent studies have shown that HB sheets are generally stable against water, in contrast to many boron hydride materials which undergo hydrolysis. This stability is attributed to the interplay between the negatively charged B and the strong B-B bond network. Despite this stability, experiments have shown that hydrolysis still takes place, albeit minimally. It is possible that the source of this minimal hydrolysis is related to regions with a less prominent B-B bond network; however, the microscopic details remain unclear. In this work, we investigated the various configurations of HB nanoribbon edges as representative of regions with less prominent B-B bond network and show their distinct region-specific behavior with water. We found that zigzag and armchair nanoribbons were generally stable against water, whereas in the case of the hydrogen-vacant armchair nanoribbon, an oxygen-boron bonding was observed, showcasing a chemisorption mechanism. Additionally, it was found that water dissociation is easier to proceed near the edge as opposed to the surface, signifying the more reactive nature of the edge. These results shed light on the mechanism of the partial hydrolysis observed in the experiment.

The measured magnetization dynamics of ferromagnetic iron-cobalt ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ alloys show a strong dependence on the alloy composition, especially near $x=0.25$. Here, we calculate from first principles the electron-phonon coupling strength in ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ alloys for compositions ranging from $x=0$ to $0.75$. We find a strong, spin-dependent variation of the electron-phonon coupling strength with alloy composition, with a minimum near $x=0.25$. We analyze the variation of the electron-phonon interaction with composition, as a function of electron spin, density of states, electron-phonon matrix elements, character of electron wave function at the Fermi level, orbital-resolved strength of the phonon perturbing potential, and phonon frequencies. We calculate the electron-phonon energy transfer coefficients, and we find that they are in qualitative agreement with the phenomenological electron-phonon energy transfer coefficient deduced from magnetization dynamics experiments. Our findings show that variations in the composition of ferromagnetic alloys can significantly alter the magnetization dynamics and transport properties.

Ultrawide bandgap (UWBG) semiconductors are poised to transform power electronics by surpassing the capabilities of established wide bandgap materials, such as GaN and SiC, owing to their capability to operate at higher voltage, frequency, and temperature ranges. While bulk group-III nitrides and their alloys have been extensively studied in the UWBG realm, their two-dimensional counterparts remain largely unexplored. Here, we examine the stability and electronic properties of monolayers of ordered boron-based group-III nitride alloys with general formula ${\mathrm{B}}_x{M}_{1-x}\mathrm{N}$, where $M=\mathrm{Al}$, Ga. On the basis of ab initio calculations we identify a number of energetically and dynamically stable structures. Instrumental to their stability is a previously overlooked out-of-plane displacement (puckering) of atoms, which induces a polar ordering and antiferroelectric ground state. Our findings reveal the energy barrier between metastable ferroelectric states is lowered by successive switching of out-of-plane displacements through an antiferroelectric state.

Innovative synthetic approaches can yield new phases containing novel structural and magnetic motifs. In this paper, we show the synthesis and magnetic characterization of three new and one previously reported layered phase in the K-Cu-Te-O(H) phase space using a tunable hydroflux technique. The hydroflux, with a roughly equal molar ratio of water and alkali hydroxide, is a highly oxidizing, low melting solvent, which can be used to isolate metastable phases unattainable through traditional solid state or flux techniques. The newly synthesized phases, ${\mathrm{K}}_2{\mathrm{Cu}}_2{\mathrm{TeO}}_6, {\mathrm{K}}_2{\mathrm{Cu}}_2{\mathrm{TeO}}_6·{\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{K}}_6{\mathrm{Cu}}_9{\mathrm{Te}}_4{\mathrm{O}}_{24}·2{\mathrm{H}}_2\mathrm{O}$, contain ${\mathrm{Cu}}^{2+}$ within ${\mathrm{CuO}}_4$ square planar plaquettes and ${\mathrm{TeO}}_6$ octahedra ordering to form structural honeycomb layers isolated by interlayer ${\mathrm{K}}^{+}$ ions and ${\mathrm{H}}_2\mathrm{O}$ molecules. We find the synthesized structures display varying tilt sequences of the ${\mathrm{CuO}}_4$ plaquettes, leading to distinct ${\mathrm{Cu}}^{2+}$ magnetic motifs on the structural honeycomb lattice and a range of effective magnetic dimensionalities. We find that ${\mathrm{K}}_2{\mathrm{Cu}}_2{\mathrm{TeO}}_6·{\mathrm{H}}_2\mathrm{O}$ does not order and displays alternating chain Heisenberg antiferromagnetic (AFM) behavior, while ${\mathrm{K}}_2{\mathrm{Cu}}_2{\mathrm{TeO}}_6$ and ${\mathrm{K}}_6{\mathrm{Cu}}_9{\mathrm{Te}}_4{\mathrm{O}}_{24}·2{\mathrm{H}}_2\mathrm{O}$ order antiferromagnetically ($T_N=100\mathrm{K}$ and $T_N=6.5\mathrm{K}$, respectively). The previously known phase, ${\mathrm{K}}_2{\mathrm{CuTeO}}_4{\left(\text{OH}\right)}_2·{\mathrm{H}}_2\mathrm{O}$, we find contains structurally and magnetically one-dimensional ${\mathrm{CuO}}_4$ plaquettes leading to uniform chain Heisenberg AFM behavior and shows no magnetic order down to $T=0.4\mathrm{K}$. We discuss and highlight the usefulness of the hydroflux technique in novel syntheses and the interesting magnetic motifs that arise in these particular phases.

We evaluate the potential of an overlooked family of materials to support both the magnetization and polarization required to be classified as multiferroics. This family of materials has a stoichiometry of ${\mathrm{A}}_2{\mathrm{BX}}_3$ and was uncovered in the Inorganic Crystal Structure Database (ICSD) while searching for structural platforms that could support low energy polarization switching. The examples here have the general chemical formula of ${\mathrm{K}}_2{\mathrm{BO}}_3$, where B is a magnetically active cation located within edge-sharing square pyramids that form a 1D chain. Density functional theory with Hubbard U corrections (DFT $+$ U) are used to determine the potential energy landscape of ${\mathrm{K}}_2{\mathrm{BO}}_3$, which include investigating multiple magnetic and polarization orderings. We analyze the ground state and electronic structures and report on how the choice of Hubbard U will affect both, which is important when predicting functional properties of low-dimensional and potentially exfoliable systems. This family contains a ferromagnetic insulator, ${\mathrm{K}}_2{\mathrm{VO}}_3$, as well as antiferromagnetic (${\mathrm{K}}_2{\mathrm{NbO}}_3$) and nonmagnetic (${\mathrm{K}}_2{\mathrm{MoO}}_3$) insulators with antipolar ground state symmetries, and accessible polar metastable states, that we predict to be antiferroelectric. This preliminary assessment of the ${\mathrm{K}}_2{\mathrm{BO}}_3$ members of the ${\mathrm{A}}_2{\mathrm{BX}}_3$ family reveals a new class of materials, that with further optimization via compositional tuning, could be multiferroic.

Characterization of grain boundaries (GBs) typically focuses exclusively on ideal minimum-energy structures, thus offering a limited perspective on the potential structural diversity and related property variations of GBs. In this study, phase field crystal (PFC) simulations are employed to systematically explore alternative GB states through $\gamma$-surface sampling. A large number of tilt and twist GBs are examined in both fcc and bcc bicrystal structures. It is demonstrated that identifying variants in GB structure necessitates considering a set of microscopic degrees of freedom (DOF), comprising the components of relative crystal translation, in addition to the GB's five macroscopic DOF. Taking GB energy and excess volume as examples of key GB properties, a significant spread in GB energy is revealed, stemming from variations in the microscopic DOF, while maintaining constant macroscopic DOF. In addition, the significant variations found in GB excess volume and energy when GB variants are considered challenge the common assumption of a strong correlation between them. Taken together, the findings underscore the importance of recognizing a range of GB structures and properties for each macroscopic GB configuration, rather than relying on singular ideal minimum-energy GB structures, as is usually done.

Light excitation in organic semiconductors produces excitons, which are tightly bound electron-hole pairs that must be separated into mobile charge carriers to produce photocurrent in organic optoelectronic devices. The details of the exciton dissociation mechanism are required to control and improve the performance of these devices. Here it is demonstrated that modulated photocurrent spectrofscopy is sensitive to elucidating this mechanism. In this technique, a probe light beam of modulated intensity and a bias light beam of fixed intensity are combined to excite a two-terminal organic semiconductor device. The resulting modulated photocurrent is measured by a lock-in amplifier. In order to analyze the modulated photocurrent spectra, a model analysis is developed that incorporates in the photogeneration process different possible exciton disassociation mechanisms. Analytical expressions are derived which reveal the essential key spectral features that are the signature of each different exciton dissociation mechanism. Among the investigated mechanisms, exciton dissociation through electron transfer to donorlike traps in the lower part of the energy gap that creates single-type carriers (holes) photocurrent was found to dominate in the pristine small molecule organic semiconductors studied here. This process becomes evident by affecting the dynamics of the interactions of holes with the above traps which are captured by the modulated photocurrent spectroscopy, highlighting its sensitivity to this mechanism. Moreover, a very useful method is proposed to extract the steady-state exciton density.

We report the continuous tuning of magnetic anisotropy in perpendicularly magnetized bismuth-substituted yttrium iron garnet (Bi-YIG) films via ${\mathrm{He}}^{+}$ ion irradiation. Our findings indicate that the magnetization direction of epitaxial Bi-YIG films on sGGG substrates transitions from out of plane in the as-grown state to in plane after ${\mathrm{He}}^{+}$ ion irradiation at a fluence exceeding $2\times {10}^{14}\mathrm{ions}/{\mathrm{cm}}^2$. The reorientation is attributed to the relaxation of tensile film strain, which reduces the perpendicular magnetic anisotropy without affecting the saturation magnetization. The Gilbert damping parameter and the inhomogeneous broadening of the ferromagnetic resonance linewidth show only minimal increases with ion irradiation. Additionally, at a fluence of $5\times {10}^{13}\mathrm{ions}/{\mathrm{cm}}^2$, we observe the formation of magnetic bubble domains in the Bi-YIG films. Micromagnetic simulations estimate a Dzyaloshinskii-Moriya interaction of $0.006\mathrm{mJ}/{\mathrm{m}}^2$, which is insufficient for stabilizing Néel-type skyrmions. Finally, we demonstrate that the effects of ${\mathrm{He}}^{+}$ ion irradiation can be largely reversed through thermal annealing in an oxygen atmosphere.

Pyrite-structure $\mathrm{Ni}{\mathrm{S}}_2$ is, in principle, a model antiferromagnetic Mott insulator that can be electron doped, hole doped, and bandwidth controlled. Despite decades of study, however, the electronic and magnetic behavior of $\mathrm{Ni}{\mathrm{S}}_2$ have proven challenging to understand. Here, we build on recent advances establishing surface conduction in $\mathrm{Ni}{\mathrm{S}}_2$ to completely reexamine the electronic phase behavior of electron- and hole-doped single-crystal ${\mathrm{Ni}}_{1-x}{\mathrm{Cu}}_x{\mathrm{S}}_2$ and ${\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{\mathrm{S}}_2$. Magnetometry, heat capacity, neutron diffraction, and electronic transport measurements suggest that prior studies missed vital details of the magnetic ordering in this system. While electron and hole doping rapidly increase the antiferromagnetic ordering temperature (by as much as fourfold by $x\approx 0.1$), signatures remain of antiferromagnetic and weak ferromagnetic ordering at the same temperatures as in undoped $\mathrm{Ni}{\mathrm{S}}_2$. As these undoped ordering temperatures remain constant, the associated magnetic moments are diminished by doping, strongly implicating electronic/magnetic phase coexistence across the Mott insulator-metal transition. Substantial structural changes and inhomogeneity accompany these evolutions, highlighting the importance of structural-chemical-electronic-magnetic coupling in $\mathrm{Ni}{\mathrm{S}}_2$. The insulator-metal transition is also strongly electron/hole asymmetric, which we interpret with the aid of complementary dynamical mean-field theory results. These findings significantly revise and advance our understanding of the electronic phase behavior of this prototypical Mott insulator, highlighting the essential role of electronic, magnetic, structural, and chemical inhomogeneity across the Mott transition.

Narrow-gap rare-earth Zintl phases frequently exhibit fascinating physical phenomena owing to their various crystal structures, complex magnetic properties, and tunable transport behaviors. Here, we report the synthesis, magnetic, thermodynamic, and transport properties of a Eu-containing Zintl arsenide, ${\mathrm{Eu}}_{11}{\mathrm{Zn}}_6{\mathrm{As}}_{12}$, which consists of infinite chains of Eu cations and anionic frameworks constructed from corner-sharing ${\mathrm{ZnAs}}_4$ tetrahedra. ${\mathrm{Eu}}_{11}{\mathrm{Zn}}_6{\mathrm{As}}_{12}$ exhibits complicated magnetic behavior owing to intricate exchange interactions mediated by the discrete anionic fragments. Two long-range magnetic transitions at 22 K ($T_{\mathrm{N}}$) and 9 K ($T^{*}$), as well as exceptionally strong ferromagnetic fluctuations around 29 K ($T_{\mathrm{F}}$), are indicated by the susceptibility, heat capacity, and resistivity measurements. Besides, ${\mathrm{Eu}}_{11}{\mathrm{Zn}}_6{\mathrm{As}}_{12}$ displays metallic behavior, attributable to the hole carriers doped by slight Eu vacancies or the mixed valence of ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$. A prominent resistivity peak occurs around $T_{\mathrm{N}}$, which is rapidly suppressed by the applied field, leading to a large negative magnetoresistance effect. A resistivity hysteresis is observed below 5 K, caused by a small net ferromagnetic component. Our study presents the distinct magnetic and transport properties of ${\mathrm{Eu}}_{11}{\mathrm{Zn}}_6{\mathrm{As}}_{12}$, and further experiments are required to elucidate the origin of these behaviors. Moreover, our findings demonstrate that Eu-based Zintl phases are a fertile ground to study the interplay between magnetism and charge transport.

High-resolution neutron powder diffraction, magnetometry, and electrotransport measurements were used to investigate the crystal and magnetic structures of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Co}}_{1-y}{\mathrm{Ni}}_y{\mathrm{O}}_{3-\gamma }$ perovskites within the temperature range of 4 to 300 K, along with their magnetic and transport properties. The symmetry of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Co}}_{1-y}{\mathrm{Ni}}_y{\mathrm{O}}_{3-\gamma }$ was identified as monoclinic $I2/a$. In contrast to models positing gradual spin-state transitions of ${\mathrm{Co}}^{3+}$ ions from low-spin (LS) to intermediate-spin (IS) and/or high-spin (HS) states, our proposed model asserts that ${\mathrm{Co}}^{3+}$ ions predominantly adopt the IS state for all ${\mathrm{Sr}}^{2+}$ and ${\mathrm{Ni}}^{2+}$ substitution levels within the investigated temperature range. Consequently, the observed transition from a semiconducting spin glass to a metallic ferromagnetic state with increasing substitution levels can be attributed to a gradual decrease in the degree of monoclinic distortions, shortening of Co–O bond lengths and straightening of Co–O–Co bond angles, enhancing the overlap between ${\mathrm{Co}}^{3+}d$ and ${\mathrm{O}}^{2-}p$ orbitals.

Understanding and manipulating the relationship between intentionally introduced disorder and material properties necessitates efficient characterization techniques. For example, single crystal diffuse scattering experiments provide insights into the driving forces behind local order phenomena. In this work, we present a time- and resource-efficient approach based on mean field theory that quantifies effective local interaction energies but, unlike other techniques, does not require computationally expensive supercell models. The method is employed to quantify competing driving forces in functionally disordered crystalline materials such as disordered rock salt cathode materials and Prussian blue analogs that share an underlying face-centered lattice.

The plastic deformation of crystalline and other heterogeneous materials often manifests in stochastic intermittent events indicating the criticality of plastic behavior. Previous studies demonstrated that the presence of short-ranged quenched disorder modifies this behavior disrupting long-range static and dynamic correlations consequently localizing dislocation avalanches. However, these observations were mostly confined to relaxed materials devoid of deformation history. In this work our focus is on how straining affects static and dynamic correlations, avalanche dynamics and local yield stresses. We demonstrate that the interplay between severe straining and confining quenched disorder induces critical behavior characterized by dislocation avalanches distinct from those at lower stresses. Namely, near the flow stress many avalanches, even if triggered locally, evolve into events affecting a larger region by exciting small clusters of dislocations all around the sample. This type of avalanches differ from the ones at low strains where plastic events typically consist of one compact cluster of dislocations which is either local or it is already quite extended at the onset of the avalanche. Furthermore, we examine the impact of avalanches on local yield stresses. It is shown in detail in this work that while some statistical features of the local yield thresholds are robust to straining, others are significantly affected by the deformation history.

We explore the impact of introducing IIIA-group element Ga in place of IIB-group element Zn in binary intermetallic GdZn on its magnetic and magnetocaloric properties, as well as explicate the modified electronic band structure of the compound. The magnetic transition temperature of the compound decreases with the increase of Ga concentration in ${\mathrm{GdZn}}_{1-x}{\mathrm{Ga}}_x(x=0–0.1)$ while the crystal structure (CsCl prototype) and lattice parameters remain unchanged. Our detailed analysis of magnetization and magnetocaloric data conclusively proves that long-ranged magnetic ordering exists in the sample, despite the magnetic interaction considerably weakening with the increase of Ga. The experimental data are rationalized using both a theoretical machine-learning model and first-principle density functional theory. The electronic band structure of GdZn is manifested with some unusual complex features which gradually diminish with Ga doping and the conventional sinusoidal feature of Ruderman-Kittel-Kasuya-Yosida-type interactions also disappears. A mean-field theory model is developed and can successfully describe the overall magnetocaloric behavior of the ${\mathrm{GdZn}}_{1-x}{\mathrm{Ga}}_x$ series of samples.

Covalent organic frameworks (COFs), recognized for their potential in vehicular methane storage, typically require comprehensive and resource-intensive evaluations for capacity determination. Herein, we introduce a novel approach that synergizes symbolic regression with machine learning, augmented by descriptive statistics, systematic feature selection, and high-throughput computational techniques, to develop predictive equations for COF methane storage capacities. By strategically selecting a concise set of three to seven features based on their interpretability and measurability, we have developed a spectrum of symbolic regression-based equations to meet diverse precision requirements in forecasting COF methane capacities. The fidelity of these equations varies: an equation with three variables achieves a root mean square error (RMSE) of $10\mathrm{c}{\mathrm{m}}^3$ (standard temperature and pressure, STP) per $\mathrm{c}{\mathrm{m}}^3$ and a mean absolute percentage error (MAPE) of 5%, while expanding to seven variables fine-tunes the RMSE to $8.1\mathrm{c}{\mathrm{m}}^3$ (STP) per $\mathrm{c}{\mathrm{m}}^3$ and the MAPE to 4.2%. Our structured approach ensures accurate predictions that correspond to the resolution of available data. Upon applying our most precise equation to an extensive dataset of $449468$ COFs, we identified numerous COFs with exceptional methane storage capacities. The adaptability of our equations to integrate new data broadens their applicability beyond COFs, making them versatile tools for optimizing methane storage across a range of nanoporous materials.