Recently developed halide double perovskites $A_2{M}^{+}{M}^{3+}{X}_6$ have attracted attention as potential lead-free perovskite solar cell absorbers. Here, the stability, electronic, and optical properties of the lead-free inorganic-organic hybrid double perovskites ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ ($X$ = halogen) are studied based on density functional theory. By investigating the thermodynamic stability against decomposition, the ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ double perovskites are shown to be stable. Furthermore, the use of a small magnitude of hydrostatic pressure is proposed to improve the thermodynamic stability of double perovskites ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$. Meanwhile, the calculated electronic band structures of ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ double perovskites exhibit suitable direct band gaps, small carrier effective masses, and three-dimensional electronic dimensionality. This, in turn, leads to strong optical absorption. Both the electronic and optical features make double perovskites ${\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)}_2\mathrm{InBi}{X}_6$ suitable for applications in optoelectronics.

Prediction models of both the electronic and ionic contributions to the static dielectric constants have been constructed using data from density functional perturbation theory calculations of approximately 1200 metal oxides via supervised machine learning. We developed two types of random forest regression models for oxides with the ground-state crystal structures: one model requires only compositional information and the other model also uses structural information. Although the training data included various atomic frameworks, the prediction models performed well even when only compositional information was used as feature descriptors. In prediction of the electronic contributions to the dielectric constants, the accuracies of the regression models with and without structural information were comparable, while the structural descriptors more clearly improved the prediction accuracy for the ionic contributions. We also analyzed the feature importance for prediction of the dielectric constants. The mean atomic mass and mass density were determined to be significant features in prediction of the electronic contributions without and with structural information, respectively. The standard deviation of the principal quantum number and mean neighbor distance variation were found to be important for the respective prediction models of the ionic contributions. The correlations between the dielectric constants and these features are discussed, along with the underlying physical mechanisms.

High-efficiency optoelectronic devices require materials with suitable band gap, excellent carrier mobility, strong light-matter interaction, and high thermal- and photostability. Herein, we report two layered materials of group VI, namely, $\mathrm{Se}{\mathrm{S}}_2$ and $\mathrm{Te}{\mathrm{S}}_2$, which meet all those desired criteria. Both materials are stable with 1T-$\mathrm{Mo}{\mathrm{S}}_2$-like structure, and exhibit in-direct gap semiconducting feature and possess extraordinary electronic transport properties with isotropic carrier mobilities outperforming their $\mathrm{Mo}{\mathrm{S}}_2$ analog. Using the $GW$ approach for determining the quasiparticle electronic structure and Bethe-Salpeter equation accounting for the many-body excitonic effects, the optical properties of both materials have been identified. We find that exciton effects are significant in both materials that is characterized by the large binding energy of 0.66 and 0.84 eV for monolayer $\mathrm{Se}{\mathrm{S}}_2$ and $\mathrm{Te}{\mathrm{S}}_2$, respectively. The absorption edges lie in the optimal energy window of $1.0\text{–}1.5\mathrm{eV}$, which enables strong photoabsorption covering the whole visible-light region. Finally and importantly, we demonstrate that the in-direct nature of those materials can be violated by capping them with 2H-$\mathrm{Mo}{\mathrm{S}}_2$ to form a van der Waals heterostructure, while keeping the gap in the energy favorable region. Those physical and optical characteristics make few-layer $\mathrm{Se}{\mathrm{S}}_2$ and $\mathrm{Te}{\mathrm{S}}_2$ highly appealing for room-temperature light-emitting devices and, particularly, solar harvesting devices.

A mechanical stretch applied to a monodomain twisted nematic elastomer thin film untwists the helical director distribution into a uniaxially aligned state. If the film thickness is less than the helical pitch, untwisting behavior is stretching direction dependent in the plane normal to the helix axis. We investigate untwisting of twisted nematic elastomers by optical, mechanical, and x-ray scattering methods. We adapt a model for cholesteric elastomers to our observations and find that the material exhibits characteristics with the combined properties of both a smectic and twisted nematic configuration.

Adhesive wear is a result of material exchange due to localized adhesive bonding between surface asperities at small scales. It has been recently debated whether a linear wear relation can be observed at the single-asperity level. Using large-scale atomistic simulations, we show that the wear relation is a direct result of the material removal mechanism at the asperity level. In the presence of weak adhesion, sliding is dominated by frictional slipping (i.e., dislocations glide in the contact plane), where occasional atomic cluster detachments and a sublinear wear relation are expected. Alternatively, a linear relation between the volume of detached material and the frictional work can be obtained when bulk plasticity dominants material removal at the asperity tip. Under this condition, high-shear stresses at the contact trigger the migration of misfit dislocations into the asperity bulk, causing severe plastic deformation via dislocation-mediated interface migration. This result highlights that the state of stresses at contact governs the process of material removal and wear relation at the asperity level.

Polarization, especially of ferroelectrics FEs, is conventionally described by ion positions, e.g., by Born effective charges, where the complete entanglement of electron polarization with that of ions is implicitly assumed. We find that such descriptions or Born effective charge polarization-type approaches break down partially in the presence of high field, owing to the partial disentanglement of electrons with ions. To overcome this, we propose a correction (non-Born effective charge polarization) that calculates both macroscopic and unit-cell-by-unit-cell total polarization accurately. The accuracy of this method is demonstrated in prototypical situations of depolarization field $E_d$ that exists in finite-size or inhomogeneous insulating FEs: paraelectric/FE, FE capacitors, and FE/vacuum. Here, FE/vacuum are shown to be electrically identical to encountering domains. This method provides simple algebraic formulas to calculate total polarization $P_{\mathrm{S}}$ and $E_d$ using conventionally estimated polarizations that are obtained from local ion positions. Therefore, it can be easily used in experimental estimations of $P_{\mathrm{S}}$ and $E_d$, including 3D cases. For example, this method reveals that $P_{\mathrm{S}}$ varies across ferroelectric/insulator far less than the conventional estimate, which explains substantially reduced $E_d$ and the absence of metallicity. In addition, vortexlike domains are discussed in view of $E_d$. The partial disentanglement of ion and electron polarization would imply limitation of Ginzburg-Landau framework of ferroelectrics under high field.

Functional van der Waals layered materials, which exhibit interesting phenomena such as magnetism and ferroelectricity, have been proposed for use in next-generation nanoscale devices and sensors. $\mathrm{CuCr}{\mathrm{P}}_2{\mathrm{S}}_6$ is a promising two-dimensional (2D) material that evinces multiferroic behavior where the $\mathrm{C}{\mathrm{u}}^{+}$ and $\mathrm{C}{\mathrm{r}}^{+3}$ cations are responsible for antiferroelectric and antiferromagnetic ordering, respectively. In this study, we use x-ray diffraction and Raman spectroscopy to map out these phase transitions in the range 70–400 K. The antiferroelectric phase transition is complex and shows a gradual transition to complete antipolar order with an intermediate quasiantipolar step. X-ray diffraction studies reveal evidence for negative thermal expansion across the antipolar phase transitions at ∼270 and ∼140 K. The latter of these is accompanied by a drastic reduction in rotational and translational mode frequencies of the anion groups in $\mathrm{CuCr}{\mathrm{P}}_2{\mathrm{S}}_6$. Our temperature-dependent structural data provide an important reference for subsequent research into this promising 2D multiferroic material.

We report on the electric-field response of 180° nanodomain walls in $\mathrm{BaTi}{\mathrm{O}}_3$ using in situ electrical biasing in transmission electron microscopy. The sample is biased on a microdevice designed for reliable testing whose key attributes are confirmed by finite element calculations. The presence of weakly charged zigzag domain walls at room temperature is attributed to the geometric confinement of the device. The motion of the domain walls under the applied electric field allows to extract local P-E loops where distinct domain-wall pinning in deep and random energy potential profiles, characteristic for “hard” and “soft” ferroelectrics, respectively, are observed. “Hard” domain-wall pinning results in asymmetrical loops typical for “hard” ferroelectrics while the “soft” domain-wall pinning follows Rayleigh-like behavior. All effects are measured locally and directly from the imaged domain structure.

In this paper, we report a direct observation of a magnetic proximity effect in an amorphous thin-film exchange-spring magnet by the use of neutron reflectometry. The exchange-spring magnet is a trilayer consisting of two ferromagnetic layers with high $T_c$ separated by a ferromagnetic layer, which is engineered to have a significantly lower $T_c$ than the embedding layers. This enables us to measure magnetization depth profiles at which the low-$T_c$ material is in a ferromagnetic or paramagnetic state, while the embedding layers are ferromagnetic. A clear proximity effect is observed 7 K above the intrinsic $T_c$ of the embedded layer, with a range extending 50 Å.

We report transport measurements of strained $\mathrm{InAs}/{\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{Sb}$ composite quantum wells (CQWs) in the quantum spin Hall phase, focusing on the control of the energy gap through structural parameters and an external electric field. For highly strained CQWs with $x=0.4$, we obtain a gap of 35 meV, an order of magnitude larger than that reported for binary InAs/GaSb CQWs. Using a dual-gate configuration, we demonstrate an electrical-field-driven topological phase transition, which manifests itself as a reentrant behavior of the energy gap. The sizable energy gap and high bulk resistivity obtained in both the topological and normal phases of a single device open the possibility of electrical switching of the edge transport.

While binary $R{E}_2\mathrm{In}$, where $RE=\mathrm{rare}\mathrm{earth}$, have been reported a few decades ago, recent investigations revealed intriguing new physical insights. For instance, the discovery of a nearly ideal first-order ferromagnetic transition in ${\mathrm{Eu}}_2\mathrm{In}$ calls for further exploration of structures and properties of $R{E}_2\mathrm{In}$, in particular for the least-documented $RE=\mathrm{Eu}$ and Yb cases. Here, we investigate ${\mathrm{Eu}}_{2\text{−}x}{\mathrm{Yb}}_x\mathrm{In}$ pseudobinaries with nominal values of $x=0.25$, 0.5, 0.75, 1, 1.5, 2 by powder x-ray diffraction (including as function of temperature from 100 to 375 K for ${\mathrm{Yb}}_2\mathrm{In}$), magnetization (5–300 K), as well as electrical resistivity (5–300 K) and calorimetric (2–150 K) measurements for ${\mathrm{Yb}}_2\mathrm{In}$. Compared to other RE, Yb or Eu always raise challenging questions linked to their valence states. From average atomic volume, Yb is anticipated to be divalent in ${\mathrm{Yb}}_2\mathrm{In}$, at least between 100 and 375 K, which is in line with the absence of $4f$ magnetism. In agreement with x-ray diffraction and magnetization data, the resistivity of ${\mathrm{Yb}}_2\mathrm{In}$ is rather featureless and typical of a metal. Establishing ${\mathrm{Yb}}_2\mathrm{In}$ as a nonmagnetic isostructural reference for ${\mathrm{Eu}}_2\mathrm{In}$ allows one to use its heat capacity to revisit that of the latter, and get experimental insights into the exceptional magnetocaloric effect of the compound with Eu. In particular, we show that a third of the total magnetic entropy ($S_{\mathrm{m}}\approx 35.6\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-1}$ at $T=100\mathrm{K}$) is concentrated in a 3 K temperature window around the $T_{\mathrm{C}}$ of ${\mathrm{Eu}}_2\mathrm{In}$. Starting from the ferromagnetic compound ${\mathrm{Eu}}_2\mathrm{In}$ $[T_{\mathrm{C}}=55.2\left(5\right)\mathrm{K}]$, we show that Yb substitutions in ${\mathrm{Eu}}_{2\text{−}x}{\mathrm{Yb}}_x\mathrm{In}$ lead to a decrease in both the Curie temperature [$T_{\mathrm{C}}=41\left(2\right)$ and 32(2) K for $x=0.25$ and 0.5] and magnetic saturation, while weakening the first-order character of the transition as $x$ increases. A significant isothermal entropy change of $5.1\left(4\right)\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-1}$ for $\mathrm{\Delta }B=2\mathrm{T}$ is found at 44 K in ${\mathrm{Eu}}_{1.75}{\mathrm{Yb}}_{0.25}\mathrm{In}$, demonstrating that the giant magnetocaloric effect of ${\mathrm{Eu}}_2\mathrm{In}$ can be tuned to lower temperatures by Yb substitutions.

Magnesium has a different crystal structure from its dihydride with hydrogenation leading to a phase transition from the hexagonal closely packed Mg into a tetragonal $\alpha \text{−}\mathrm{Mg}{\mathrm{H}}_2$ rutile type structure. Such materials exhibit complex hydrogen uptake and release kinetics because hydrogen diffusivities significantly change when the crystal structure changes. To provide a foundational understanding of (de)hydrogenation kinetics that is applicable to all stages of the reaction, we performed statistically averaged molecular dynamics simulations to derive hydrogen diffusivities as a function of temperature and hydrogen content for both magnesium and magnesium hydride. Our studies confirm that hydrogen diffusivities in magnesium hydride are much lower than in magnesium, in agreement with experimental data. Additionally, we observe that in either magnesium or magnesium hydride, higher hydrogen compositions result in reduced diffusivities. The latter was not revealed by prior experiments, which were conducted at fixed hydrogen composition. Finally, we discover a non-Arrhenius behavior in magnesium hydride. The physical origin of this behavior is also discussed.

Nanosize vacancy clusters, characterized in metals after plastic deformation, irradiation or specific heat treatments are suspected to participate in materials hardening through their interactions with mobile dislocations. Our numerical simulations made from combining three different simulation techniques, i.e., molecular statics, kinetic Monte Carlo and elastic line models allow us to compute the dislocations velocity in realistic conditions of applied shear stress, temperature, concentration, and size of the vacancy clusters, in face–centered-cubic aluminium. We show that the clusters behave as sources of vacancies that follow a reaction path along the dislocation line, which is recognized as a pipe diffusion process. The accumulation of vacancies in the dislocation stacking fault ribbon yields jogs that participate in the dislocation climb. Both vacancy leaks from clusters and climb of dislocation segments contribute to the dislocation crossing, which remains thermally activated. We integrated the ensemble of the thermally activated processes: Diffusion, emission, absorption processes, as well as dislocation-cluster crossing, into the same simulation allowing us to predict the dislocation mobility in good agreement with experimental deformation tests.

To shed a new light on the complex microstructural evolution in the Ti–Al–Mo system, we employ ab initio calculations to study bcc-fcc structural transformations of ordered ${\beta }_{\mathrm{o}}-\mathrm{TiAl}(+\mathrm{Mo})$ and disordered $\beta -\mathrm{TiAl}(+\mathrm{Mo})$ to ordered $\gamma -\mathrm{TiAl}(+\mathrm{Mo})$ and hypothetically assumed disordered ${\gamma }_{\mathrm{dis}}-\mathrm{TiAl}(+\mathrm{Mo})$ alloys, respectively. In particular, tetragonal (Bain's path) and trigonal transformations are combined with the concept of special quasirandom structures (SQS) and examined. Our calculations of the ordered phases show that the ${\beta }_{\mathrm{o}}\to \gamma$ tetragonal transformation of TiAl is barrierless, i.e., proceeds spontaneously, reflecting the genuine structural instability of the ${\beta }_{\mathrm{o}}$ phase. Upon alloying of $\approx 7.4\mathrm{at}.\%$ Mo, a small barrier between ${\beta }_{\mathrm{o}}$ and $\gamma$-related local energy minima is formed. Yet a higher Mo content of $\approx 9\mathrm{at}.\%$ leads to an opposite-direction barrierless transformation $\gamma \to {\beta }_{\mathrm{o}}$, i.e., fully stabilizing the ${\beta }_{\mathrm{o}}$ phase. Considering the disordered phases, the $\beta -{\mathrm{Ti}}_{0.5}{\mathrm{Al}}_{0.5-x}{\mathrm{Mo}}_x$ and ${\gamma }_{\mathrm{dis}}-{\mathrm{Ti}}_{0.5}{\mathrm{Al}}_{0.5-x}{\mathrm{Mo}}_x$ are energetically very close. Importantly, for all here-considered compositions up to $11\mathrm{at}.\%$ of Mo, a small energy barrier separates $\beta -\mathrm{TiAl}(+\mathrm{Mo})$ and ${\gamma }_{\mathrm{dis}}-\mathrm{TiAl}(+\mathrm{Mo})$ energy minima. Finally, a trigonal path was studied as an alternative transformation connecting disordered $\beta$ and ${\gamma }_{\mathrm{dis}}$-TiAl phases, but it turns out that it exhibits an energy barrier over $60\mathrm{meV}/\mathrm{at}.$ which, in comparison to the Bain's path with $9\mathrm{meV}/\mathrm{at}.$ barrier, effectively disqualifies the trigonal transformation for the TiAl system.

The existence of a charge density wave (CDW) in transition-metal dichalcogenide (TMDC) ${\mathrm{CuS}}_2$ has remained undetermined since its first experimental synthesis nearly 50 years ago. Despite conflicting experimental literature regarding its low-temperature structure, there exists no theoretical investigation of the phonon properties and lattice stability of this material. By studying the first-principles electronic structure and phonon properties of ${\mathrm{CuS}}_2$ at various electronic temperatures, we identify temperature-sensitive soft phonon modes which unveil a previously unreported Kohn anomaly at approximately 100 K. Variation of the electronic temperature shows the presence of two distinct phases, characterized at low electronic temperature by a $2\times 2\times 2$ periodic charge modulation associated with the motion of the ${\mathrm{S}}_2$ dimers. We find this is driven by a slight orbital occupation imbalance of the copper $d$ and sulfur $p$ orbitals, reminiscent of the Jahn-Teller effect in finite systems. Investigation of the Fermi surface presents a potential Fermi surface nesting vector related to the location of the Kohn anomaly and observed band splittings in the unfolded band structure. The combination of these results suggests a strong possibility of CDW order in ${\mathrm{CuS}}_2$. Further study of ${\mathrm{CuS}}_2$ in monolayer form finds no evidence of a CDW phase, as the identified bulk periodic distortions cannot be realized in two dimensions. This behavior sets this material apart from other transition-metal dichalcogenide materials, which exhibit a charge density wave phase down to the two-dimensional limit. As CDW in TMDC materials is considered to compete with superconductivity, the lack of a CDW in monolayer ${\mathrm{CuS}}_2$ suggests the possibility of enhanced superconductivity relative to other transition-metal dichalcogenides. Overall, our work identifies ${\mathrm{CuS}}_2$ as a previously unrealized candidate to study the interplay of superconductivity, CDW order, and dimensionality.

We report an atomistic investigation, based on density functional theory calculations within the D3 van der Waals correction, of the adsorption properties of ${\mathrm{H}}_2, {\mathrm{N}}_2, {\mathrm{O}}_2, \mathrm{CO}, \mathrm{NO}, {\mathrm{CO}}_2, {\mathrm{NO}}_2$, and ${\mathrm{CH}}_4$ on the semiconductor ${\mathrm{Sn}}_3{\mathrm{O}}_4\left(010\right)$ monolayer surface. Except for ${\mathrm{NO}}_2$ and $\mathrm{NO}$ molecules, the adsorption energies are from $-64\mathrm{m}\mathrm{e}\mathrm{V}$ (${\mathrm{H}}_2$) up to $-167\mathrm{m}\mathrm{e}\mathrm{V}$ (${\mathrm{CO}}_2$) with the molecule-surface distances larger than $3.30\text{Å}$ for all molecules, and hence, minor effects were observed on the ${\mathrm{Sn}}_3{\mathrm{O}}_4\left(010\right)$ surface electronic structure upon adsorption. ${\mathrm{NO}}_2$ has the largest adsorption energy ($-525\mathrm{m}\mathrm{e}\mathrm{V}$), which can be explained by closer approach of the two $\mathrm{O}$ atoms towards the surface, while $\mathrm{NO}$ binds to the surface with about half of the ${\mathrm{NO}}_2$ adsorption energy (e.g., $-279\mathrm{m}\mathrm{e}\mathrm{V}$). From Bader analysis, we found substantial charge transfer from the surface to the molecules, $-0.52e$ (${\mathrm{NO}}_2$) and $-0.23e$ ($\mathrm{NO}$), which is consistent with the smaller distances to the surface, 2.46 and $2.82\text{Å}$, respectively. Thus, those results suggest an improved detection performance of ${\mathrm{Sn}}_3{\mathrm{O}}_4$ towards ${\mathrm{NO}}_2$, which can help to design sensor devices based on the ${\mathrm{Sn}}_3{\mathrm{O}}_4\left(010\right)$ monolayers.

High-strength metal alloys achieve their performance via careful control of precipitates and solutes. The nucleation, growth, and kinetics of precipitation, and the resulting mechanical properties, are inherently atomic scale phenomena, particularly during early-stage nucleation and growth. Atomistic modeling using interatomic potentials is a desirable tool for understanding the detailed phenomena involved in precipitation and strengthening, which requires length and timescales far larger than those accessible by first-principles methods. Current interatomic potentials for alloys are not, however, sufficiently accurate for such studies. Here a family of neural-network potentials (NNPs) for the Al-Cu system are presented as a first example of a machine learning potential that can achieve near-first-principles accuracy for many different metallurgically important aspects of this alloy. High-fidelity predictions of intermetallic compounds, elastic constants, dilute solid-solution energetics, precipitate-matrix interfaces, generalized stacking fault energies and surfaces for slip in matrix and precipitates, antisite defect energies, and other quantities, are shown. The NNPs also captures the subtle entropically induced transition between ${\theta }^{\prime }$ and $\theta$ at temperatures around 600 K. Many comparisons are made with the state-of-the-art angular-dependent potential for Al-Cu, demonstrating the significant quantitative benefit of a machine learning approach. A preliminary kinetic Monte Carlo study shows the NNP to predict the emergence of GP zones in Al-4at%Cu at $T=300$ K in agreement with experiments. These studies show that the NNP has significant transferability to defects and properties outside the structures used to train the NNP but also shows some errors highlighting that the use of any interatomic potential requires careful validation in application to specific metallurgical problems of interest.

Interatomic potentials are essential for studying fundamental mechanisms of deformation and failure in metals and alloys because the relevant defects (dislocations, cracks, etc.) are far above the scales accessible to first-principles studies. Existing potentials for non-fcc metals and nearly all alloys are, however, not sufficiently quantitative for many crucial phenomena. Here machine learning in the Behler-Parrinello neural-network framework is used to create a broadly applicable potential for pure hcp magnesium (Mg). Lightweight Mg and its alloys are technologically important while presenting a diverse range of slip systems and crystal surfaces relevant to both plasticity and fracture that present a significant challenge for any potential. The machine learning potential is trained on first-principles density-functional theory (DFT) computable metallurgically relevant properties and is then shown to well predict metallurgically crucial dislocation and crack structures and competing phenomena. Extensive comparisons to an existing very good modified embedded atom method potential are made. These results demonstrate that a single machine learning potential can represent the wide scope of phenomena required for metallurgical studies. The DFT database is openly available for use in any other machine learning method. The method is naturally extendable to alloys, which are necessary for engineering applications but where ductility and fracture are controlled by complex atomic-scale mechanisms that are not well predicted by existing potentials.

The solid solution $\mathrm{Ba}{\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{\mathrm{S}}_2$ exhibits an insulator-to-metal transition close to $x=0.21$. Questions of whether this transition is coupled with structural changes remain open. Here we follow the structural evolution as a function of the Ni content $x$ using synchrotron powder x-ray diffraction and pair distribution function analyses to reveal significant basal sulfide anion displacements occurring preferentially along the ${\mathrm{CoS}}_5$ pyramidal edges comprising the edge-connected bond network in $\mathrm{Ba}{\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{\mathrm{S}}_2$. These displacements decrease in magnitude as $x$ increases and are nearly quenched in $x=1{\mathrm{BaNiS}}_2$. Density-functional-theory-based electronic structure calculations on $x=0{\mathrm{BaCoS}}_2$ suggest that these displacements arise as a dynamic first-order Jahn-Teller effect owing to partial occupancy of nominally degenerate ${\mathrm{Co}}^{2+}{d}_{xz}$ and $d_{yz}$ orbitals, leading to local structural symmetry breaking in the $xy$-plane of the Co-rich phases. The Jahn-Teller instability is associated with the opening of a band gap that is further strengthened by electronic correlation. The Jahn-Teller effect is reduced upon increased electron filling as $x\to 1$, indicating that the local structure and band filling cooperatively result in the observed insulator-to-metal transition.