Materials adopting the diamond structure possess useful properties in atomic and colloidal systems and are a popular target for synthesis in colloids where a photonic band gap is possible. The desirable photonic properties of the diamond structure pose an interesting opportunity for reconfigurable matter: Can we create a crystal able to switch reversibly to and from the diamond structure with a photonic band gap in the visible light range? Drawing inspiration from high-pressure transitions of diamond-forming atomic systems, we design a system of polyhedrally shaped colloidal particles with spherical cores that transitions from diamond to a tetragonal diamond derivative upon a small pressure change. The transition can alternatively be triggered by changing the shape of the particle in situ. We propose that the transition provides a reversible reconfiguration process for a potential new colloidal material and draw parallels between this transition and the phase behavior of the atomic transitions from which we take inspiration.

Lithium intercalation in graphite is an important process in the context of anodes for lithium ion batteries. With increasing demands on lithium ion batteries to operate at lower temperatures and higher currents, it is crucial to understand lithium intercalation in graphite due to issues associated with lithium plating. Lithium intercalation into graphite has been extensively studied theoretically using density functional theory (DFT) calculations, complemented by experimental studies through x-ray diffraction, spectroscopy, optical imaging, and other techniques. In this work, we present a first-principles-based model using DFT calculations, employing the Bayesian error estimation functional van der Waals (BEEF-vdW) as the exchange correlation functional, and Ising model to determine the phase transformations and, subsequently, the thermodynamic intercalation potential diagram. We explore a configurational phase space of about $1\times {10}^9$ structures by accurately determining the important interactions for the Ising model. The BEEF-vdW exchange correlation functional employed accurately captures a range of interactions including vdW, covalent, and ionic interactions. We incorporate phonon contributions at finite temperatures and configurational entropy to get high accuracy in free energy and potentials. We utilize the built-in error estimation capabilities of the BEEF-vdW exchange correlation functional and develop a methodological framework for determining the uncertainty associated with DFT calculated phase diagrams and intercalation potentials. The framework also determines the confidence of each predicted stable phase. The confidence value of a phase can help us to identify regions of solid solutions and phase transformations accurately. Given the subtle differences in energy between lithium intercalation into graphite and lithium plating $\left(<0.1\mathrm{eV}\right)$, we believe such an error estimation framework is crucial to know the reliability of DFT predictions.

Lead halide hybrid perovskites consist of an inorganic framework hosting a molecular cation located in the interstitial space. These compounds have been extensively studied as they have been identified as promising materials for photovoltaic applications with the interaction between the molecular cation and the inorganic framework implicated as influential for the electronic properties. ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbCl}}_3$ undergoes two structural transitions from a high temperature cubic unit cell to a tetragonal phase at 177 K and then a subsequent orthorhombic transition at 170 K. We have measured the low-frequency lattice dynamics using neutron spectroscopy and observe an energy broadening in the acoustic phonon linewidth towards the high-symmetry point ${\mathbf{Q}}_X=(2,\frac{1}{2},0)$ when approaching the transitions. Concomitant with these zone boundary anomalies is a hardening of the entire acoustic phonon branch measured in the $\mathbf{q}\to 0$ limit near the (2, 0, 0) Bragg position with decreasing temperature. Measurements of the elastic scattering at the Brillouin zone edges ${\mathbf{Q}}_X=(2,\frac{1}{2},0),{\mathbf{Q}}_M=(\frac{3}{2},\frac{1}{2},0)$, and ${\mathbf{Q}}_R=(\frac{3}{2},\frac{3}{2},\frac{5}{2})$ show Bragg peaks appearing below these structural transitions. Based on selection rules of neutron scattering, we suggest that the higher 177 K transition is displacive with a distortion of the local octahedral environment and the lower transition is a rigid tilt transition of the octahedra. We do not observe any critical broadening in energy or momentum, beyond resolution, of these peaks near the transitions. We compare these results to the critical properties reported near the structural transitions in other perovskites and particularly ${\mathrm{CsPbCl}}_3$ [Y. Fujii, S. Hoshino, Y. Yamada, and G. Shirane, Phys. Rev. B 9, 4549 (1974)]. We suggest that the simultaneous onset of static resolution-limited Bragg peaks at the zone boundaries and the changes in acoustic phonon energies near the zone center is evidence of a coupling between the inorganic framework and the molecular cation. The results also highlight the importance of displacive transitions in organic-inorganic hybrid perovskites.

The second-order nonlinear optical coefficients in the static limit are evaluated using density functional perturbation theory from the electronic response to a static electric field for the group-III nitrides and several $\mathrm{II}\text{−}\mathrm{IV}\text{−}{\mathrm{N}}_2$ ternary nitrides. They are compared with literature results using the sum over states approach including local field effects. The effects of the scissor correction are evaluated. Good agreement is obtained for GaN, AlN, and w-BN. For InN, the small or even negative gap in the LDA at $\mathrm{\Gamma }$ causes an extreme sensitivity to the k point summation and pseudopotentials. Similar problems occur for other very small gap $\mathrm{II}\text{−}\mathrm{IV}\text{−}{\mathrm{N}}_2$ semiconductors. The nonlinear optics coefficients are showing a general trend of increasing values with smaller gaps but no clear scaling relation with the direct gaps is obtained. In addition, unexpected changes in sign are found for the Si-based compounds, similar to the case of AlN, compared to the expected signs of $d_{33}$ in GaN. The Pockels coefficient, which includes in addition to the electronic response the phonon and piezoelectric response, is also evaluated. They show that the phonon and electronic contributions to the response in these materials are comparable in magnitude but in many cases of opposite sign.

Discovering novel, multicomponent crystalline materials is a complex task owing to the large space of feasible structures. Here we demonstrate a method to significantly accelerate materials discovery by using a machine learning (ML) model trained on density functional theory (DFT) data from the Open Quantum Materials Database (OQMD). Our ML model predicts the stability of a material based on its crystal structure and chemical composition, and we illustrate the effectiveness of the method by application to finding new quaternary Heusler (QH) compounds. Our ML-based approach can find new stable materials at a rate 30 times faster than undirected searches and we use it to predict 55 previously unknown, stable QH compounds. We find the accuracy of our ML model is higher when trained using the diversity of crystal structures available in the OQMD than when training on well-curated datasets which contain only a single family of crystal structures (i.e., QHs). The advantage of using diverse training data shows how large datasets, such as OQMD, are particularly valuable for materials discovery and that we need not train separate ML models to predict each different family of crystal structures. Compared to other proposed ML approaches, we find that our method performs best for small $\left(<{10}^3\right)$ and large $\left(>{10}^5\right)$ training set sizes. The excellent flexibility and accuracy of the approach presented here can be easily generalized to other types of crystals.

Electrical control of magnetism is a long-standing goal in science and technology, with the potential to enable a next generation of low-power memory and logic devices. Recently developed electrolyte gating techniques provide a promising route to realization, although the ultimate limits on modulation of magnetic properties remain unknown. Here, guided by a recent theoretical prediction, we demonstrate large enhancement of electrostatic modulation of ferromagnetic order in ion-gel-gated ultrathin films of the perovskite $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ by thickness tuning to the brink of percolation. Application of only 3–4 V is then shown capable of inducing a clear percolation transition from a short-range magnetically ordered insulator to a robust long-range ferromagnetic metal with perpendicular magnetic anisotropy. This realizes giant electrostatic Curie temperature modulation over a 150 K window, outstanding values for both complex oxides and electrolyte gating. In operando polarized neutron reflectometry confirms gate-controlled ferromagnetism, additionally demonstrating, surprisingly, that electrostatically induced magnetic order can penetrate substantially deeper than the Thomas-Fermi screening length.

We have investigated the band alignment at the interface of amorphous aluminum oxide (am-${\mathrm{Al}}_2{\mathrm{O}}_3$) grown by atomic layer deposition on p-Ge(100) and the effects of postgrowth annealing using hard x-ray photoelectron spectroscopy and density function theory (DFT). Accurate determination of the valence-band offsets was obtained by comparing the experimentally measured valence bands with DFT-calculated densities of states. The am-${\mathrm{Al}}_2{\mathrm{O}}_3$ density of states calculated from a weighted ensemble of crystalline ${\mathrm{Al}}_2{\mathrm{O}}_3$ structures gives excellent agreement with experiment, sufficiently capturing the nonlinear shape of the valence-band edge. We report a valence-band offset of $2.60\pm 0.1$ eV for am-${\mathrm{Al}}_2{\mathrm{O}}_3$/p-Ge, which is reduced by 0.20 eV upon annealing as interfacial ${\mathrm{GeO}}_x$ is formed.

$\mathrm{N}{\mathrm{a}}_{0.35}{\left({\mathrm{C}}_3{\mathrm{N}}_2{\mathrm{H}}_{10}\right)}_{0.426}\mathrm{F}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2$ exhibits a large orthorhombic distortion at room temperature without magnetic ordering, hence providing an ideal platform to investigate the correlation between nematicity and superconductivity. We show that its nematicity is stabilized by “nematic” ordered 1,3-diaminopropane (1,3- DIA) molecules in between the FeSe layers, and the C2 twofold rotational symmetry is preserved down to 10 K without phase transition. The superconductivity occurs at 46.5 K even though the orthorhombic distortion is much larger than that for β-FeSe and most of the other pnictide superconductors at low temperatures. First principles calculations reveal that this “artificial” orthorhombic distortion has a selective effect on the band structure, which behaves differently on splitting the iron $3d_{xz}$ and $3d_{yz}$ orbitals around the Г and M points in the Brillouin zone. Our finding strongly supports that the nematic state is irrelevant to the emergent superconductivity, at least in high-$T_c$ iron selenide superconductors.

Using a combination of high-resolution transmission electron microscopy (HRTEM), geometric phase analysis of HRTEM images, and scanning TEM, we study the structural transition and strain-field evolution at the grain boundary in an $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ bicrystal with a misorientation relationship of $\left[0001\right]/{30}^{\circ }$. The specimens are annealed at 300, 500, and 700 °C in the TEM. The grain boundaries in the specimens before annealing and annealed at 300 °C are observed to be faceted, but the grain boundaries in those annealed at 500 and 700 °C become defaceted, exhibiting meandering morphology. Interestingly, for the as-received specimen and that annealed at 300 °C, there occurs a periodic strain fluctuation along the grain boundary. However, such a periodic fluctuation along the grain boundary is not observed in the specimens annealed at 500 and 700 °C. These results suggest an unexplored possibility that the roughening transition of a grain boundary is enhanced by strain relaxation occurring at the grain boundary.

A systematic study of the electronic properties of single-layer Sb (antimonene) nanoribbons is presented. By using a six-orbital tight-binding Hamiltonian, we study the electronic band structure of finite ribbons with zigzag or armchair termination. We show that there is good agreement between ab initio calculations and the tight-binding model. We study how the size of the gap can be controlled by applying an external bias potential. An electric field applied perpendicular to the antimonene layer is found to increase the band gap, while a transverse bias potential leads to a position-dependent reduction of the band gap. Both kinds of bias potential break inversion symmetry of the crystal. This, together with the strong intrinsic spin-orbit coupling of antimonene, leads to spin splitting of the valence-band states.

Hydrogen exhibits qualitatively different charge states depending on the host material, as nicely explained by the state-of-the-art impurity-state calculation. Motivated by a recent experiment [Nature (London) 546, 124 (2017)], we show that the complex oxide ${\mathrm{SrCoO}}_{2.5}$ represents an interesting example, in which the interstitial H appears as a deep-level center according to the commonly-used transition level calculation, but no bound electron can be found around the impurity. Via a combination of charge difference analysis, density of states projection, and constraint magnetization calculation, it turns out that the H-doped electron is spontaneously trapped by a nonunique Co ion and is fully spin polarized by the onsite Hund's rule coupling. Consequently, the doped system remains insulating, whereas the antiferromagnetic exchange is slightly perturbed locally.

We study the structure and dynamics of water confined in a nanoporous graphitic carbon structure using molecular-dynamics (MD) simulations. The carbon structure is generated by a reactive MD simulation of oxidation of a silicon carbide nanoparticle. We embed water molecules in the nanopores and study structural and dynamical properties of nanoconfined water as a function of temperature. MD simulation results indicate the presence of high-density water (HDW) and low-density water. Radial distribution functions and spatial density functions indicate that the second solvation shell of the HDW is broken. We calculate the self-diffusion coefficient of confined water molecules as a function of temperature and find a significant decrease in the diffusion of water molecules around $T=190\mathrm{K}$. The cage correlation function $c\left(t\right)$ of confined water molecules at $T=200\mathrm{K}$ exhibits stretched exponential decay, $c\left(t\right)=\exp [-{\left(\mathrm{t}/\tau \right)}^{\beta }]$, with $\beta =0.43$, which matches exactly with the theoretical prediction $\beta =3/7$. Furthermore, the self-intermediate scattering function at $T=200\mathrm{K}$ indicates differences in small-scale and large-scale dynamics of water molecules.

Here, we provide a link which shows how the compositional change affects the liquid and glass stabilities by measuring temperature-time transformation (TTT) diagram of Cu-Zr alloys, revealing the origin of glass forming ability (GFA) with composition. From the TTT study, we find that the compositional change yields a different origin of GFA for the three best bulk metallic glasses $({\mathrm{Cu}}_{64}{\mathrm{Zr}}_{36},{\mathrm{Cu}}_{56}{\mathrm{Zr}}_{44}$, and ${\mathrm{Cu}}_{50}{\mathrm{Zr}}_{50})$ due to varying stabilities of liquid and glass, which explains maxima observed in undercoolability, persisting time at nose temperature, and crystal-liquid interfacial free energy.

The addition of Bi isoelectronic dopants to GaAs provides an attractive avenue for tailoring its electronic band structure, yet it also introduces less appealing and very strong hole localization. The origin of the localization is still not thoroughly understood, which has in part inhibited the practical use of $\mathrm{GaA}{\mathrm{s}}_{1-x}\mathrm{B}{\mathrm{i}}_x$ alloys. In this study, the evolution of hole localization was evaluated as a function of composition. We find that spatial overlap of Bi-related bound states at concentrations $>0.6\%$ Bi effectively enables holes to be channeled to those at the lowest energies, thereby aiding localization of excitons $\ge 150\mathrm{meV}$ below the band gap. The large energy gap between these bound states and the GaAs valence-band edge combined with the slow upward movement of the valence band with composition causes deep localization to persist to high concentrations $>6\%$ Bi. The results provide important insight into the optical and transport behavior of $\mathrm{GaA}{\mathrm{s}}_{1-x}\mathrm{B}{\mathrm{i}}_x$ and its implications for device applications.

The extended charge carrier lifetime in hybrid halide perovskites was attributed to a quasi-indirect band gap that arises due to a Rashba splitting in both conduction and valence band edges. In this paper, we present results for an effective relativistic band structure of $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right){\mathrm{PbI}}_3$ with the focus on the dispersion of electronic states near the band edges of $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right){\mathrm{PbI}}_3$ affected by thermal structural fluctuations. We establish a relationship between the magnitude of the Rashba splitting and a deviation of the Pb atom from its centrosymmetric site position in the ${\mathrm{PbI}}_6$ octahedron. For the splitting energy to reach the thermal energy $k_{\text{B}}T\approx 26$ meV (room temperature), the displacement should be on the order of $0.3\AA$, which is far above the static displacements of Pb atoms in the tetragonal phase of $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right){\mathrm{PbI}}_3$. The significant dynamic enhancement of the Rashba splitting observed at earlier simulation times (less than 2 ps) later weakens and becomes less than the thermal energy despite the average displacement of Pb atoms remaining large (0.37 Å). A randomization of Pb-displacement vectors and associated cancellation of the net effective magnetic field acting on electrons at the conduction band edge is responsible for reduction of the Rashba splitting. The lattice dynamics also leads to deterioration of a Bloch character for states in the valence band leading to the subsequent localization of holes, which affects the bipolar mobility of charge carriers in $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right){\mathrm{PbI}}_3$. These results call into question the quasi-indirect band gap as a reason for the long carrier lifetime observed in $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right){\mathrm{PbI}}_3$ at room temperature. Analysis of spin projections and the spin overlap at the band edges also rules out the spin helicity as a possible cause for a long lifetime of optical excitations in perovskite structures. An alternative mechanism involves dynamic localization of holes and their reduced overlap with electrons in reciprocal space.

Barlowite, ${\mathrm{Cu}}_4{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$, has attracted much attention as the parent compound of a new series of quantum spin-liquid candidates, ${\mathrm{Zn}}_x{\mathrm{Cu}}_{4-x}{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$. While it is known to undergo a magnetic phase transition to a long-range ordered state at $T_N=15$ K, there is still no consensus over either its nuclear or magnetic structures. Here, we use comprehensive, high-flux powder neutron diffraction studies on deuterated samples of barlowite to demonstrate that the only space group consistent with the observed nuclear and magnetic diffraction at low temperatures is the orthorhombic $Pnma$ space group. We furthermore conclude that the magnetic intensity at $T<T_N$ is correctly described by the $P{n}^{\prime }{m}^{\prime }a$ magnetic space group, which crucially allows the ferromagnetic component observed in previous single-crystal and powder magnetization measurements. As such, the magnetic structure of barlowite resembles that of the related material clinoatacamite, ${\mathrm{Cu}}_4{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$, the parent compound of the well-known quantum spin-liquid candidate hebertsmithite, ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$.