We study the properties of NiTi shape-memory nanoparticles coherently embedded in TiV matrices using three-dimensional atomistic simulations based on the modified embedded-atom method. To this end, we develop and present a suitable NiTiV potential for our simulations. Employing this potential, we identify the conditions under which the martensitic phase transformation of such a nanoparticle is triggered—specifically, how these conditions can be tuned by modifying the size of the particle, the composition of the surrounding matrix, or the temperature and strain state of the system. Using these insights, we establish how the transformation temperature of such particles can be influenced and discuss the practical implications in the context of shape-memory strengthened alloys.

We quantify the accuracy of different non-self-consistent and self-consistent spin-orbit coupling (SOC) treatments in Kohn-Sham and hybrid density functional theory by providing a band-structure benchmark set for the valence and low-lying conduction energy bands of 103 inorganic compounds, covering chemical elements up to polonium. Reference energy band structures for the PBE density functional are obtained using the full-potential (linearized) augmented plane wave code wien2k, employing its self-consistent treatment of SOC including Dirac-type $p^{1/2}$ orbitals in the basis set. We use this benchmark set to benchmark a computationally simpler, non-self-consistent all-electron treatment of SOC based on scalar-relativistic orbitals and numeric atom-centered orbital basis functions. For elements up to $Z\approx 50$, both treatments agree virtually exactly. For the heaviest elements considered (Tl, Pb, Bi, Po), the band-structure changes due to SOC are captured with a relative deviation of 11% or less. For different density functionals (PBE versus the hybrid HSE06), we show that the effect of spin-orbit coupling is usually similar but can be dissimilar if the qualitative features of the predicted underlying scalar-relativistic band structures do not agree. All band structures considered in this work are available online via the NOMAD repository to aid in future benchmark studies and methods development.

CaOFeS is a semiconducting oxysulfide with a polar layered triangular structure. Here a comprehensive theoretical study has been performed to reveal its physical properties, including magnetism, electronic structure, phase transition, magnetodielectric effect, as well as optical absorption. Our calculations confirm the Ising-like G-type antiferromagnetic ground state driven by the next-nearest neighbor exchanges, which breaks the trigonal symmetry and is responsible for the magnetodielectric effect driven by exchange striction. In addition, a large coefficient of visible light absorption is predicted, which leads to promising photovoltaic effect with the maximum light-to-electricity energy conversion efficiency up to $24.2\%$.

Hematite ($\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$) is a promising photocatalyst material for water splitting, where photoinduced holes lead to the oxidation of water and the release of molecular oxygen. In this work, we investigate the properties of holes in hematite using density functional theory (DFT) calculations with hybrid functionals. We find that holes form small polarons and, depending on the fraction of exact exchange included in the PBE0 functional, the site where the holes localize changes from Fe to O. We find this result to be independent of the size and structure of the system: small ${\mathrm{Fe}}_2{\mathrm{O}}_3$ clusters with tetrahedral coordination, larger clusters with octahedral coordination, ${\mathrm{Fe}}_2{\mathrm{O}}_3$(001) surfaces in contact with water, and bulk ${\mathrm{Fe}}_2{\mathrm{O}}_3$ display a very similar behavior in terms of hole localization as a function of the fraction of exact exchange. We then use wave-function-based methods such as coupled cluster with single and double excitations and Møller-Plesset second-order perturbation theory applied on a cluster model of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ to shed light on which of the two solutions is correct. We find that these high-level quantum chemistry methods suggest holes in hematite are localized on oxygen atoms. We also explore the use of the $\mathrm{DFT}+U$ approach as a computationally convenient way to overcome the known limitations of generalized gradient approximation functionals and recover a gap in line with experiments and hole localization on oxygen in agreement with quantum chemistry methods.

Topological superconductors as characterized by Majorana surface states have been actively searched for their significance in fundamental science and technological implication. The large spin-orbit coupling in Bi-Pd binaries has stimulated extensive investigations on the topological surface states in these superconducting compounds. Here we report a study of normal-state electronic structure in a centrosymmetric $\alpha \text{−}{\mathrm{PdBi}}_2$ within density functional theory calculations. By investigating the electronic structure from the bulk to slab geometries in this system, we predict that $\alpha \text{−}{\mathrm{PdBi}}_2$ can host orbital-dependent and asymmetric Rashba surface states near the Fermi energy. This study suggests that $\alpha \text{−}{\mathrm{PdBi}}_2$ will be a good candidate to explore the relationship between superconductivity and topology in condensed matter physics.

We show the potential of cross-sectional scanning tunneling microscopy to address structural properties of dilute III-V bismides by investigating Bi:InP. Bismuth atoms down to the second monolayer below the $\left\{110\right\}$ InP surfaces, which give rise to three classes of distinct contrast, are identified with the help of density functional theory calculations. Based on this classification, the pair-correlation function is used to quantify the ordering of Bi atoms on the long range. In a complementary short-ranged study, we investigate the Bi ordering at the atomic level. An enhanced tendency for the formation of first-nearest-neighbor Bi pairs is found. In addition, the formation of small Bi clusters is observed whose geometries appear to be related to strong first-nearest-neighbor Bi pairing. We also identify growth related crystal defects, such as In vacancies, P antisites, and Bi antisites.

Methane activation is one of the biggest challenges for chemical conversion of hydrocarbons and fundamental science. We systematically screen $d$-block transition metal elements as potential candidates of single-atom catalysts (SACs) for methane dissociation. The adsorption of methane on free metal atoms strongly depends on the number of $d$ electrons of SAC, where the maximum binding energy is formed with the Ni group (electronic configuration $d^8{s}^2$ or $d^9{s}^1)$. Interestingly, the magnetic moment of the SACs decreases by $2{\mu }_{\mathrm{B}}$ for strong interactions, suggesting that the methane-metal bond forms a spin singlet state involving two electrons of opposite spins. To examine the effect of substrates, the screened transition metals, Ni, Rh, and Pt are further put onto prototype metal oxide surfaces. The substrate dramatically modifies the discrete energy levels of a single metal and its catalytic properties. Single Ni atoms supported on an O-terminated $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\left(0001\right)$ surface $\left(\mathrm{N}{\mathrm{i}}_1/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\right)$ show superior catalytic properties, with a low activation barrier of 0.4 eV (0.11 eV after zero-point energy correction) for the C-H bond dissociation and simultaneously an extreme stability with a high binding energy of $\sim 9.39\mathrm{eV}$ for the Ni anchor. This work identifies $\mathrm{N}{\mathrm{i}}_1/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ catalyst as an optimal SAC and offers new atomistic insights into the mechanism of methane activation on SACs.

Surfaces and interfaces of bulk materials with liquids are of importance for a wide range of chemical processes. In this work, we systematically explore reconstructions on the (100) surface of calcium fluoride $\left({\mathrm{CaF}}_2\right)$ and other fluorites $\left(M{\mathrm{F}}_2\right)$, $M=\{\text{Sr},\text{Cd},\text{Ba}\}$ by sampling the configurational space with the minima hopping structure prediction method in conjunction with density functional theory calculations. We find a large variety of structures that are energetically very close to each other and are connected by very low barriers, resulting in a high mobility of the topmost surface anions. This high density of configurational states makes the ${\mathrm{CaF}}_2$ (100) surface a very dynamic system. The majority of the surface reconstructions found in ${\mathrm{CaF}}_2$ are also present in ${\mathrm{SrF}}_2,{\mathrm{CdF}}_2$, and ${\mathrm{BaF}}_2$. Furthermore, we investigate in detail the influence of these reconstructions on the crystal growth of ${\mathrm{CaF}}_2$ in solvents by modeling the fluorite-water interface and its wetting properties. We perform a global structural search both by explicitly including water molecules and by employing a recently developed soft-sphere solvation model to simulate an implicit aqueous environment. The implicit approach correctly reproduces both our findings with the explicit-water model and the experimentally reported contact angles for the partial-hydrophobic (111) and hydrophilic (100) surfaces. Our simulations show that the high anion mobility and the low coordination of the (100) surface atoms strongly favors the adsorption of water molecules over the (111) surface. The aqueous environment makes terminations with low-coordination surface atoms more stable, promoting (100) growth instead of the (111).

We demonstrate that differences in the microstructure of chemically identical materials can lead to distinct triboelectric charging behavior. Contact charging experiments are carried out between strained and unstrained polytetrafluoroethylene samples. Whereas charge transfer is random between samples of identical strain, when one of the samples is strained, systematic charge transfer occurs. No significant changes in the molecular-level structure of the polymer are observed by XRD and micro-Raman spectroscopy after deformation. However, the strained surfaces are found to exhibit void and craze formation spanning the nano- to micrometer length scales by molecular dynamics simulations, SEM, UV-vis spectroscopy, and naked-eye observations. This suggests that material microstructure (voids and crazes) can govern the triboelectric charging behavior of materials.

We report a strain glass state (B19′ strain glass) in a Ni-rich TiNi shape memory alloy produced by cold rolling. As compared to previously reported strain glasses, this strain glass state has outstanding properties including quasilinear superelasticity with a large recoverable strain (∼4%), and slim hysteresis and high strength (∼1.0 GPa) over a wide temperature range (∼200 K). The existence of the B19′ strain glass state is confirmed by (i) frequency dispersion of storage modulus, (ii) continuous decrease of electrical resistivity, and (iii) continuous growth of B19′ nanodomains upon cooling. This study proves that the effect of defect strength on the creation of a strain glass state is in parallel to the effect of cooling rate on the creation of a structural glass, e.g., any strain crystal (i.e., martensite) can be turned into a strain glass if strong enough defects could be engineered.

We have studied $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ thin films grown on (3×1)-reconstructed $\mathrm{SrTi}{\mathrm{O}}_3$ (110) substrates. Films with thicknesses less than the critical thickness of ${\theta }_{\mathrm{c}}\cong 8$ unit cells are insulating in the measured temperature ($T$) range (2–400 K). However, films with thicknesses slightly over ${\theta }_{\mathrm{c}}$ exhibit reentrant nonmetallic behavior at low temperatures in addition to the normally observed metal-insulator transition at higher temperatures. In contrast, the magnetization does not show signs of low-$T$ transitions. Such reentrance of a low-$T$ nonmetallic phase is affected by the film thickness as well as the density of oxygen vacancies. The electrical resistivity analysis reveals that localization effects are responsible for the reentrant nonmetallic behavior, which is enhanced with reduced film thickness.

Transition-metal carbides (TMCs) possess many intriguing properties and inspiring application potentials, and recently the study of a two-dimensional form of TMCs has attracted great attention. Herein, we report successful fabrication of continuous $\mathrm{M}{\mathrm{o}}_2\mathrm{C}$ ultrathin single-crystalline films at $700{}^{\circ }\mathrm{C}$ with an approach of plasma-enhanced pulsed-laser deposition. By sophisticated structural analyses, the $\mathrm{M}{\mathrm{o}}_2\mathrm{C}$ films are characterized as single crystal with a rarely reported face-centered cubic structure. In further electrical transport measurements, superconductivity observed in the $\mathrm{M}{\mathrm{o}}_2\mathrm{C}$ films demonstrates a typical two-dimensional feature, which is consistent with Berezinskii-Kosterlitz-Thouless transitions. Besides, large upper critical magnetic fields are discovered in this system. Our work offers an approach to grow large-area and high-quality TMCs at relatively low temperatures. This study may stimulate more related investigations on the synthesis, characterizations, and applications of two-dimensional TMCs.

We report on growth and characterization of individual ${\mathrm{InAs}}_x{\mathrm{P}}_{1-x}$/GaInP quantum dots with variable nominal As molar fraction. Magnetophotoluminescence experiments reveal quantum dot emission in a wide range from 1.3 to 1.8 eV, confirming successful incorporation of As into the quantum dots. Transmission electron microscopy reveals a core-cap structure of InAsP quantum dots with an InAs-rich core capped by an InP-rich layer. Inside the core, an average As molar fraction up to $x\approx 0.15$ is observed. The heavy-hole $g$ factors are found to be strongly dependent on As molar fraction, while the electron $g$ factors are close to the InP values. This suggests type-II carrier confinement in the studied InAsP dots with holes (electrons) localized in the core (cap) region. Finally, dynamic nuclear polarization is observed, which allows for further insight into structural properties using nuclear magnetic resonance.

${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ is an exemplary correlated electron metal that features two domes of unconventional superconductivity in its temperature-pressure phase diagram. The first dome surrounds an antiferromagnetic quantum critical point, whereas the more exotic second dome may span the termination point of a line of $f\text{-electron}$ valence transitions. This behavior has received intense interest, but what has been missing are ways to access the high pressure behavior under milder conditions. Here we study Si $\to$ P chemical substitution, which compresses the unit cell volume but simultaneously weakens the hybridization between the $f$- and conduction electron states and encourages complex magnetism. At concentrations that show magnetism, applied pressure suppresses the magnetic ordering temperature and superconductivity is recovered for samples with low disorder. These results reveal that the electronic behavior in this system is controlled by a nontrivial combination of effects from unit cell volume and electronic shell filling. Guided by this topography, we discuss prospects for uncovering a valence fluctuation quantum phase transition in the broader family of Ce-based ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type materials through chemical substitution.

${\mathrm{Ta}}_3{\mathrm{N}}_5$ is a promising photoanode for solar water splitting. However, it often suffers from high onset potential for water oxidation, which may be partially ascribed to the oxygen impurities in ${\mathrm{Ta}}_3{\mathrm{N}}_5$. Oxygen impurities, which are always introduced into ${\mathrm{Ta}}_3{\mathrm{N}}_5$ during the preparation process, are difficult to remove due to the low formation energy of O-doped ${\mathrm{Ta}}_3{\mathrm{N}}_5$. The valence- and conduction-band-edge positions shift almost in parallel towards more positive potentials with addition of oxygen impurities, which may increase the onset potential for water oxidation. In this study, the hybrid-DFT (density functional theory) calculations of Ti-doped ${\mathrm{Ta}}_3{\mathrm{N}}_5$ show that as Ti doping concentration increases, both the valence- and conduction-band-edge positions of ${\mathrm{Ta}}_3{\mathrm{N}}_5$ move towards more negative potentials, which is opposite to the role of oxygen impurties. In the case of Ti-O codoped ${\mathrm{Ta}}_3{\mathrm{N}}_5$, Ti doping can compensate the effect of oxygen impurities and may reduce the onset potential for water oxidation. Defect formation energies reveal that Ti-O codoped ${\mathrm{Ta}}_3{\mathrm{N}}_5$ is thermodynamically stable. Therefore we propose that by controlling the amount of O and Ti, the band-edge positions can be modified to a proper level so that better photoelectrochemical performances for solar water splitting can be achieved.