Bulk metastable phases can be stabilized during thin-film growth by employing substrates with similar crystal structure and lattice parameter, albeit over a thickness range limited by coherency-strain relaxation. Expanding that strategy, growth of superlattices comprising one stable and another metastable compound with similar crystal structure and lattice parameters are known to yield epitaxial stabilization over a few nanometers of thickness. In this work, the high-pressure rocksalt (B1) phase of $\mathrm{A}{\mathrm{l}}_{0.72}\mathrm{S}{\mathrm{c}}_{0.28}\mathrm{N}$ was stabilized epitaxially in a multilayer with TiN with thicknesses of up to 26 nm. In order to investigate the microstructural changes leading to the phase transformation of the metastable B1 phase to its wurtzite allomorph, we demonstrate a design based on a multilayer architecture with systematically varying thicknesses of the metastable compound within a constant-thickness lattice of stable metallic TiN with the cubic rocksalt structure. The multilayer films show an increasing hardness and elastic modulus for decreasing period thickness, in correspondence with both coherency-strain and Koehler hardening. The phase transition is accompanied by an increase of lattice strain with increasing multilayer periods, and resulting ultimately in coherency-strain relaxation upon phase transformation. Further, we show that the phase transformation is mediated by voids decorating the $\left\{130\right\}$ planes that separate regions of different growth rates and act as additional growth fronts for wurtzite growth during the phase transformation. The TiN/(Al,Sc)N interfaces themselves remain atomically sharp and smooth until the interface structure roughens along with the epitaxial rocksalt to wurtzite transition of (Al,Sc)N. These results show the strong influence of the voids on controlling the target thickness of epitaxially stabilized thin-film growth to the range relevant for applications, such as coatings, plasmonic materials, and electronic device technology, where the mechanical integrity of the material is critical.

Bent-core molecules can form numerous polar and symmetry broken liquid crystalline phases with fascinating properties. Here we report the characteristics of a previously unknown polar synclinic smectic phase, $\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{X}}$, found in the phase sequence of an achiral bent-core material with a 4-cyanoresorcinol bisbenzoate core, terminated by long linear alkyl chains ($n=18$) on both ends. This phase exists over a narrow range of temperatures and is sandwiched in between the random polar synclinic smectic phase ($\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{R}}$) and polar synclinic ferroelectric ($\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{F}}$) phase with ${\mathit{P}}_S\sim 250\mathrm{nC}/\mathrm{c}{\mathrm{m}}^2$. In a planar-aligned cell it exhibits only chirality flipping on application of a conventional ac field but it also exhibits optical switching by rotation of the molecular directors on the tilt cone subjected to a modified sequence of bipolar pulses. This changeover is discussed in terms of the model given by Nakata et al. [Phys. Rev. Lett. 96, 067802 (2006)], involving a competition between the two forms of switching: the rotation around the long molecular axis and the switching through rotation of the molecular directors on the tilt cone. The model is modified to take account of the azimuthal pretilt and the molecular tilt angles. In addition to it, characteristics of $\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{R}}$ are also discussed.

$\alpha$-Quartz-type gallium phosphate and representative compositions in the $\mathrm{AlP}{\mathrm{O}}_4\text{−}\mathrm{GaP}{\mathrm{O}}_4$ solid solution were studied by x-ray powder diffraction and absorption spectroscopy, Raman scattering, and by first-principles calculations up to pressures of close to 30 GPa. A phase transition to a metastable orthorhombic high-pressure phase along with some of the stable orthorhombic $Cmcm\mathrm{CrV}{\mathrm{O}}_4$-type material is found to occur beginning at 9 GPa at $320{}^{\circ }\mathrm{C}$ in $\mathrm{GaP}{\mathrm{O}}_4$. In the case of the $\mathrm{AlP}{\mathrm{O}}_4\text{−}\mathrm{GaP}{\mathrm{O}}_4$ solid solution at room temperature, only the metastable orthorhombic phase was obtained above 10 GPa. The possible crystal structures of the high-pressure forms of $\mathrm{GaP}{\mathrm{O}}_4$ were predicted from first-principles calculations and the evolutionary algorithm USPEX. A predicted orthorhombic structure with a $Pmn{2}_1$ space group with the gallium in sixfold and phosphorus in fourfold coordination was found to be in the best agreement with the combined experimental data from x-ray diffraction and absorption and Raman spectroscopy. This method is found to very powerful to better understand competition between different phase transition pathways at high pressure.

Commonly used antiferromagnets contain expensive precious metals, which limits their applicability. Novel materials that are made of abundant elements are thus required for a large scale application, e.g., in spintronic devices. We propose a combinatorial, high-throughput approach based on density functional theory calculations to search for such new antiferromagnets. The power of the method is demonstrated by screening the ternary Heusler compounds for antiferromagnetic phases. We utilize the AFLOWLib, a computational materials database that contains over one million ternary phases. Among these we identify 291 potentially stable magnetic Heusler compounds. By explicitly checking for antiferromagnetic configurations we identify 70 antiferromagnetic Heusler compounds. Comparison with available experimental data shows that the method has excellent selectivity: all known antiferromagnetic Heusler compounds are correctly identified and no material is erroneously assigned an antiferromagnetic ground state. By calculating the Néel temperatures we predict 21 antiferromagnetic Heusler compounds with a Néel temperature above room temperature.

We report on electrical measurements of spin-dependent transport of holes in all-epitaxial CoFe/$p$-type germanium $(p\text{−}{\mathrm{Ge})/\mathrm{Fe}}_3\mathrm{Si}$ spin valves, where the hole concentration $\left(p_{\mathrm{h}}\right)$ of the $p$-Ge layer is estimated to be $\sim {10}^{18}{\mathrm{cm}}^{-3}$. Spin-accumulation output voltages can electrically be detected in the antiparallel magnetization state between CoFe and ${\mathrm{Fe}}_3\mathrm{Si}$ ferromagnetic electrodes. The room-temperature spin lifetime of holes in the $p$-Ge layers can tentatively be discussed in terms of the theory by Fert and Jaffrès. We propose that the use of (111)-oriented $p$-Ge with a hole concentration of $\sim {10}^{18}{\mathrm{cm}}^{-3}$ enables the transport of spin-polarized holes in bulk Ge even at room temperature.

Structural, dynamical, and electronic properties of the liquid and amorphous phase of the ${\mathrm{In}}_2{\mathrm{Te}}_3$ compound have been studied by means of density functional molecular dynamics simulations. This system is of interest as a phase change material, undergoing a fast and reversible change between the crystalline and amorphous phases upon heating. It can be seen as a constituent of ternary InSbTe alloys which are receiving attention for application in electronic phase change memories. Amorphous models of ${\mathrm{In}}_2{\mathrm{Te}}_{3}300$-atom large have been generated by quenching from the melt by using different exchange and correlation functionals and different descriptions of the van der Waals interaction. It turns out the local bonding geometry of the amorphous phase is mostly tetrahedral with corner and edge sharing tetrahedra similar to those found in the crystalline phases of the InTe, ${\mathrm{In}}_2{\mathrm{Te}}_3$, and ${\mathrm{In}}_2{\mathrm{Te}}_5$ compounds. Benchmark calculations on the crystalline $\alpha$ phase of ${\mathrm{In}}_2{\mathrm{Te}}_3$ in the defective zincblend geometry have also been performed. The calculations reveal that the high symmetric $F\overline{4}3m$ structure inferred experimentally from x-ray diffraction for the $\alpha$ phase must actually result from a random distribution of Te-Te bonds in different octahedral cages formed by the coalescence of vacancies in the In sublattice.

Ferrihydrite, with a ‘‘two-line’’ x-ray diffraction pattern (2L-Fh), is the most amorphous of the iron oxides and is ubiquitous in both terrestrial and aquatic environments. It also plays a central role in the regulation and metabolism of iron in bacteria, algae, higher plants, and animals, including humans. In this study, we present a single-phase model for ferrihydrite that unifies existing analytical data while adhering to fundamental chemical principles. The primary particle is small (20–50 Å) and has a dynamic and variably hydrated surface, which negates long-range order; collectively, these features have hampered complete characterization and frustrated our understanding of the mineral's reactivity and chemical/biochemical function. Near and intermediate range neutron diffraction (NIMROD) and first-principles density functional theory (DFT) were employed in this study to generate and interpret high-resolution data of naturally hydrated, synthetic 2L-Fh at standard temperature. The structural optimization overcomes transgressions of coordination chemistry inherent within previously proposed structures, to produce a robust and unambiguous single-phase model.

The computer aided discovery of structural alloys is a burgeoning but still challenging area of research. A primary challenge in the field is to identify computable screening parameters that embody key structural alloy properties. Here, an elastic anisotropy parameter that captures a material's susceptibility to solute solution strengthening is identified. The parameter has many applications in the discovery and optimization of structural materials. As a first example, the parameter is used to identify alloys that might display the super elasticity, super strength, and high ductility of the class of TiNb alloys known as gum metals. In addition, it is noted that the parameter can be used to screen candidate alloys for shape memory response, and potentially aid in the optimization of the mechanical properties of high-entropy alloys.

A first-principles investigation of the influence of lattice softening on lithium-magnesium alloys near the body-centered-cubic (bcc)/hexagonal close-packed (hcp) transition composition is presented. Results show that lithium-magnesium alloys display a softening of the shear modulus $C_{11}-C_{12}$, and an acoustic phonon branch between the $\mathrm{\Gamma }$ and $N$ high symmetry points, as the composition approaches the stability limit for the bcc phase. This softening is accompanied by an increase in the size of the dislocation core region. Ideal tensile strength calculations predict that ordered phases of lithium-magnesium alloys are intrinsically brittle. Methods to make the alloys more ductile are discussed, and the propensity for these alloys to display gum-metal-like behavior is assessed.

We investigated the influence of water molecules on the diffusion, dynamics, and electrosorption of a room temperature ionic liquid (RTIL), $\left[\mathrm{BMI}{\mathrm{m}}^{+}\right]\left[\mathrm{T}{\mathrm{f}}_2{\mathrm{N}}^{-}\right]$, confined in carbide-derived carbon with a bimodal nanoporosity. Water molecules in pores improved power densities and rate handling abilities of these materials in supercapacitor electrode configurations. We measured the water-dependent microscopic dynamics of the RTIL cations using quasielastic neutron scatting (QENS). The ionic liquid demonstrated greater mobility with increasing water uptake, facilitated by the nanoporous carbon environment, up to a well-defined saturation point. We concluded that water molecules displaced RTIL ions attached to the pore surfaces and improved the diffusivity of the displaced cations. This effect consequently increased capacitance and rate handling of the electrolyte in water-containing pores. Our findings suggest the possible effect of immiscible co-solvents on energy and power densities of energy storage devices, as well as the operating viability of nonaqueous supercapacitor electrolytes in humid environments.

In specific conditions, grain-boundary (GB) migration occurs in polycrystalline materials as an alternative vector of plasticity compared to the usual dislocation activity. The shear-coupled GB migration, the expected most efficient GB based mechanism, couples the GB motion to an applied shear stress. Stresses on GB in polycrystalline materials seldom have, however, a unique pure shear component. This work investigates the influence of a normal strain on the shear coupled migration of a $\mathrm{\Sigma }13\left(320\right)\left[001\right]$ GB in a copper bicrystal using atomistic simulations. We show that the yield shear stress inducing the GB migration strongly depends on the applied normal stress. Beyond, the application of a normal stress on this GB qualitatively modifies the GB migration: while the $\mathrm{\Sigma }13\left(320\right)\left[001\right]$ GB shear couples following the $\langle 110\rangle$ migration mode without normal stress, we report the observation of the $\langle 010\rangle$ mode under a sufficiently high tensile normal stress. Using the nudge elastic band method, we uncover the atomistic mechanism of this $\langle 010\rangle$ migration mode and energetically characterize it.

We report on an experimental study on the structural and magnetic properties of the intermetallic compound CrAlGe. The material has been investigated in detail by x-ray diffraction, dc magnetization, ac susceptibility, and electrical resistivity measurements. X-ray diffraction and associated Rietveld analysis show that the compound crystallizes in the orthorhombic $\mathrm{TiS}{\mathrm{i}}_2\text{−}\mathrm{type}$ structure, with lattice parameters $a=4.7531\left(1\right)\AA ,b=8.2182\left(1\right)\AA$, and $c=8.6951\left(1\right)\AA$. The magnetization measurements demonstrate that the alloy is ferromagnetic with a Curie temperature of $T_{\mathrm{C}}\approx 80\mathrm{K}$. At 5 K the magnetization saturates at 50 kOe with a saturation moment of $M_{\mathrm{S}}=19\mathrm{emu}/\mathrm{g}$. Below $T_{\mathrm{C}}$, the ac susceptibility data exhibit two distinct frequency dependent peaks, which are also strongly dependent on ac field amplitude and dc magnetic fields. The experimental results strongly suggest the coexistence of ferromagnetic and reentrant spin glass type states in CrAlGe, which is due to the effect of the structural disorder in the $16f$ site (mixed Al/Ge occupation) on the crystallographically ordered $8a$ site of Cr. To our knowledge, CrAlGe is the first example where such unusual magnetic behavior is brought about not by structural disorder on the site occupied by the magnetic atom, but is due to the disorder in the atomic sites of the neighboring atoms.

In multinary compound semiconductors, cation disorder can decisively alter the electronic properties and impact potential applications. $\mathrm{ZnSn}{\mathrm{N}}_2$ is a ternary nitride of interest for photovoltaics, which forms in a wurtzite-derived crystal structure. In the ground state, every N anion is coordinated by two Zn and two Sn cations, thereby observing the octet rule locally. Using a motif-based model Hamiltonian, we performed Monte Carlo simulations that provide atomistic representations of $\mathrm{ZnSn}{\mathrm{N}}_2$ with varying degrees of cation disorder. Subsequent electronic structure calculations describe the evolution of band gaps, optical properties, and carrier localization effects as a function of the disorder. We find that octet-rule conserving disorder is practically impossible to avoid but perfectly benign, with hardly any effects on the electronic structure. In contrast, a fully random cation distribution would be very detrimental, but fortunately it is energetically highly unfavorable. A degree of disorder that can realistically be expected for nonequilibrium thin-film deposition leads to a moderate band-gap reduction and to moderate carrier localization effects. Comparing the simulated structures with experimental samples grown by sputtering, we find evidence that these samples indeed incorporate a certain degree of octet-rule violating disorder, which is reflected in the x-ray diffraction and in the optical absorption spectra. This study demonstrates that the electronic properties of $\mathrm{ZnSn}{\mathrm{N}}_2$ are dominated by changes of the local coordination environments rather than long-range ordering effects.

This paper reports a generalized theory of the smallest diameter of metallic nanorods from physical vapor deposition. The generalization incorporates the effects of nanorod separation and those of van der Waals interactions on geometrical shadowing. The generalized theory relies on approximations to be in closed form. Numerical solutions of governing equations with no approximations verify the accuracy of the closed-form theory. Further, experiments of physical vapor deposition validate the theory in terms of the diameter as a function of the separation of nanorods. In contrast, the previous theory for idealized geometrical shadowing [X. B. Niu et al., Phys. Rev. Lett. 110, 136102 (2013)] excludes any dependence on nanorod separation and predicts the diameter to be about ½ to ⅓ of what the generalized theory does.

We demonstrate a kinetic Monte Carlo simulation tool, based on published data using first-principles quantum mechanics, applied to answer the question: under which conditions of stress, temperature, and nominal hydrogen concentration does the presence of hydrogen in iron increase or decrease the screw dislocation velocity? Furthermore, we examine the conditions under which hydrogen-induced shear localization is likely to occur. Our simulations yield quantitative data on dislocation velocity and the ranges of hydrogen concentration within which a large gradient of velocity as a function of concentration is expected to be observed and thereby contribute to a self-perpetuating localization of plasticity—a phenomenon that has been linked to hydrogen-induced fracture and fatigue failure in ultrahigh strength steel. We predict the effect of hydrogen in generating debris made up of edge dipoles trailing in the wake of gliding screw dislocations and their role in pinning. We also simulate the competing effects of softening by enhanced kink-pair generation and hardening by solute pinning. Our simulations act as a bridge between first-principles quantum mechanics and discrete dislocation dynamics, and at the same time offer the prospect of a fully physics-based dislocation dynamics method.

This is the second of two papers that present a theoretical analysis of the phenomenon of hydrogen embrittlement of $\alpha$-Fe. We make contact between the thermodynamic-kinetic continuum and cohesive zone models and the quantum-mechanical magnetic tight-binding approximation to interatomic forces. We are able to solve a coupled set of equations using quantum mechanically obtained atomistic data to follow the decohesion process in time as traction is applied to a hydrogen charged crystal and decohesion occurs between two (111) crystal planes. This scheme will be readily extended from transgranular to intergranular failure, although the complexities of the trapping sites in the cohesive zone associated with a grain boundary will greatly complicate the calculation of the configurational energy. Hydrogen-enhanced decohesion postulated widely in the field has not yet been demonstrated experimentally, although our calculations find a reduction in the ideal cohesive strength as a result of dissolved hydrogen in $\alpha$-Fe from 30 to 22 GPa. Because of the well-known steep and nonlinear relation between plastic and ideal elastic work of fracture, this represents a very significant reduction in toughness as a result of a hydrogen concentration of less than ten atomic parts per million.

To investigate dislocations or heterostructures across interfaces is now of great interest to condensed matter and materials scientists. With the advances in aberration-corrected electron optics, the scanning transmission electron microscope has demonstrated its excellent capability of characterizing atomic structures within nanomaterials, and well-resolved atomic-resolution images can be obtained through long-exposure data acquisition. However, the sample drifting, carbon contamination, and radiation damage hinder further analysis, such as deriving three-dimensional (3D) structures from a series of images. In this study, a method for obtaining atomic-resolution images with significantly reduced exposure time was developed, using which an original high-resolution image with approximately one tenth the electron dose can be obtained by combining a fast-scan high-magnification image and a slow-scan low-magnification image. The feasibility of obtaining 3D atomic structures using the proposed approach was demonstrated through multislice simulation. Finally, the feasibility and accuracy of image restoration were experimentally verified. This general method cannot only apply to electron microscopy but also benefit to image radiation-sensitive materials using various light sources.

The formation of vortices is usually considered to be the main mechanism of angular momentum disposal in superfluids. Recently, it was predicted that a superfluid can acquire angular momentum via an alternative, microscopic route, namely, through interaction with rotating impurities, forming so-called “angulon quasiparticles” [R. Schmidt and M. Lemeshko, Phys. Rev. Lett. 114, 203001 (2015)]. The angulon instabilities correspond to transfer of a small number of angular momentum quanta from the impurity to the superfluid, as opposed to vortex instabilities, where angular momentum is quantized in units of $\hslash$ per atom. Furthermore, since conventional impurities (such as molecules) represent three-dimensional (3D) rotors, the angular momentum transferred is intrinsically 3D as well, as opposed to a merely planar rotation which is inherent to vortices. Herein we show that the angulon theory can explain the anomalous broadening of the spectroscopic lines observed for ${\mathrm{CH}}_3$ and ${\mathrm{NH}}_3$ molecules in superfluid helium nanodroplets, thereby providing a fingerprint of the emerging angulon instabilities in experiment.