Magnetic properties of dilute single crystals of ${\mathrm{Gd}}_{1-x}{\mathrm{Y}}_x{\mathrm{B}}_4$ ($x=0$, 0.2, and 0.4) were systematically studied. These antiferromagnetic Shastry-Sutherland compounds display a remnant magnetization $M{}_{\mathrm{R}}$ when field cooled below the Néel temperature $T_{\mathrm{N}}$. The magnitude of $M{}_{\mathrm{R}}$ at low magnetic fields is magnetic field dependent, increasing with increasing magnetic fields, and it saturates for $H\le 30$ Oe. The $M_R\left(T\right)$ data for both $x=0.2$ and 0.4 compositions overlap, and the remnant magnetization follows a universal behavior. We argue that these findings are explained by taking into account the formation of magnetic domains within the crystals, originating from the dilution of a magnetically frustrated, antiferromagnetic sublattice.

We predict two new nickel hydrides, $\mathrm{N}{\mathrm{i}}_2{\mathrm{H}}_3$ ($C2/m$) and $\mathrm{Ni}{\mathrm{H}}_2$ ($I4/mmm$), to be thermodynamically stable at 60 GPa by DFT calculations. The calculated structure of $\mathrm{Ni}{\mathrm{H}}_2$ is similar to the one observed earlier for $\mathrm{Fe}{\mathrm{H}}_2$. However, to the best of our knowledge, the monoclinic structure predicted for $\mathrm{N}{\mathrm{i}}_2{\mathrm{H}}_3$ was never reported in the other hydrides. We successfully synthesized the monoclinic $\mathrm{N}{\mathrm{i}}_2{\mathrm{H}}_3$ phase using a laser-heating technique in a diamond anvil cell at pressures around 60 GPa. The $\mathrm{N}{\mathrm{i}}_2{\mathrm{H}}_3$ phase can be retained down to 17 GPa, and DFT calculations predict it to be nonmagnetic and nonsuperconducting. However, we did not find in our experiments the theoretically predicted $\mathrm{Ni}{\mathrm{H}}_2$ phase. Higher pressures and/or different synthesis conditions might be needed to synthesize this and other polyhydrides of nickel. Our results show that the Ni-H system behaves differently in comparison to the Fe-H system: Ni extends its own chemical identity to pressures as high as 60–80 GPa. This finding may have significant implications for the high-pressure behavior of Fe-Ni alloys at the high-pressure conditions relevant for the Earth and planetary sciences.

Adsorption-induced deformation of porous materials is the generation of strains in a solid due to its interaction with adsorbing fluids. The theoretical description of adsorption-induced deformation often relies on the so-called solvation pressure, the normal component of a pressure tensor in the liquid adsorbed in the pore. Recent measurements of adsorption-induced strains in two dimensions require a description that allows for the deformation to be anisotropic. Here, we present such a description. We refrain from using the solvation pressure concept and instead base the discussion on a phenomenological description of coupled mechanics and adsorption that has well-established links to continuum mechanics. We find that our approach captures all relevant features of anisotropic sorption strain; the approach thus provides a useful alternative to the solvation pressure concept. We derive analytical expressions for the stress-strain relations in a model porous material with an array of parallel channel-like pores of high aspect ratio (length/width). These relations include separate terms from the liquid pressure, from the surface stress at the liquid-solid interface, and from a spreading tension at the solid-liquid-vapor triple line. Surface stress and liquid pressure contribute to the strains along and normal to the pore axis in a qualitatively different manner. The underlying discussion of capillary forces sheds light on the variation of the surface stress during adsorption and capillary condensation.

Dislocations can climb out of their glide plane by absorbing (or emitting) point defects [vacancies and self-interstitial atoms (SIAs)]. In contrast with conservative glide motion, climb relies on the point defects' thermal diffusion and hence operates on much longer timescales, leading to some forms of creep. While equilibrium point defect concentrations allow dislocations to climb to relieve nonglide stresses, point defect supersaturations also lead to osmotic forces, driving dislocation motion even in the absence of external stresses. Self-interstitial atoms typically have significantly higher formation energies than vacancies, so their contribution to climb is usually ignored. However, under irradiation conditions, both types of defect are athermally created in equal numbers. In this paper, we use simple thermodynamic arguments to show that the contribution of interstitials cannot be neglected in irradiated materials and that the osmotic force they induce on dislocations is many orders of magnitude larger than that caused by vacancies. This explains why the prismatic dislocation loops observed by in situ transmission electron microscope irradiations are more often of interstitial rather than vacancy character. Using discrete dislocation dynamics simulations, we investigate the effect on dislocation-obstacle interactions and find reductions in the depinning time of many orders of magnitude. This has important consequences for the strength of particle-reinforced alloys under irradiation.

Resistively heated diamond-anvil cells have been used together with synchrotron x-ray diffraction to investigate the phase diagram of calcium up to 50 GPa and 800 K. The phase boundaries between the Ca-I (fcc), Ca-II (bcc), and Ca-III (simple cubic, sc) phases have been determined at these pressure-temperature conditions, and the ambient temperature equation of state has been generated. The equation of state parameters at ambient temperature have been determined from the experimental compression curve of the observed phases by using third-order Birch-Murnaghan and Vinet equations. A thermal equation of state was also determined for Ca-I and Ca-II by combining the room-temperature Birch-Murnaghan equation of state with a Berman-type thermal expansion model.

We have used differential scanning calorimetry and thermometry techniques under applied magnetic field and compressive uniaxial stress to determine isothermal entropy and adiabatic temperature changes that quantify the caloric effects associated with the magnetostructural transition of an ${\mathrm{Fe}}_{49}{\mathrm{Rh}}_{51}$ alloy. It is found that the transition temperature increases with increasing compressive stress while it decreases for increasing tensile stress. This behavior gives rise to a conventional elastocaloric effect for compressive stresses in contrast to the reported inverse elastocaloric effect for tensile stresses. The combined effect of stress and magnetic field does not lead to a significant increase of the maximum temperature and entropy changes associated with magnetocaloric and elastocaloric effects, but there is a modification of the temperature window where the sample exhibits giant caloric responses.

We perform a thorough structural search with the minima hopping method (MHM) to explore low-energy structures of methylammonium lead iodide. By combining the MHM with a forcefield, we efficiently screen vast portions of the configurational space with large simulation cells containing up to 96 atoms. Our search reveals two structures of methylammonium iodide perovskite (MAPI) that are substantially lower in energy than the well-studied experimentally observed low-temperature $Pnma$ orthorhombic phase according to density functional calculations. Both structures have not yet been reported in the literature for MAPI, but our results show that they could emerge as thermodynamically stable phases via compression at low temperatures. In terms of the electronic properties, the two phases exhibit larger band gaps than the standard perovskite-type structures. Hence, the pressure-induced phase selection at technologically achievable pressures (i.e., via thin-film strain) is a viable route towards the synthesis of several MAPI polymorph with variable band gaps.

Increasing the storage capacity of lithium electrodes, without altering their cyclability, is one of the key challenges for modern ion-based batteries. For graphite-based anodes, the well-known capacity limit is $\sim 370$ mAh/g, which corresponds to a lithium composition of ${\mathrm{Li}}_1{\mathrm{C}}_6$. Lithium intercalation is accompanied by a volume expansion of $\sim 10\%$. In the present work, accurate first-principles methods are used to investigate the performance of different bulk ${sp}^2$ carbon allotropes as anodes in lithium-ion batteries. Compared to graphite, which is an alternated stack of graphene layers (Bernal stacking) exhibiting a perfect hexagonal tiling, the layers of the other stacked systems considered are constructed from various polygonal carbon rings, such as squares, pentagons, hexagons, heptagons, octagons, and dodecagons. These ${sp}^2$ allotropes, which appear locally in defective graphene and grain boundaries, can exhibit a substantial increase in specific capacity with respect to graphite (up to a factor of two, i.e., ${\mathrm{Li}}_2{\mathrm{C}}_6$) with only a relatively small volume expansion (at most 25%). The mechanisms for this predicted increase in the number of lithium atoms that can be hosted in these still hypothetical carbon crystals are analyzed in detail, yielding global strategies for improving lithium capacity in ${sp}^2$ carbon-based batteries. In addition, these results offer an insight on the local mechanism of Li incorporation in randomly defective graphite.

Understanding diffusion of large solutes such as hydrogen and lithium in solids is of paramount importance for energy storage in metal hydrides and advanced batteries. Due to its high gravimetric and volumetric densities, magnesium is a material of great potential for solid-state hydrogen storage. However, the slow hydrogen diffusion kinetics and the deleterious blocking effect in magnesium have hampered its practical applications. Here, we demonstrate fast lateral hydrogen diffusion in quasifree magnesium films without the blocking effect. Massive concomitant lattice expansion leads to the formation of remarkable self-organized finger patterns extending over tens of micrometers. Detailed visualization of diffusion fronts reveals that the fingers in these patterns follow locally the direction of hydrogen diffusion. Thus, the streamlines of the diffusion process are self-recorded by means of the finger pattern. By inclusion of fast hydrogen diffusion objects or local gaps, the resulting streamlines exhibit a clear analogy to optical rays in geometric optics. The possibility to spatially manipulate hydrogen diffusion opens an avenue to build advanced hydrogen storage systems, cloaking and active plasmonic devices, as well as prototype systems for computational models.

We combine density functional theory (DFT) formation energies and empirical potential calculations of vibrational free energies to calculate the free energies of formation of point defects and clusters of oxygen interstitials, and we use a dilute defect model to calculate the concentrations of defects as a function of temperature and composition. We find that at high temperature oxygen interstitials are dominant, either in isolated form or in clusters depending on the deviation from stoichiometry. At temperatures lower than 1300 K we predict uranium vacancies to be dominant in the stoichiometric material. The disorder in $\mathrm{U}{\mathrm{O}}_2$ therefore changes from Schottky to Frenkel type with increasing temperature. Uranium vacancies remain dominant up to deviations form stoichiometry as large as 0.045 at 800 K. Moreover, the concentration of uranium vacancies proves to be non-monotonous as a function of composition. These results are consistent with some experimental data on the evolution with stoichiometry of lattice constant, diffusion coefficients of uranium, positron lifetime and dilatometry measurements.

Motivated by the novel properties of topological nodal-line semimetal and its compatibility with the well-developed Si-based semiconductor technology, we propose a three-dimensional all-silicon topological nodal-line semimetal composed of interpenetrating silicene networks (ISN). Using state-of-the-art first-principles calculations and the cluster expansion method, we find the ISN structure to be dynamically, thermally, and mechanically stable. In addition, it exhibits linearly dispersive band crossings along two perpendicular high-symmetry directions due to the band inversion and shows two nontrivial nodal lines, traversing in the Brillouin zone near the Fermi level. Furthermore, ISN has the potential as an anode material for sodium-ion batteries with an extremely low diffusion energy barrier and a small volume change during the charging/discharging process, which would lead to a high rate and stable cycling performance.

Heterogeneous nucleation of the $\alpha$ to $\beta$ phase transition at ${\langle 111\rangle }_{\beta }$-type screw dislocations in pure titanium is examined through a combination of elasticity theory and molecular dynamics simulations using a modified embedded atom method potential. These screw dislocations act as heterogeneous nucleation sites and increase the $\alpha$ phase growth rate but also restrict the orientation of the $\alpha$ nuclei to certain directions, along which the strain field of the dislocation aligns with the strain required to complete the Burgers transformation path. Simulations and elasticity theory predict the same three $\alpha$ phase variants along the same preferential directions for $\alpha$ nucleus growth in the early stages of transformation. Previous elasticity theory calculations indicate that this growth does not result in the elastically preferred habit plane for the $\alpha$ nucleus. Molecular dynamics simulations on many-layer supercells presented here show that large $\alpha$ plates will change their growth direction toward the predicted habit plane, but this rotation is resisted by the line tension of the dislocation until the $\alpha$ precipitate detaches from the dislocation.

We investigate the magnetocrystalline anisotropy of ${\mathrm{YCo}}_5$ and ${\mathrm{ZrCo}}_5$ compounds from first-principles calculations using a real-space formalism of pseudopotentials. We study not only the experimentally observed phases but also other possible crystal structures in order to examine the impact of structural differences on the magnetic properties. Our results indicate that it may be difficult to enhance magnetocrystalline anisotropy constant $K_1$ and saturation magnetization $M_s$ simultaneously in ${\mathrm{YCo}}_5$ compounds in a hexagonal crystal family. We find that ${\mathrm{ZrCo}}_5$ compounds have moderate $K_1$ and sufficient $M_s$, which can be much better than those of conventional ferrite magnets. We expect that ${\mathrm{ZrCo}}_5$ compounds, consisting of affordable elements, are promising rare-earth-free materials that will be useful for permanent magnet applications.

We present a real-space pseudopotential method for calculating magnetocrystalline anisotropy within relativistic density-functional theory. The spin-orbit interaction, a fundamental origin of magnetocrystalline anisotropy, is incorporated with norm-conserving pseudopotentials expressed on a real-space grid. We demonstrate the utility of our method by calculating the magnetocrystalline anisotropy constant, $K_1$, of ${\mathrm{YCo}}_5$, which is a prototype compound with large $K_1$. Our calculated $K_1$ agrees with previous theoretical work. We show that our formalism also works for describing magnetocrystalline anisotropy in other transition-metal compounds, such as ${\mathrm{Mn}}_2\mathrm{Ga}$ and FeNi, which have modest values for $K_1$. Our real-space pseudopotential method is well suited for a parallel computing environment and is an efficacious approach to solving the relativistic Kohn-Sham equations, which include spin-orbit effects as well as noncollinear magnetism.

Liquid-liquid transitions are interesting to many researchers since they occur in systems as diverse as monoatomic liquids, multicomponent oxides, and metallic glass formers. In some cases, the crossover is accompanied by changes in the dynamical properties. By combining state-of-the-art synchrotron techniques, we followed the structure and atomic motion during quasistatic cooling of the $\mathrm{A}{\mathrm{u}}_{49}\mathrm{C}{\mathrm{u}}_{26.9}\mathrm{S}{\mathrm{i}}_{16.3}\mathrm{A}{\mathrm{g}}_{5.5}\mathrm{P}{\mathrm{d}}_{2.3}$ metallic glass former from the low-temperature supercooled liquid. With this thermal protocol, we were able to lower the glass transition temperature far enough to reveal a liquid-liquid crossover between two amorphous structures corresponding to two ultraviscous liquids with different kinetic behavior. This transition is in competition with vitrification, which occurs at conventional cooling rates, and is accompanied by structural changes not affecting the average density. Our results provide a direct connection between polyamorphism and dynamical crossover, and an alternative case to add to the highly debated topic on the low-temperature divergence of the dynamics in supercooled liquids.

We have synthesized ceramic specimens of the tetragonal tungsten bronze ${\mathrm{K}}_3\mathrm{L}{\mathrm{i}}_2\mathrm{T}{\mathrm{a}}_5{\mathrm{O}}_{15}$ (KLT) and characterized its phase transition via x-ray diffraction, dielectric permittivity, resonant ultrasonic spectroscopy, and heat capacity measurements. The space group of KLT is reported as both $P4/mbm$ and $Cmmm$ with the orthorhombic distortion occurring when there are higher partial pressures of volatile K and Li used inside the closed crucibles for the solid state synthesis. The data show strong relaxor behavior, with the temperature at which the two dielectric relative permittivity peaks decreasing, with $104\ge T_{m1}\ge 69\mathrm{K}$ and $69\ge T_{m2}\ge 46\mathrm{K}$ as probe frequency $f$ is reduced from 1 MHz to 316 Hz. $F$ tests show that the data satisfies a Vogel-Fulcher model better than Arrhenius with an extrapolated freezing temperature for ${\varepsilon }^{\prime }$ and ${\varepsilon }^{″}$ of $T_{f1}=+15.8$ and $-11.8\mathrm{K}$ and $T_{f2}=-5.0$ and $-15.0\mathrm{K}$ for $f\to 0$ (tending to dc). This difference between $T_f$ from real and imaginary values, albeit counterintuitive, is mandatory, according to the theory of Tagantsev. Therefore, by tuning frequency, the transition could be shifted to absolute zero, suggesting KLT has a relaxor-type quantum critical point. In addition, we have reanalyzed the conflicting literature for $\mathrm{P}{\mathrm{b}}_2\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_7$ pyrochlore which suggests that this also has a relaxor-type quantum critical point since the freezing temperature from the Vogel-Fulcher fitting is below absolute zero. Since the transition temperature evidenced in the dielectric data at approximately 100 kHz shifts below 0 K for very low frequencies, this transition would not be seen with heat capacity data collected in the zero-frequency (dc) limit. Both of these materials show promise for possible new relaxor-type quantum critical points with nonperovskite based structures.

While it is well established that elevated temperatures can induce surface roughening of metal surfaces, the effect of a high electric field on the atomic structure at ambient temperature has not been investigated in detail. Here we show with atomic resolution using in situ transmission electron microscopy how intense electric fields induce reversible switching between perfect crystalline and disordered phases of gold surfaces at room temperature. Ab initio molecular dynamics simulations reveal that the mechanism behind the structural change can be attributed to a vanishing energy cost in forming surface defects in high electric fields. Our results demonstrate how surface processes can be directly controlled at the atomic scale by an externally applied electric field, which promotes an effective decoupling of the topmost surface layers from the underlying bulk. This opens up opportunities for development of active nanodevices in, e.g., nanophotonics and field-effect transistor technology as well as fundamental research in materials characterization and of yet unexplored dynamically controlled low-dimensional phases of matter.

Density functional theory calculations elucidate crystallographic and electronic structural responses of the fcc delta phase of plutonium (δ-Pu) to point defects. Discussed in this paper are responses that vary from a defect exhibiting a local structural instability common in fcc metals to an unusual defect structure that mimics properties similar to the monoclinic α phase. In the prior case, the existence of a self-vacancy when ionic relaxation is allowed induces in the fcc lattice a local tetragonal instability in nearest-neighbor atoms, slight electronic charge increase, and a spin-density decrease. However, in the latter case, after ionic relaxation, the defect is not recognizable as the structure of a vacant site common to many simple fcc metals, but is a complex extended defect involving neighboring atoms to collectively exhibit loss of lattice symmetry through formation of short Pu-Pu bonds and associated narrow bands at the Fermi energy. Partial density of states (PDOS) indicates that these narrow bands form as spectral weighting from the less energetic $6d$ electronic states is shifted to the $5f$ electronic states by means of the spin density, which occupies states at the Fermi energy. The PDOS of a $5f$ system exhibiting narrow bands at the Fermi energy is associated with the formation of an unusual defect structure in the lattice.