The phases of $A_2{\mathrm{Mn}}_8{\mathrm{O}}_{16}$ hollandite group oxides emerge from the competition between ionic interactions, Jahn-Teller effects, charge ordering, and magnetic interactions. Their balanced treatment with feasible computational approaches can be challenging for commonly used approximations in density functional theory. Three examples ($A$ = Ag, Li, and K) are studied with a sequence of different approximate exchange-correlation functionals. Starting from a generalized gradient approximation (GGA), an extension to include van der Waals interactions and a recently proposed meta-GGA are considered. Then local Coulomb interactions for the Mn $3d$ electrons are more explicitly considered with the DFT + $U$ approach. Finally, selected results from a hybrid functional approach provide a reference. Results for the binding energy of the A species in the parent oxide highlight the role of van der Waals interactions. Relatively accurate results for insertion energies can be achieved with a low-$U$ and a high-$U$ approach. In the low-$U$ case, the materials are described as band metals with a high-symmetry, tetragonal crystal structure. In the high-$U$ case, the electrons donated by A result in formation of local ${\mathrm{Mn}}^{3+}$ centers and corresponding Jahn-Teller distortions characterized by a local order parameter. The resulting degree of monoclinic distortion depends on charge ordering and magnetic interactions in the phase formed. The reference hybrid functional results show charge localization and ordering. Comparison to low-temperature experiments of related compounds suggests that charge localization is the physically correct result for the hollandite group oxides studied here. Finally, while competing effects in the local magnetic coupling are subtle, the fully anisotropic implementation of DFT + $U$ gives the best overall agreement with results from the hybrid functional.

Small-scale mechanics of solute atom segregation and incipient plasticity in nanotwinned Ag containing trace concentrations of Cu were studied by using large-scale hybrid Monte Carlo and molecular-dynamic simulations. It is found that solute Cu atoms are segregated concurrently to grain boundaries and intrinsic twin-boundary kink-step defects during thermal annealing. Low Cu dopant contents below 1 at. % are predicted to substantially increase twin stability in nanotwinned Ag, accompanied with a pronounced rise in yield strength at 300 K. Incipient plasticity is associated with kink-step migration, grain-boundary sliding, and dislocation nucleation from grain boundaries and twin-boundary defects, which are affected by doping. Cu-dependent yield strengthening in doped nanotwinned Ag is shown to correlate with the critical stress required to initiate crystal slip emitted from grain boundaries and twin-boundary defects. These findings provide fundamental insight into the roles of twin-boundary imperfections on plastic yielding, and offer clues to further extend the extraordinary stability and strength of nanotwinned metals by microalloying.

Magnetically doped topological insulators (TIs) are key to realizing the quantum anomalous Hall (QAH) effect, with the prospect of enabling dissipationless electronic devices in the future. Doping of the well-established three-dimensional TIs of the (Bi,${\mathrm{Sb})}_2$(Se,${\mathrm{Te})}_3$ family with the transition metals Cr and V is now an established approach for observing the QAH state at very low temperatures. While the magnetic transition temperatures of these materials are on the order of tens of degrees Kelvin, full quantization of the QAH state is achieved below $\sim 100$ mK, governed by the size of the magnetic gap and thus the out-of-plane magnetic moment. In an attempt to raise the size of the magnetic moment and transition temperature, we carried out a structural and magnetic investigation of codoped (V,Cr):${\mathrm{Sb}}_2{\mathrm{Te}}_3$ thin films. Starting from singly doped Cr:${\mathrm{Sb}}_2{\mathrm{Te}}_3$ films, free of secondary phases and with a transition temperature of $\sim 72$ K, we introduced increasing fractions of V and found a doubling of the transition temperature, while the magnetic moment decreases. In order to separate the properties and contributions of the two transition metals in the complex doping scenario independently, we employed spectroscopic x-ray techniques. Surprisingly, already small amounts of V lead to the formation of the secondary phase ${\mathrm{Cr}}_2{\mathrm{Te}}_3$. No V was detectable in the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ matrix. Instead, it acts as a surfactant and can be found in the near-surface layers at the end of the growth. Our paper highlights the importance of x-ray-based studies for the doping of van der Waals systems, for which the optimization of magnetic moment or transition temperature alone is not necessarily a good strategy.

Most magnetic refrigeration materials showing a large and reversible magnetocaloric effect (MCE) undergo a second-order ferromagnetic (FM) transition involving large-moment magnetic species on one sublattice. A stronger MCE is expected near a cooperative FM order of two or more magnetic species with large magnetic moments residing on different sublattices, but experimental realizations are rare. Here we report on the discovery of large MCE in the cubic pyrochlore $\mathrm{E}{\mathrm{r}}_2\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_7$ near its second-order FM transition at $T_c\approx 34\mathrm{K}$; under the magnetic field change of 1 and 5 T, the maximum magnetic entropy change $-\mathrm{\Delta }{S}_M$ is 5.27 and $16.1\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$, and the estimated magnetic refrigerant capacity reaches 68 and $522\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}$, respectively. These latter values are among the largest for the known MCE materials. The observed giant and reversible MCE in $\mathrm{E}{\mathrm{r}}_2\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_7$ is mainly attributed to the large saturation moment of $18.9{\mu }_{\mathrm{B}}$ per formula unit owing to a simultaneous FM ordering of the rear-earth ${\mathrm{Er}}^{3+}$ and transition-metal ${\mathrm{Mn}}^{4+}$ localized moments. Our results suggest that $\mathrm{E}{\mathrm{r}}_2\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_7$ pyrochlore is a promising candidate for magnetic refrigeration applications in the temperature range 20–80 K. More importantly, this work provides a new material system for developing high-performance MCE materials that can exhibit a strongly coupled FM transition involving two magnetic sublattices of large local moments in a single-phase material.

In InN, a genuine band gap opening observed after hydrogenation has been explained by means of the “solitary cation” model, a multi-H complex in which the central cation, In*, is fully separated from the structure [Pettinari et al., Adv. Funct. Mater. 25, 5353 (2015)]. Similar effects of H on the host band gap have been observed in In-rich ${\text{In}}_{1-x}{\text{Ga}}_x\text{N}$ alloys. Paying attention to these materials, we have theoretically investigated the In* properties against three kinds of disorder, structural, compositional, and configurational, all of them possibly occurring in ${\text{In}}_{1-x}{\text{Ga}}_x\text{N}$ alloys. As a first major result we have found that a same, general solitary-cation model and mechanism explain the effects of hydrogenation on the electronic properties of both InN and In-rich ${\text{In}}_{1-x}{\text{Ga}}_x\text{N}$ alloys. Even more interestingly, in these alloys, both the energetics of the In* solitary cations and their effects on the band gap result to be thoroughly independent of their atomic neighborhood, in particular, of the number and spatial distribution of their cation neighbors. Significantly, this implies that band-gap opening effects can be safely predicted in whatever hydrogenated In-rich nitride alloy containing different In companions (e.g., B, Al, or Ga) as well as in InN-containing, unconventional compounds (e.g., ZnO-InN), thus offering novel opportunities for material engineering.

In this paper, we study the mechanisms of growth of Ag nanoclusters in a solid Ar matrix and the emission of these nanoclusters from the matrix by a combination of experimental and theoretical methods. The molecular dynamics simulations show that the cluster growth mechanism can be described as “thermal spike-enhanced clustering” in multiple sequential ion impact events. We further show that experimentally observed large sputtered metal clusters cannot be formed by direct sputtering of Ag mixed in the Ar. Instead, we describe the mechanism of emission of the metal nanocluster that, at first, is formed in the cryogenic matrix due to multiple ion impacts, and then is emitted as a result of the simultaneous effects of interface boiling and spring force. We also develop an analytical model describing this size-dependent cluster emission. The model bridges the atomistic simulations and experimental time and length scales, and allows increasing the controllability of fast generation of nanoclusters in experiments with a high production rate.

The constituents of oxide glasses are typically classified as network formers, which form the rigid backbone of glasses, and network modifiers, which tend to either charge stabilize tetrahedral network formers or depolymerize the network. Although it is well known that the properties of glasses depend on their degree of polymerization, little is known about the role of the type of elements used as network modifiers. Here, based on a series of aluminoborate glasses comprising varying alkali oxide modifiers, we show that the glasses’ structural and mechanical properties are controlled by the field strength (ratio of charge to size) of the modifiers. Namely, we show that the stiffness, hardness, and toughness depend on a fine balance between the atomic bonding energy, the packing efficiency of the atoms, and the ability of the network to densify reversibly or irreversibly, with each of these features showing a different dependence on the modifier field strength. This opens a new degree of freedom in the optimization of glass properties.

We report on the successful high-pressure synthesis of single-phase powders of the quadruple perovskite $\left({\mathrm{YMn}}_3\right){\mathrm{Mn}}_4{\mathrm{O}}_{12}$, where the chemical pressure exerted by the small ${\mathrm{Y}}^{3+}$ ion is expected to enhance the exchange interaction between ${\mathrm{Mn}}^{3+}$ ions. According to this expectation, powder neutron diffraction, specific heat, and magnetization measurements give evidence of a C-type antiferromagnetic ordering of the ${\mathrm{Mn}}^{3+}$ ions in the octahedrally coordinated $B$ sites at $T_{N,B}=108\mathrm{K}$, 30 K higher than in the isostructural and isovalent compound, $\left({\mathrm{LaMn}}_3\right){\mathrm{Mn}}_4{\mathrm{O}}_{12}$. Surprisingly, we found no evidence of long-range magnetic order of the square-coordinated $A^{\prime }$ sites, although an indication of latent magnetic order is given by a broad peak of the DC magnetization between 70 and 40 K and by a sharp peak in the AC susceptibility at $T^{*}=70\mathrm{K}$. A further unexpected feature of $\left({\mathrm{YMn}}_3\right){\mathrm{Mn}}_4{\mathrm{O}}_{12}$, not found in previously reported $(A{\mathrm{Mn}}_3){\mathrm{Mn}}_4{\mathrm{O}}_{12}$ compounds, is a second-order structural phase transition at $T_s=200\mathrm{K}$, attributed to a partial redistribution of charge involving both the $B$ and $A^{\prime }$-site ${\mathrm{Mn}}^{3+}$ ions. The present results suggest the existence of competing magnetic orderings, which may be linked to the occurrence of magnetic ferroelectricity in the $(A{\mathrm{Mn}}_3){\mathrm{Mn}}_4{\mathrm{O}}_{12}$ system.

Herein we report a theoretical investigation based on density-functional theory (DFT) calculations devoted to the nature of charge carriers in $\mathrm{CdI}{\mathrm{n}}_2{\mathrm{S}}_4$. Our simulations led to unambiguous results concerning the origin of $n$-type semiconductivity and its magnitude in this material. Namely, the calculated defect formation energies demonstrate that the cadmium substoichiometry is more favorable than the indium one to account for $n$-type conductivity. Moreover, the anionic vacancies would not be the driving force of the occurring processes but a sulfur-poor atmosphere has to be privileged compared to a sulfur-rich one to favor the formation of $\mathrm{CdI}{\mathrm{n}}_2{\mathrm{S}}_4$ with understoichiometry in Cd(II) and thus achieve a higher concentration of free electrons at room temperature.

Moiré patterns in van der Waals (vdW) heterostructures bring novel physical effects to the materials. We report theoretical investigations of the Moiré pattern formed by graphene (Gr) on hexagonal boron nitride $\left(h\text{BN}\right)$. For both the two-dimensional (2D) flat-sheet and the freestanding three-dimensional (3D) wavelike film geometries, the behaviors of Dirac electrons are strongly modulated by the local high-symmetry stacking configurations of the Moiré pattern. In the 2D flat sheet, the secondary Dirac cone (SDC) dispersion emerges due to the stacking-selected localization of SDC wave functions, while the original Dirac cone (ODC) gap is suppressed due to an overall effect of ODC wave functions. In the freestanding 3D wavelike Moiré structure, we predict that a specific local stacking in the Moiré superlattice is promoted at the expense of other local stackings, leading to an electronic structure more similar to that of the perfectly matching flat Gr/$h\text{BN}$ than that of the flat-sheet 2D Moiré pattern. To capture the overall picture of the Moiré superlattice, supercells containing 12 322 atoms are simulated by first principles.

The origin of highly efficient iodine doping of carbon nanotubes is not well understood. Relying on first-principles calculations, we found that iodine molecules $\left({\mathrm{I}}_2\right)$ in contact with a carbon nanotube interact to form monoiodide or/and polyiodide from two and three ${\mathrm{I}}_2$ as a result of removing electrons from the carbon nanotube $(p$-type doping). Charge per iodine atom for monoiodide ion or iodine atom at end of iodine chain is significantly higher than that for ${\mathrm{I}}_2$. This atomic analysis extends previous studies showing that polyiodide ions are the dominant dopants. Moreover, we observed isolated I atoms in atomically resolved transmission electron microscopy, which proves the production of monoiodide. Finally, using Raman spectroscopy, we quantitatively determined the doping level and estimated the number of conducting channels in high electrical conductivity fibers composed of iodine-doped double-wall carbon nanotubes.

Diffusion quantum Monte Carlo results are used as a reference to analyze properties related to phase stability and magnetism in vanadium dioxide computed with various formulations of density functional theory. We introduce metrics related to energetics, electron densities and spin densities that give us insight on both local and global variations in the antiferromagnetic M1 and R phases. Importantly, these metrics can address contributions arising from the challenging description of the $3d$ orbital physics in this material. We observe that the best description of energetics between the structural phases does not correspond to the best accuracy in the charge density, which is consistent with observations made recently by Medvedev et al. [Science 355, 371 (2017)] in the context of isolated atoms. However, we do find evidence that an accurate spin density connects to correct energetic ordering of different magnetic states in ${\mathrm{VO}}_2$, although local, semilocal, and meta-GGA functionals tend to erroneously favor demagnetization of the vanadium sites. The recently developed SCAN functional stands out as remaining nearly balanced in terms of magnetization across the M1-R transition and correctly predicting the ground state crystal structure. In addition to ranking current density functionals, our reference energies and densities serve as important benchmarks for future functional development. With our reference data, the accuracy of both the energy and the electron density can be monitored simultaneously, which is useful for functional development. So far, this kind of detailed high accuracy reference data for correlated materials has been absent from the literature.

The present high-resolution inelastic neutron scattering experiment on ice XVII, containing molecular hydrogen with a different ortho/para ratio, allows one to assign the ${\mathrm{H}}_2$ motion spectral bands to rotational and center-of-mass translational transitions of either para- or ortho-${\mathrm{H}}_2$. Due to its structure, ice XVII confines ${\mathrm{H}}_2$ molecules to move in spiral channels of molecular size. Reported data demonstrate that ${\mathrm{H}}_2$ molecules rotate almost freely in these nanometric channels, though showing larger perturbation than in clathrate hydrates, and perform a translational motion exhibiting two low-frequency excitations. The agreement between the experimental spectra and the corresponding molecular dynamics results clearly enables one to portray a picture of the confined motions of a hydrophobic guest within a metastable ice framework, i.e., ice XVII.

The adsorption energy of benzene on various metal substrates is predicted using the random phase approximation (RPA) for the correlation energy. Agreement with available experimental data is systematically better than 10% for both coinage and reactive metals. The results are also compared with more approximate methods, including van der Waals density functional theory (DFT), as well as dispersion-corrected DFT functionals. Although dispersion-corrected DFT can yield accurate results, for instance, on coinage metals, the adsorption energies are clearly overestimated on more reactive transition metals. Furthermore, coverage dependent adsorption energies are well described by the RPA. This shows that for the description of aromatic molecules on metal surfaces further improvements in density functionals are necessary, or more involved many-body methods such as the RPA are required.

Epitaxial strain and chemical doping are two different methods that are commonly used to tune the physical properties of epitaxial perovskite oxide films, but their cooperative effects are less addressed. Here we try to tune the phase separation (PS) in ${\left(\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{P}{\mathrm{r}}_x\right)}_{2/3}\mathrm{C}{\mathrm{a}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ $(0\le x\le 0.4$, LPCMO) films via cooperatively controlling the anisotropic epitaxial strain (AES) and the Pr doping. These films are grown simultaneously on $\mathrm{NdGa}{\mathrm{O}}_3\left(110\right),{\left(\mathrm{LaAl}{\mathrm{O}}_3\right)}_{0.3}{\left(\mathrm{SrA}{\mathrm{l}}_{0.5}\mathrm{T}{\mathrm{a}}_{0.5}{\mathrm{O}}_3\right)}_{0.7}\left(001\right)$, and $\mathrm{NdGa}{\mathrm{O}}_3\left(001\right)$ substrates with progressively increased in-plane AES, and probed by x-ray diffraction, magnetotransport, and magnetic force microscopy (MFM) measurements. Although it is known that for $x=0$ the AES can enhance the orthorhombicity of the films yielding a phase diagram with the antiferromagnetic charge-ordered insulator (AF-COI) state induced, which is quite different from the bulk one, we illustrate that the Pr doping can further drive the films towards a more robust COI state. This cooperative effect is reflected by the increasing magnetic fields needed to melt the COI phase as a function of AES and the doping level. More strikingly, by directly imaging the phase competition morphology of the $\mathrm{LPCMO}/\mathrm{NdGa}{\mathrm{O}}_3\left(001\right)$ films via MFM, we find that during COI melting the PS domain structure is subject to both AES and the quenched disorder. However, in the reverse process, as the magnetic field is decreased, the COI phase reappears and the AES dominates leaving a crystalline-orientation determined self-organized microstructure. This finding suggests that the PS states and the domain configurations can be selectively controlled by the AES and/or the quenched disorder, which may shed some light on the engineering of PS domains for device fabrications.

Two-dimensional (2D) carbides and nitrides of early transition metals (MXenes) combine high conductivity with hydrophilic surfaces, which make them promising for energy storage, electrocatalysis, and water desalination. The effects of intercalated metal ions on the vibrational states of water confined in ${\mathrm{Ti}}_3{\mathrm{C}}_2{T}_x$ MXenes have been explored using inelastic neutron scattering (INS) and molecular-dynamics simulations to better understand the mechanisms that control MXenes’ behavior in aqueous electrolytes, water purification, and other important applications. We observe an INS signal from water in all samples, pristine and with lithium, sodium, or potassium ions intercalated between the 2D ${\mathrm{Ti}}_3{\mathrm{C}}_2{T}_x$ layers. However, only a small amount of water is found to reside in ${\mathrm{Ti}}_3{\mathrm{C}}_2{T}_x$ intercalated with metal ions. Water in pristine ${\mathrm{Ti}}_3{\mathrm{C}}_2{T}_x$ is more disordered, with bulklike characteristics, in contrast to intercalated ${\mathrm{Ti}}_3{\mathrm{C}}_2{T}_x$, where water is more ordered, irrespective of the metal ions used for intercalation. The ordering of the confined water increases with the ion size. This finding is further confirmed from molecular-dynamics simulation, which showed an increase in interference of water molecules with increasing ion size resulting in a concomitant decrease in water mobility, therefore providing guidance to tailor MXene properties for energy and environmental applications.

We use density functional theory calculations to show nanofilms of covalent ${\mathrm{MoSi}}_2$ exhibit tunable carrier densities and high metallicity at the Fermi level. We determine the nanofilm stabilities by assessing their formation energies, surface energies, and phonon spectra. The dependence of metallicity on the nanofilm geometry is explained using surface-termination-induced electronic structure changes and confinement effects. Understanding this metallicity not only is important to further improve the oxidation resistance of related alloys, but also will facilitate the identification of various thin films of ${\mathrm{MoSi}}_2$ alloys and should lead to their use in nanoelectronic, electrochemical catalysis and low-dimensional transport experiments.