${\mathrm{Sb}}_2{\mathrm{S}}_3$ has attracted great attention recently as a prospective solar cell absorber material. In this work, intrinsic defects, dopants, and their complexes in ${\mathrm{Sb}}_2{\mathrm{S}}_3$ are systematically investigated by using hybrid functional theory. ${\mathrm{V}}_{\mathrm{Sb}}$ and ${\mathrm{V}}_{\mathrm{S}}$ are dominant native defects and pin the Fermi level near the midgap, which is consistent with the high resistivity observed experimentally. Both ${\mathrm{V}}_{\mathrm{Sb}}$ and ${\mathrm{V}}_{\mathrm{S}}$ introduce deep levels inside the band gap, which can trap free carriers. Our calculated deep transition levels of ${\mathrm{V}}_{\mathrm{Sb}}$ and ${\mathrm{Sb}}_{\mathrm{S}}$ are consistent well with the results of the deep-level transient spectroscopy measurement. We further study dopants (including Cu, Ti, Zn, Br, and Cl) in ${\mathrm{Sb}}_2{\mathrm{S}}_3$ and find that Zn and Br/Cl are shallow acceptors and donors, respectively, which may be used to control the carrier and trap densities in ${\mathrm{Sb}}_2{\mathrm{S}}_3$. In addition, the defect complexes, i.e., $\mathrm{Cu}{\left(\mathrm{Zn}\right)}_{\mathrm{Sb}}+{\mathrm{V}}_{\mathrm{S}}$ and $\mathrm{Cl}{\left(\mathrm{Br}\right)}_{\mathrm{S}}+{\mathrm{V}}_{\mathrm{Sb}}$ are also investigated. The interaction between the donor and acceptor defects makes the defect levels of complexes shallower and less detrimental to carrier transport.

A- and B-site ordered quadruple perovskites with the chemical formula ${\mathrm{AA}}_3^{\prime }{\mathrm{B}}_2{\mathrm{B}}_2^{\prime }{\mathrm{O}}_{12}$ can form with 1:3 ratio at the A site. A unique feature of this specially ordered perovskite is that three different atomic sites $({\mathrm{A}}^{\prime }, \mathrm{B}, \mathrm{and} {\mathrm{B}}^{\prime }$ sites) can all accommodate magnetic transition metals. As a consequence, multiple magnetic and electronic interactions can occur at ${\mathrm{A}}^{\prime }$, B, and/or ${\mathrm{B}}^{\prime }$ sites, giving rise to a series of intriguing physical phenomena. ${\mathrm{CaCu}}_3{\mathrm{Fe}}_2{\mathrm{Re}}_2{\mathrm{O}}_{12}$ is a good example, which shows half-metallic electronic structure, large magnetization, and a very high Curie temperature. Here we investigated a series of ferrimagnetic (FiM) compounds ${\mathrm{ACu}}_3{\mathrm{Fe}}_2{\mathrm{Re}}_2{\mathrm{O}}_{12}$ (A=Ca, Sr, Ba, Pb, Sc, Y, La) by using density functional theory and Monte Carlo simulations. We found that all compounds with ${\mathrm{A}}^{2+}$ ions exhibit high Curie temperatures (above 405 K), and the compounds with ${\mathrm{A}}^{3+}$ substitution yield even higher $T_C$ (above 502 K). By examining interatomic exchange parameters, we found that the antiferromagnetic exchange couplings between Re and Cu as well as Re and Fe are responsible for this very high Curie temperature. For the compounds with ${\mathrm{A}}^{3+}$ substitution, electron doping in bands around the Fermi level dominated by the Re ions strengthens the Re-Cu, Re-Fe, and Re-Re exchange interactions, which cause an increase in the critical temperature. Finally, we calculated the formation energies of the quadruple perovskites with respect to the possible decomposition pathways and have found that the values are reasonable for the synthesis of these compounds under the conditions of high pressure and high temperature. In summary, this work demonstrates a design strategy of enhancing the spin ordering temperature by replacing A-site nonmagnetic ions.

The magnetic anisotropy in MgO-capped MnPt films and its voltage control are studied using first-principles calculations. Sharp variation of the magnetic anisotropy with film thickness, especially in the Pt-terminated film, suggests that it may be widely tuned by adjusting the film thickness. In thick films the linear voltage control coefficient is as large as 1.5 and $-0.6$ pJ/Vm for Pt-terminated and Mn-terminated interfaces, respectively. The combination of a widely tunable magnetic anisotropy energy and a large voltage-control coefficient suggest that MgO-capped MnPt films can serve as a versatile platform for magnetic memory and antiferromagnonic applications.

In the region of a martensitic transformation, conventional elastocaloric effect causes cooling when the forward parent to martensite transition is induced by loading under adiabatic conditions and by heating when the load is removed and the reverse transition occurs. An inverse elastocaloric effect might be expected if the reverse transition could be induced upon loading. However, this behavior is quite uncommon and inverse elastocaloric effect rarely occurs. Herein, we have successfully manufactured a Ti-50.8 at.% Ni crystal in a high proportion of favorable $R$ phase (martensite) variant containing single-variant ${\mathrm{Ti}}_3{\mathrm{Ni}}_4$ nanoprecipitates aligned along the ${\langle 111\rangle }_{B2}$ direction of the $B2$ parent phase, by means of a compressive stress associated aging along the ${\langle 111\rangle }_{B2}$ direction. The material displays inverse elastocaloric effect induced by compressive stress along the ${\left[111\right]}_{B2}$ direction in the temperature range of stability of the $R$ phase. This inverse elastocaloric effect is caused by the stress-induced reverse transformation of the high proportion of favorable $R$-phase variant that occurs when the external compressive stress is applied along the direction of the internal tensile stress induced by the aligned ${\mathrm{Ti}}_3{\mathrm{Ni}}_4$ nanoprecipitates. The mechanism giving rise to this inverse elastocaloric effect is explained by the transformation strain in the ${\left[111\right]}_{B2}$ and ${\left[1\overline{1}0\right]}_{B2}$ directions for different $R$-phase variants based on crystallography. Results indicate that the stress-associated aging process is an effective way of patterning materials to display inverse elastocaloric effect through controlling the alignment of the ${\mathrm{Ti}}_3{\mathrm{Ni}}_4$ nanoprecipitate in the Ni-rich Ti-Ni shape memory alloy.

Thermodynamic properties of bcc uranium with point defects are studied using ab initio molecular dynamics (MD) simulations at 1100 K. The simulations were performed with canonical ensembles of ${\mathrm{U}}_{127}{\mathrm{M}}_1$, ${\mathrm{U}}_{128}{\mathrm{M}}_1$, and ${\mathrm{U}}_{126}{\mathrm{M}}_1{\square }_1$ for M = $\square$, He, Ne, Ar, Kr, Xe, Sr, Zr, I, Cs, and Pu disposed on a bcc lattice lying within a $4\times 4\times 4$ cubic supercell. This work provides formation energies of substitutional, self, and solute interstitial atom defects as well as binding energies of M-$\square$ pair defects. This work demonstrates that our computational scheme based on MD simulations gives reliable formation and binding energies of atomic defects in bcc uranium compared to conventional density functional theory calculations. The equilibrium volume, bulk modulus, and thermal expansion coefficient of pure bcc uranium obtained from our MD simulations compare very well with corresponding experimental results. The vacancy formation energy is predicted to be 0.88 eV. The experimental vacancy formation energy remains uncertain. Experimental study of the formation and binding energies of other point defects as well as the bulk modulus and thermal expansion coefficients of uranium with these defects is also not found in the literature. This work shows that point defects tend to decrease the bulk modulus and increase the thermal expansion coefficient of bcc uranium. The solute formation energies of noble gas atoms show a bearing on their size. A large solute (Xe) has a high formation energy, and vice versa. This size effect is not quite evident for the chemically reactive solutes, namely, Sr, Zr, I, Cs, and Pu. Our MD simulations further show that vacancies are the favorable point defects in bcc uranium rather than both vacancies and self interstitials as predicted by earlier calculations. The formation energies of self interstitial atoms are found to be lower than those of solute interstitial atoms, each calculated in six different basic interstitial dumbbell configurations. That is, bcc U accommodates self interstitials more easily than decay or fission gas interstitials (He, Kr, and Xe). Further, He atoms are found to have comparable formation energies in the substitutional and interstitial locations. The fission product atoms Kr and Xe prefer to occupy vacant substitutional lattice sites rather than interstitial sites. Binding energies of divacancy and solute-vacancy pairs (0.31 vs $-0.69$ eV for the Xe-$\square$ pair, for instance) from our MD simulation show that nucleation and growth of fission gas bubbles are supported by a thermodynamic driving force, whereas vacancies tend to stay apart. This is in agreement with literature reporting that bcc uranium softens and swells mainly by agglomeration of noble gas bubbles.

Ultrahigh lattice thermal conductivity materials hold great importance since they play a critical role in the thermal management of electronic and optical devices. Models using machine learning can search for materials with outstanding higher-order properties like thermal conductivity. However, the lack of sufficient data to train a model is a serious hurdle. Herein we show that big data can complement small data for accurate predictions when lower-order feature properties available in big data are selected properly and applied to transfer learning. The connection between the crystal information and thermal conductivity is directly built with a neural network by transferring descriptors acquired through a pretrained model for the feature property. Successful transfer learning shows the ability of extrapolative prediction and reveals descriptors for lattice anharmonicity. The resulting model is employed to screen over $60000$ compounds to identify novel crystals that can serve as alternatives to diamond. Even though most materials in the top list are superhard materials, we reveal that superhard property does not necessarily lead to high lattice thermal conductivity. Large hardness means high elastic constants and group velocity of phonons in the linear dispersion regime, but the lattice thermal conductivity is determined also by other important factors such as the phonon relaxation time. What is more, the average or maximum dipole polarizability and the van der Waals radius are revealed to be the leading descriptors among those that can also be qualitatively related to anharmonicity.

This paper reports on first-principles calculations of the electronic and magnetic structure of tetragonal Heusler compounds with the composition ${\mathrm{Rh}}_2{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}\mathrm{Sb}$ $(0\le x\le 1)$. It is found that the magnetic moments increase from 2 to 3.4 ${\mu }_B$ and the Curie temperature decreases from 500 to 464 K with increasing Fe content $x$. The $3d$ transition metals make the main contribution to the magnetic moments, whereas Rh contributes only approximately 0.2 ${\mu }_B$ per atom, independent of the composition. The paper focuses on the magnetocrystalline anisotropy of the borderline compounds ${\mathrm{Rh}}_2\mathrm{FeSb}$, ${\mathrm{Rh}}_2{\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}\mathrm{Sb}$, and ${\mathrm{Rh}}_2\mathrm{CoSb}$. A transition from easy-axis to easy-plane anisotropy is observed when the composition changes from ${\mathrm{Rh}}_2\mathrm{CoSb}$ to ${\mathrm{Rh}}_2\mathrm{FeSb}$. The transition occurs at an iron concentration of approximately 40%. The difference in the anisotropy is caused by differences in the charge and spin distributions, mainly at Fe and Co. The results of the calculations are in good agreement with recent experiments.

Lead halide perovskites undergo pressure-induced phase transition, facilitated by bond compressions and different tilt patterns of the ${\mathrm{PbI}}_6$ octahedra. Utilizing first-principles calculations we investigated 14 possible tilt systems of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ under compression, derived from the high-symmetry $Pm\overline{3}m$ structure. Fr substitution is adopted to mimic the rotational disorder effect of the ${\mathrm{CH}}_3{\mathrm{NH}}_3$ cation with the similar size (at room temperature or higher). Analyses of these phases reveal an interplay between tilting and distortion of the ${\mathrm{PbI}}_6$ octahedra. Dynamical fluctuations at finite temperature provide additional insight into the phases' stability. Drawing from the trends observed in $\mathrm{Fr}\mathrm{Pb}{\mathrm{I}}_3$ and additional calculations for perovskites involving differently ordered organic cations, we propose that phase transition in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ occurs from tetragonal at ambient pressure to orthorhombic under high pressure via the pathway I4/mcm$\to$P4/mbm$\to$Imm2. The distinct discontinuity in the calculated volume-pressure curve of I4/mcm and band-gap evolution of the proposed phases are consistent with photoluminescence shifts observed in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ under pressure.

We report the structure and properties of ${(\mathrm{Ca},\mathrm{Ba})}_x{\mathrm{La}}_{2-x}{\mathrm{CuIrO}}_6$. The rock-salt double perovskite structure goes through a $P\overline{1}$ to $P{2}_1/n$ phase transition with increasing Ba content but not with Ca content. In both cases, the Ir ion is oxidized from $4+$ to $5+$ with increasing substitution. Transport and magnetic properties measurements reveal that all compositions are insulating with hysteretic spin-freezing magnetic behavior. Seebeck coefficient measurements reveal a large thermopower change from $+176\mu \mathrm{V}$/K in ${\mathrm{La}}_2{\mathrm{CuIrO}}_6$ to $-223\mu \mathrm{V}$/K with increasing Ba content in ${\mathrm{Ba}}_x{\mathrm{La}}_{2-x}{\mathrm{CuIrO}}_6$. This behavior is modeled well by the Heikes formula for correlated hopping conduction. ${(\mathrm{Ca},\mathrm{Ba})}_x{\mathrm{La}}_{2-x}{\mathrm{CuIrO}}_6$ provides a model system for investigations of other systems with similar carrier sign tuning, with applications toward semiconductors and thermoelectric materials.

To develop a high-quality Weyl magnet ${\mathrm{Mn}}_3\mathrm{Sn}$ thin film, polycrystalline Mn-Sn was sputter deposited onto an fcc or hexagonal close-packed metallic seed layer of Pt, Cu, Ir, Ru, Cr, or Al. Among the tested seed materials, Pt and Cu enabled fabrication of highly $c$-plane oriented textured $D{0}_{19}$-type ${\mathrm{Mn}}_3\mathrm{Sn}$ films with a smooth surface (mean roughness below 0.3 nm) after postannealing at $350{}^{\circ }\mathrm{C}$ or below. The ${\mathrm{Mn}}_3\mathrm{Sn}$ films exhibited substantial signals of the anomalous Nernst effect (ANE), which is one characteristic of a Weyl magnet. Microstructural analysis revealed that the Cu seed layer significantly diffused into the ${\mathrm{Mn}}_3\mathrm{Sn}$ layer during postannealing, while the Pt seed layer changed little. The results suggested that a small amount of Cu mixture (up to 2.5 at. %) did not degrade the ANE signal but effectively lowered the postannealing temperature to form the $D{0}_{19}$ structure. The developed films and techniques are beneficial for antiferromagnetic spintronics in terms of the smoothness and process consistency in a standard process for magnetic random-access memory.

We use density functional theory to identify several previously unknown polar and nonpolar structures within the low-energy phase space of perovskite-structure bismuth ferrite, $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$. Bismuth ferrite is one of the most widely studied multiferroic materials because of its large ferroelectric polarization coexisting with magnetic order at room temperature. Of particular interest is a series of noncentrosymmetric structures with polarization along one lattice vector, combined with antipolar distortions, reminiscent of ferroelectric domains, along a perpendicular direction. We discuss possible routes to stabilising the new phases using biaxial heteroepitaxial strain or interfacial electrostatic control in heterostructures.

Materials with van der Waals bonding are known to exhibit a quantum confinement effect, in which the electronic band gap of the three-dimensional realization of a material is lower than that of its two-dimensional (2D) counterpart. However, the possibility of an anomalous quantum confinement effect (AQCE) exists, where the band gap trend is reversed. In this work, we computationally identify materials for which such AQCE occurs. Using density-functional theory, we compute ∼1000 OptB88vdW (semilocal functional), ∼50 HSE06, and ∼50 PBE0 (hybrid functional) band gaps for bulk and their corresponding monolayers in the JARVIS-DFT database. OptB88vdW identifies 65 AQCE materials, but the hybrid functionals only confirm such findings in 14 cases. Some of the AQCE systems identified through HSE06 and PBE0 are hydroxides or oxide hydroxide compounds [$\mathrm{AlO}{\mathrm{H}}_2$, $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$, ${\mathrm{Mg}}_2{\mathrm{H}}_2{\mathrm{O}}_3$, $\mathrm{Ni}{\left(\mathrm{OH}\right)}_2$, and $\mathrm{Sr}{\mathrm{H}}_2{\mathrm{O}}_3$], as well as Sb-halogen-chalcogenide compounds (SbSBr and SbSeI) and alkali-chalcogenides (RbLiS and RbLiSe). A detailed electronic structure analysis, based on band structure and projected density of states, shows AQCE is often characterized by lowering of the conduction band in the monolayer and corresponding changes in the $p_z$ electronic orbital contribution, with $z$ being the nonperiodic direction in the 2D case. We investigated the possibility of these materials being topological through spin-orbit spillage calculations and determined that they are not. We believe our computational results would spur the effort to validate the results experimentally and will have an impact on band-gap engineering applications based on low-dimensional materials. Our systematic approach also shows that data-driven approaches can be used to discover a new phenomenon such as AQCE.

We investigate the roles of disorder on low-temperature transport in $\mathrm{Sm}{\mathrm{B}}_6$ crystals grown by both the Al flux and floating zone methods. We used the inverted resistance method with Corbino geometry to investigate whether low-temperature variations in the standard resistance plateau arise from a surface or a bulk channel in floating zone samples. The results show significant sample-dependent residual bulk conduction, in contrast to smaller amounts of residual bulk conduction previously observed in Al flux grown samples with Sm vacancies. We consider hopping in an activated impurity band as a possible source for the observed bulk conduction, but it is unlikely that the large residual bulk conduction seen in floating zone samples is solely due to Sm vacancies. We therefore propose that one-dimensional defects, or dislocations, contribute as well. Using chemical etching, we find evidence for dislocations in both flux and floating zone samples, with higher dislocation density in floating zone samples than in Al flux grown samples. In addition to the possibility of transport through one-dimensional dislocations, we also discuss our results in the context of recent theoretical models of $\mathrm{Sm}{\mathrm{B}}_6$.

Quasicrystalline materials possess a unique structure that is ordered yet not periodic. Despite their special configuration and many useful properties, they are typically very brittle at temperatures below ∼75% of their melting points, rendering them difficult to process and often unsuitable for practical implementations. Micro-compression offers an opportunity to unveil the fundamental mechanisms of quasicrystal plasticity. Here, we study the mechanical behavior of a typical icosahedral quasicrystal (i-Al-Pd-Mn) using microthermomechanical techniques over a temperature in the range 25–500 °C. We observe a few interesting phenomena, including micropillar shrinkage, phase transformations, grain refinement, and thermally induced transitions in deformation behavior (from brittle fracture at room temperature to serrated plastic flows and then to homogeneous flows at elevated temperatures). Furthermore, we discuss the multiple underlying mechanisms on the mechanical behavior of the quasicrystal in this temperature regime, exploring surface evaporation/diffusion, diffusion-enhanced plasticity, dislocation activities, and grain boundary rotation/sliding. Our study bridges the gap between room-temperature and high-temperature plasticity in quasicrystals, demonstrating an opportunity to study complex intermetallic phases in broad size and temperature regimes.

The chemical bath deposition (CBD) of ZnO nanowires (NWs) is of high interest, but their formation occurs in a growth medium containing many impurities including carbon, nitrogen, and hydrogen, rendering the accurate determination of predominant crystal defects as highly debated. In addition to the typical interstitial hydrogen in bond-centered sites $\left({\mathrm{H}}_{\mathrm{BC}}\right)$ and zinc vacancy-hydrogen $\left(V_{\mathrm{Zn}}\text{−}n\mathrm{H}\right)$ complexes, we reveal that nitrogen-related defects play a significant role on the physical properties of unintentionally doped ZnO NWs. We show by density functional theory that the $V_{\mathrm{Zn}}\text{−}{\mathrm{N}}_{\mathrm{O}}\text{−}\mathrm{H}$ defect complex acts as a deep acceptor with a relatively low formation energy and exhibits a prominent Raman line at $3078\mathrm{c}{\mathrm{m}}^{\text{–}1}$ along with a red-orange emission energy of ∼1.82 eV in cathodoluminescence spectroscopy. The nature and concentration of the nitrogen- and hydrogen-related defects are found to be tunable using thermal annealing under oxygen atmosphere, but a rather complex, fine evolution including successive formation and dissociation processes is highlighted as a function of annealing temperature. ZnO NWs annealed at the moderate temperature of 300 °C specifically exhibit one of the smallest free charge carrier densities of $5.6\times {10}^{17}\mathrm{c}{\mathrm{m}}^{\text{–}3}$ along with a high mobility of $\sim 60\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$ following the analysis of longitudinal optical phonon-plasmon coupling. These findings report a comprehensive diagram showing the complex interplay of each nitrogen- and hydrogen-related defect during thermal annealing and its dependence on annealing temperature. They further reveal that the engineering of the nitrogen- and hydrogen-related defects as the major source of crystal defects in ZnO NWs grown by CBD is capital to precisely control their electronic structure properties governing their electrical and optical properties in any devices.

We identify highly correlated hole pocket on the Fermi surface of colossal thermoelectric material ${\mathrm{K}}_{0.65}{\mathrm{RhO}}_2$, studied using high-resolution angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT) calculations. Most importantly, two kinks at binding energies of 75 and 195 meV have been observed below the Fermi level. While the low-energy kink at 75 meV can be understood as a result of the electron-phonon interaction, the high-energy kink at 195 meV is a discovery of this system, leading to anomalous band renormalization, possibly originated from the bosonic excitations at higher frequencies. We further notice that the high-energy anomaly has important implications on the colossal thermoelectric power of ${\mathrm{K}}_{0.65}{\mathrm{RhO}}_2$.

We study realizations of spirals and skyrmions in two-dimensional antiferromagnets with a triangular lattice on an inversion-symmetry-breaking substrate. As a possible material realization, we investigate the adsorption of transition-metal atoms (Cr, Mn, Fe, or Co) on a monolayer of ${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, or ${\mathrm{WSe}}_2$ and obtain the exchange, anisotropy, and Dzyaloshinskii-Moriya interaction parameters using first-principles calculations. Using energy minimization and parallel-tempering Monte Carlo simulations, we determine the magnetic phase diagrams for a wide range of interaction parameters. We find that skyrmion lattices can appear even with weak Dzyaloshinskii-Moriya interactions, but their stability is hindered by magnetic anisotropy. However, a weak easy plane magnetic anisotropy can be beneficial for stabilizing the skyrmion phase. Our results suggest that Cr/${\mathrm{MoS}}_2$, Fe/${\mathrm{MoS}}_2$, and Fe/${\mathrm{WSe}}_2$ interfaces can host spin spirals formed from the ${120}^{\circ }$ antiferromagnetic states. Our results further suggest that for interfaces, such as Fe/${\mathrm{MoS}}_2$, the Dzyaloshinskii-Moriya interaction is strong enough to drive the system into a three-sublattice skyrmion lattice in the presence of experimentally feasible external magnetic field.

We present a detailed study of the growth of Bi films on superconducting Nb(110) substrates in dependence on the Bi coverages and the Nb surface quality. We find that Bi grows in a (110) orientation at low coverage equivalent to about five pseudomorphic monolayers (${\mathrm{ML}}^{\mathrm{ps}}$) on clean Nb(110), but then undergoes a structural transition to Bi(111) below about 8 ${\mathrm{ML}}^{\mathrm{ps}}$. Comparison with two oxygen-reconstructed Nb(110) surfaces, the $\mathrm{Nb}{\mathrm{O}}_x$ phases I and II, reveals that the film thickness at which the (110)-to-(111) transition takes place depends on the surface quality. Whereas it is observed at lower coverage for the $\mathrm{Nb}{\mathrm{O}}_x$ phase I, our results indicate that Bi(110) remains stable on $\mathrm{Nb}{\mathrm{O}}_x$ phase II up to the largest film thickness studied here, i.e., 18 ML. The quality and smoothness of the thin Bi films considerably depends on the cleanliness of the Nb substrate, revealing the most flat and defect-free Bi films grown on the oxygen-free clean Nb(110) surface.