It has been shown that oxide ceramics containing multiple transition and/or rare-earth elements in equimolar ratios have a strong tendency to crystallize in simple single-phase structures, stabilized by the high configurational entropy. In analogy to the metallic alloy systems, these oxides are denoted high-entropy oxides (HEOs). The HEO concept allows to access hitherto uncharted areas in the multielement phase diagram. Among the already realized structures there is the highly complex class of rare-earth transition element perovskites. This fascinating class of materials generated by applying the innovative concept of high-entropy stabilization provides a new and manyfold research space with promise of discoveries of unprecedented properties and phenomena. The present study provides a first investigation of the magnetic properties of selected compounds of this novel class of materials. Comprehensive studies by DC and AC magnetometry are combined with element specific spectroscopy in order to understand the interplay between magnetic exchange and the high degree of chemical disorder in the systems. We observe a predominant antiferromagnetic behavior in the single-phase materials, combined with a small ferromagnetic contribution. The latter can be attributed to either small ferromagnetic clusters or configurations in the antiferromagnetic matrix or a possible spin canting. In the long term perspective it is proposed to screen the properties of this family of compounds with high throughput methods, including combined experimental and theoretical approaches.

Chalcogenide phase change materials, such as $\mathrm{G}{\mathrm{e}}_1\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_4$ (GST), are of tremendous importance in emerging data storage technology, which takes advantage of rapid and reversible switching of GST between the amorphous and crystalline phases. To date, however, the atomic arrangement of the crystalline GST structure has not been fully resolved, resulting in a controversial understanding of the polymorphic transition mechanism. Here, the atomic and chemical arrangements of stable hexagonal structures of GST are determined by state-of-the-art aberration-corrected scanning transmission electron microscopy. A partially ordered Ge/Sb atomic stacking is resolved in the hexagonal structure to balance the enthalpy and entropy, differing from completely disordered Ge/Sb intermixing arrangement in a metastable rock-salt structure. The transition mechanism between these two phases is proposed, achieved by opening the van de Waals gap like a zipper triggered by the Ge/Sb hopping near vacancy grooves rather than the interplanar random atomic migration. The present results shed light on the understanding of the atomic arrangement and polymorphic phase transitions as well as the control of disorder in GST phase change memory.

We report on the fabrication of conducting interfaces between ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ by ${90}^{\circ }$ off-axis sputtering in an Ar atmosphere. At a growth pressure of 0.04 mbar the interface is metallic, with a carrier density of the order of $1\times {10}^{13}{\mathrm{cm}}^{-2}$ at 3 K. By increasing the growth pressure, we observe an increase of the out-of-plane lattice constants of the ${\mathrm{LaAlO}}_3$ films while the in-plane lattice constants do not change. Also, the low-temperature sheet resistance increases with increasing growth pressure, leading to an insulating interface when the growth pressure reaches 0.10 mbar. We attribute the structural variations to an increase of the La/Al ratio, which also explains the transition from metallic behavior to insulating behavior of the interfaces. Our research shows that the control which is furnished by the Ar pressure makes sputtering as versatile a process as pulsed laser deposition, and emphasizes the key role of the cation stoichiometry of ${\mathrm{LaAlO}}_3$ in the formation of the conducting interface.

The influence of the mechanical strain and chemical substitution on the magnetic anisotropy energy (MAE) of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ is studied using first-principles calculations. Dzyaloshinskii-Moriya interaction contributes substantially to MAE by inducing spin canting when the antiferromagnetic order parameter is not aligned with the hexagonal axis. Nearly cubic crystal field results in a very small MAE in pure ${\mathrm{Cr}}_2{\mathrm{O}}_3$ at zero strain, which is incorrectly predicted to be negative (in-plane) on account of spin canting. The MAE is strongly modified by epitaxial strain, which tunes the crystal-field splitting of the $t_{2g}$ triplet. The contribution from magnetic dipolar interaction is very small at any strain. The effects of cation (Al, Ti, V, Co, Fe, Nb, Zr, Mo) and anion (B) substitutions on MAE are examined. Al increases MAE thanks to the local lattice deformation. In contrast, the electronic configuration of V and Nb strongly promotes easy-plane anisotropy, while other transition-metal dopants have only a moderate effect on MAE. Substitution of oxygen by boron, which has been reported to increase the Néel temperature, has a weak effect on MAE, whose sign depends on the charge state of B. The electric field applied along the (0001) axis has a weak second-order effect on the MAE.

Based on first-principles calculations, we show that chemically active metal $\mathrm{n}{\mathrm{s}}^2$ lone pairs play an important role in exciton relaxation and dissociation in low-dimensional halide perovskites. We studied excited-state properties of several recently discovered luminescent all-inorganic and hybrid organic-inorganic zero-dimensional (0D) Sn and Pb halides. The results show that, despite the similarity in ground-state electronic structure between Sn and Pb halide perovskites, the chemically more active $\mathrm{S}{\mathrm{n}}^{2+}$ lone pair leads to stronger excited-state structural distortion and larger Stokes shift in Sn halides. The enhanced Stokes shift hinders excitation energy transport, which reduces energy loss to defects and increases the photoluminescence quantum efficiency. The presence of the $\mathrm{n}{\mathrm{s}}^2$ metal cations in the 0D halide perovskites also promotes the exciton dissociation into electron and hole polarons especially in all-inorganic compounds, in which the coupling between metal-halide clusters is significant.

Chemical decomposition of compound phases has been observed in many engineering alloys subjected to plastic deformation. However, little is known about the role of dislocations moving within compounds in their deformation-induced chemical decomposition. We report observations of chemical decomposition of compound crystals along dislocations with atomic resolution, which is inconsistent with the well-established dislocation theory. Transition-metal (TM) and rare-earth element (RE) atoms in the wake of moving partial dislocations are removed out of Mg-TM-RE compounds in magnesium alloys, producing magnesium nanoslices inside. Glide of partial dislocations forces local transformation from face-centered cubic to hexagonal structures with supersaturated TM and RE in Mg-TM-RE compounds. TM and RE atoms behind moving partial dislocations are then relocated into adjacent magnesium grains through pipe diffusion, under the driving force of supersaturated chemical potential. Our observations demonstrate atomically spatial coupling of individual moving dislocations and chemical decomposition within bulk crystals during plastic deformation, providing insights into dislocations and their roles in materials.

Promising multifunctional magnetoresponsive effects such as magnetoresistance, magnetostrain, and the magnetocaloric effect have recently been extensively studied in Ni-Mn-based metamagnetic shape memory alloys, but large reversible magnetoresponsive effects are usually obtained under high magnetic fields, which is an obstacle for practical applications. Here, through manipulating magnetostructural transition, we achieved large reversible magnetoresponsive effects under a relatively low magnetic field of 3 T in a Ni-Co-Mn-In alloy. By systematically tuning the Mn/In ratio and Co substitution, an optimum composition ${\mathrm{Ni}}_{49}{\mathrm{Co}}_3{\mathrm{Mn}}_{34}{\mathrm{In}}_{14}$ with a low thermal hysteresis (8 K), a narrow transformation interval (7 K) and a high sensitivity of transformation temperature to field change ($6\mathrm{K}{\mathrm{T}}^{-1}$), was obtained. Good geometric compatibility between austenite and martensite was revealed by in situ synchrotron high-energy x-ray diffraction experiment, which accounts for the low hysteresis and narrow transformation interval. A reversible transformation between pure austenite and pure martensite is induced by a relatively low field of 3 T, which was directly evidenced by in situ neutron diffraction experiments. As a result, a large reversible magnetocaloric effect with entropy change of $16.5\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$, a large reversible magnetostrain of 0.26%, and a large reversible magnetoresistance of 60%, under a relatively low field of 3 T, were simultaneously achieved. These reversible magnetoresponsive effects are comparable to the maximum reversible values obtained under high fields in other Ni-Mn-based alloys, but the magnetic field we applied is much lower. This study may guide the design of metamagnetic shape memory alloys with low-field-induced magnetoresponsive properties for magnetic refrigeration, magnetic sensing, and magnetic recording applications.

High-quality defect-free lonsdaleite Si and Ge can now be grown on hexagonal nanowire substrates. These hexagonal phases of group-IV semiconductors have been predicted to exhibit improved electronic and optical properties for optoelectronic applications. While lonsdaleite Si is a well-characterized indirect semiconductor, experimental data and reliable calculations on lonsdaleite Ge are scarce and not consistent regarding the nature of its gap. Using ab initio density-functional theory, we calculate accurate structural, electronic, and optical properties for hexagonal Ge. Given the well-known sensitivity of electronic-structure calculations for Ge to the underlying approximations, we systematically test the performance of several exchange-correlation functionals, including meta-GGA and hybrid functionals. We first validate our approach for cubic Ge, obtaining atomic geometries and band structures in excellent agreement with available experimental data. Then, the same approach is applied to predict electronic and optical properties of lonsdaleite Ge. We portray lonsdaleite Ge as a direct semiconductor with only weakly dipole-active lowest optical transitions, small band gap, huge crystal-field splitting, and strongly anisotropic effective masses. The unexpectedly small direct gap and the oscillator strengths of the lowest optical transitions are explained in terms of symmetry and backfolding of energy bands of the diamond structure.

Epitaxial growth of III-V semiconductors on Si is a promising route for silicon photonics. Threading dislocations and the residual thermal stress generated during growth are expected to affect the thermal conductivity of the III-V semiconductors, which is crucial for efficient heat dissipation from photonic devices built on this platform. In this work, we combine a noncontact laser-induced transient thermal grating technique with ab initio phonon simulations to investigate the in-plane thermal transport of epitaxial GaAs-based buffer layers on Si, employed in the fabrication of III-V quantum dot lasers. Surprisingly, we find a significant reduction of the in-plane thermal conductivity of GaAs, up to $19\%$, as a result of a small in-plane biaxial stress of $\sim 250\mathrm{MPa}$. Using ab initio phonon calculations, we attribute this effect to the enhancement of phonon-phonon scattering caused by the in-plane biaxial stress, which breaks the cubic crystal symmetry of GaAs. Our results indicate the importance of eliminating the residual thermal stress in the epitaxial III-V layers on Si to avoid the reduction of thermal conductivity and facilitate heat dissipation. Additionally, our results showcase potential means of effectively controlling thermal conductivity of solids with external strain/stress.

The force-electric effect in ferroelectrics is characterized by the release of bound charge during pressure/shock-induced depolarization. In contrast to other electrical energy storage systems, the charge-storage/release by the force-electric effect of ferroelectrics is determined by polarization switching or polar-nonpolar phase transition. This offers a further set of options for materials design in the realm of energy conversion, especially for the high power density applications. Here, we report that a ferroelectric ceramic, $\mathrm{N}{\mathrm{a}}_{0.5}\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ (NBT), can generate a high power output $(3.04\times {10}^8\mathrm{W}/\mathrm{kg})$ under shock compression, which is one of the highest values achieved by the force-electric effect. The in situ synchrotron x-ray diffraction studies reveal that this power output mainly arises from a polar-nonpolar phase transition (rhombohedral-orthorhombic). First-principles calculations show that this is a first-order phase transition that undergoes two-step structure changes. These results extend the application of the force-electric effect and are a key step in understanding the phase transition behaviors of NBT under high pressure.

We develop a method to efficiently construct phase diagrams using machine learning. Uncertainty sampling (US) in active learning is utilized to intensively sample around phase boundaries. Here, we demonstrate constructions of three known experimental phase diagrams by the US approach. Compared with random sampling, the US approach decreases the number of sampling points to about 20%. In particular, the reduction rate is pronounced in more complicated phase diagrams. Furthermore, we show that using the US approach, undetected new phases can be rapidly found, and smaller numbers of initial sampling points are sufficient. Thus, we conclude that the US approach is useful to construct complicated phase diagrams from scratch and will be an essential tool in materials science.

Transition-metal compounds pose serious challenges to first-principles calculations based on density-functional theory (DFT), due to the inability of most approximate exchange-correlation functionals to capture the localization of valence electrons on their $d$ states, essential for a predictive modeling of their properties. In this work we focus on two representatives of a well known family of cathode materials for Li-ion batteries, namely the orthorhombic $\mathrm{Li}M{\text{PO}}_4$ olivines ($M$ = Fe, Mn). We show that extended Hubbard functionals with on-site ($U$) and intersite ($V$) interactions (so called DFT+$U$+$V$) can predict the electronic structure of the mixed-valence phases, the formation energy of the materials with intermediate Li contents, and the overall average voltage of the battery with remarkable accuracy. We find, in particular, that the inclusion of intersite interactions in the corrective Hamiltonian improves considerably the prediction of thermodynamic quantities when electronic localization occurs in the presence of significant interatomic hybridization (as is the case for the Mn compound), and that the self-consistent evaluation of the effective interaction parameters as material- and ground-state-dependent quantities allows the prediction of energy differences between different phases and concentrations.

Crackling noise of ferroelectric lead zirconate titanate samples during ferroelectric switching is demonstrated to be compatible with avalanche statistics. The peaks of the slew rate (time derivative of current $dI/dt$ squared), defined as “jerks,” were statistically analyzed and shown to obey power laws. The critical exponent obtained is $1.64\pm 0.15$, in agreement with predictions from avalanche theory. The exponent is independent of temperature within experimental error margins.

The microscopic origin of the large negative thermal expansion of cubic scandium trifluorides (${\mathrm{ScF}}_3$) is investigated by performing a set of anharmonic free-energy calculations based on density functional theory. We demonstrate that the conventional quasiharmonic approximation (QHA) completely breaks down for ${\mathrm{ScF}}_3$ and the quartic anharmonicity, treated nonperturbatively by the self-consistent phonon theory, is essential to reproduce the observed transition from negative to positive thermal expansivity and the hardening of the ${\mathrm{R}}^{4+}$ soft mode with heating. In addition, we show that the contribution from the cubic anharmonicity to the vibrational free energy, evaluated by the improved self-consistent phonon theory, is significant and as important as that from the quartic anharmonicity for robust understandings of the temperature dependence of the thermal expansion coefficient. The first-principles approach of this study enables us to compute various thermodynamic properties of solids in the thermodynamic limit with the effects of cubic and quartic anharmonicities. Therefore, it is expected to solve many known issues of the QHA-based predictions particularly noticeable at high temperature and in strongly anharmonic materials.

We report on the pressure-driven superconductor-insulator transition in heavily boron-doped nanodiamond films. By systematically increasing the pressure, we suppress the Josephson coupling between the superconducting nanodiamond grains. The diminished intergrain coupling gives rise to an overall insulating state in the films, which is interpreted in the framework of a parallel-series circuit model to be the result of bosonic insulators with preserved localized intragrain superconducting order parameters. Our investigation opens up perspectives for the application of high pressure in research on quantum confinement and coherence. Our data unveil the percolative nature of the electrical transport in nanodiamond films, and highlight the essential role of grain boundaries in determining the electronic properties of this material.

An atomistic study of the thermal properties of lithiated molybdenum disulfide (${\mathrm{MoS}}_2$) is presented and an explanation for the experimentally determined anisotropic behavior of the in- and through-plane thermal conductivity is proposed. Configurations with different levels of lithium concentration are simulated using density functional theory and their structural, electronic, and thermal transport properties are evaluated as a function of the degree of lithiation. Comparisons between regular and disordered lithium distributions as well as to the experimental data reveal that the measured ratio of the in-plane to through-plane thermal conductivity can only be reproduced if the disorder of the lithium ions is taken into account. These results suggest that it is not only possible to modify the thermal properties of ${\mathrm{MoS}}_2$ by changing the degree of lithiation, but also by controlling its level of disorder.

High-performance $p$-type transparent conducting materials (TCMs) must exhibit a rare combination of properties including high mobility, transparency, and $p$-type dopability. The development of high-mobility/conductivity $p$-type TCMs is necessary for many applications such as solar cells or transparent electronic devices. Oxides have been traditionally considered as the most promising chemical space to dig out novel $p$-type TCMs. However, nonoxides might perform better than traditional $p$-type TCMs (oxides) in terms of mobility. We report on a high-throughput computational search for nonoxide $p$-type TCMs from a large data set of more than 30 000 compounds which identified CaTe and ${\mathrm{Li}}_3\mathrm{Sb}$ as very good candidates for high-mobility $p$-type TCMs. From our calculations, both compounds are expected to be $p$-type dopable: intrinsically for ${\mathrm{Li}}_3\mathrm{Sb}$ while CaTe would require extrinsic doping. Using electron-phonon computations, we estimate hole mobilities at room temperature to be about 20 and $70{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ for CaTe and ${\mathrm{Li}}_3\mathrm{Sb}$, respectively. These are “upper bound” values as only scattering with phonons is taken into account. The computed hole mobility for ${\mathrm{Li}}_3\mathrm{Sb}$ is quite exceptional and comparable with the electron mobility in the best $n$-type TCMs.

We report an investigation of the ground-state properties of americium dioxide and its sesquioxides using first-principles calculations. In order to take into account strong $5f$ electronic correlations, we apply the generalized gradient approximation (GGA)+U as well as the hybrid functionals to these compounds. We present a systematic study of several bulk properties such as structural, elastic, energetic, electronic, and magnetic properties as a function of the effective Coulomb U and exchange J GGA+U parameters in the range of 0.0–8.0 and 0.00–0.75 eV, respectively. The values $\left(U,J\right)=\left(6.00\mathrm{eV},0.75\mathrm{eV}\right)$ are those providing a correct description of bulk properties by comparison to the available experimental data. We discuss the effect of loss of symmetry caused by DFT+U on several properties. We also discuss the effect of the spin-orbit coupling on these properties. Furthermore, we highlight that in order to reach the ground state of americium dioxide and sesquioxides using GGA+U the monitoring of the occupation matrices of $5f$ correlated orbitals is crucial. Our computational scheme provides reliable and accurate results concerning several bulk properties that have not been studied experimentally yet. For instance, our elastic constants calculated for $\mathrm{Am}{\mathrm{O}}_2$ follow the same trend as those of other actinide oxides ($\mathrm{U}{\mathrm{O}}_2$, $\mathrm{Np}{\mathrm{O}}_2$, and $\mathrm{Pu}{\mathrm{O}}_2$) and can be used as reference. We have also computed the electronic and static dielectric constants, as well as the Am and O Born effective charge of americium dioxide. Finally, we find the A-type hexagonal structure to be the most stable structure at low temperature for americium sesquioxides. For this phase, we predict the internal structural parameters as well as the bulk modulus. We provide data concerning the formation enthalpy of the $\mathrm{A}{\mathrm{m}}_2{\mathrm{O}}_3$ bcc cubic structure.