One atom or molecule binds to another through various types of bond, the strengths of which range from several meV to several eV. Although some computational methods can provide accurate descriptions of all bond types, those methods are not efficient enough for many studies (for example, large systems, ab initio molecular dynamics and high-throughput searches for functional materials). Here, we show that the recently developed non-empirical strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (meta-GGA) within the density functional theory framework predicts accurate geometries and energies of diversely bonded molecules and materials (including covalent, metallic, ionic, hydrogen and van der Waals bonds). This represents a significant improvement at comparable efficiency over its predecessors, the GGAs that currently dominate materials computation. Often, SCAN matches or improves on the accuracy of a computationally expensive hybrid functional, at almost-GGA cost. SCAN is therefore expected to have a broad impact on chemistry and materials science.
A notable exception is Bi 2 Te 3 [ 2,3 ] and compounds based on it that have comparatively high hexagonal symmetry. This severely restricts the exploration of TE materials to a small percentage of semiconductors that possess high-symmetry cubic structures, and thus excludes a large number of low-symmetry non-cubic materials even though they might manifest ideal bandgaps and low thermal conductivities. Directly determined by the chemical nature of its constituents, the crystal structure of a given material is rigid and rarely can be turned from low symmetry to high symmetry short of external stimuli such as pressure. It remains a key challenge to discover or design novel high-performance TE compounds among non-cubic materials. In this work, taking a hint from the recently emerging chalcopyrite TE materials with reasonable zT values,  we report on our successful approach of rationally tuning crystal structures to design pseudocubic or cubic-like structure blocks in non-cubic materials that lead directly to cubic-like degenerate band-edge electronic states and thus high power factors and enhanced zT values in a few carefully selected chalcopyrites (see Figure 1 ).The pseudocubic structure approach is here understood as a realization of cubic-like, highly degenerate electronic bands at band edges of non-cubic materials through a complex architecture containing an inherently long-range, nearly cubic framework as well as localized short-range non-cubic lattice distortions (see Figure 1 b). Electronic transport processes are dominated by the long-range cubic framework displaying cubic-like highly degenerate band edges and prospects for multi-valley carrier pockets, while the heat conduction is blocked by the presence of large, locally non-cubic lattice distortions. Thus, a very special character of the pseudocubic structure allows the design of high-performance novel TE materials with the ability to simultaneously optimize electrical and thermal transport properties. Binary zinc blende materials have a typical cubic structure with degenerate electron band edges (see Figure 1 a). Starting from non-cubic tetragonal chalcopyrites, the cation sublattice can be tuned to show cubic or nearly cubic framework, while the anion sublattice shows a locally distorted non-cubic framework with two types of irregular tetrahedra in ternary chalcopyrites, leading to a periodic supercell with a cubic framework (Figure 1 b). The high symmetry cubic supercell results in cubic-like degenerate electron bands at the gamma point of the folded Brillouin zone in tetragonal chalcopyrites, representing an ideal pseudocubic structure (see Figure 1 b). Furthermore, through a rationally designed mixing strategy, the emerging complex solid solution chalcopyrites might show an increased randomness of the locally irregular tetrahedra while maintaining the cubic-like Energy harvesting requires clean and highly effi cient energyconversion technologies. Thermoelectricity (TE) is one such technology that achieves thermal-t...
The question of material stability is of fundamental importance to any analysis of system properties in condensed matter physics and materials science. The ability to evaluate chemical stability, i.e., whether a stoichiometry will persist in some chemical environment, and structure selection, i.e. what crystal structure a stoichiometry will adopt, is critical to the prediction of materials synthesis, reactivity and properties. Here, we demonstrate that density functional theory, with the recently developed strongly constrained and appropriately normed (SCAN) functional, has advanced to a point where both facets of the stability problem can be reliably and efficiently predicted for main group compounds, while transition metal compounds are improved but remain a challenge. SCAN therefore offers a robust model for a significant portion of the periodic table, presenting an opportunity for the development of novel materials and the study of fine phase transformations even in largely unexplored systems with little to no experimental data.
In recent years, stem cell-based approaches have attracted more attention from scientists and clinicians due to their possible therapeutical effect on stroke. Animal studies have demonstrated that the beneficial effects of stem cells including embryonic stem cells (ESCs), inducible pluripotent stem cells (iPSCs), neural stem cells (NSCs), and mesenchymal stem cell (MSCs) might be due to cell replacement, neuroprotection, endogenous neurogenesis, angiogenesis, and modulation on inflammation and immune response. Although several clinical studies have shown the high efficiency and safety of stem cell in stroke management, mainly MSCs, some issues regarding to cell homing, survival, tracking, safety, and optimal cell transplantation protocol, such as cell dose and time window, should be addressed. Undoubtably, stem cell-based gene therapy represents a novel potential therapeutic strategy for stroke in future.
Originating from a broken spatial inversion symmetry, ferroelectricity is a functionality of materials with an electric dipole that can be switched by external electric fields. Spontaneous polarization is a crucial ferroelectric property, and its amplitude is determined by the strength of polar structural distortions. Density functional theory (DFT) is one of the most widely used theoretical methods to study ferroelectric properties, yet it is limited by the levels of approximations in electron exchangecorrelation. On the one hand, the local density approximation (LDA) is considered to be more accurate for the conventional perovskite ferroelectrics such as BaTiO 3 and PbTiO 3 than the generalized gradient approximation (GGA), which suffers from the so-called super-tetragonality error. On the other hand, GGA is more suitable for hydrogen-bonded ferroelectrics than LDA, which largely overestimates the strength of hydrogen bonding in general. We show here that the recently developed general-purpose strongly constrained and appropriately normed (SCAN) meta-GGA functional significantly improves over the traditional LDA/GGA for structural, electric, and energetic properties of diversely-bonded ferroelectric materials with a comparable computational effort, and thus enhances largely the predictive power of DFT in studies of ferroelectric materials. We also address the observed system-dependent performances of LDA and GGA for ferroelectrics from a chemical bonding point of view.
The carrier transport and optical properties of the hybrid organic-inorganic perovskite CH3NH3PbI3 are investigated using first-principles approaches. We found that the electron and hole mobilities could reach surprisingly high values of 7-30 × 10(3) and 1.5-5.5 × 10(3) cm(2) V(-1) s(-1), respectively, and both are estimated to be much higher than the current experimental measurements. The high carrier mobility is ascribed to the intrinsically small effective masses of anti-bonding band-edge states. The above results imply that there is still space to improve the performance of related solar cells. This material also has a sharp photon absorption edge and an absorption coefficient as high as 10(5) cm(-1), both of which contribute to effective utilization of solar radiation. Although band-edge states are mainly derived from the inorganic ions of Pb and I, thermal movement of the organic base has indirect influences on the bandgap and carrier effective masses, resulting in the temperature-dependent solar cell efficiencies.
High entropy alloys (HEAs) usually possess weak liquidity and castability, and considerable compositional inhomogeneity, mainly because they contain multiple elements with high concentrations. As a result, large-scale production of HEAs by casting is limited. To address the issue, the concept of eutectic high entropy alloys (EHEAs) was proposed, which has led to some promise in achieving good quality industrial scale HEAs ingots, and more importantly also good mechanical properties. In the practical large-scale casting, the actual composition of designed EHEAs could potentially deviate from the eutectic composition. The influence of such deviation on mechanical properties of EHEAs is important for industrial production, which constitutes the topic of the current work. Here we prepared industrial-scale HEAs ingots near the eutectic composition: hypoeutectic alloy, eutectic alloy and hypereutectic alloy. Our results showed that the deviation from eutectic composition does not significantly affect the mechanical properties, castability and the good mechanical properties of EHEAs can be achieved in a
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