William Yi Wang

A mixed-space approach to first-principles calculations of phonon frequencies for polar materials

We propose a mixed-space approach using the accurate force constants calculated by the direct approach in real space and the dipole-dipole interactions calculated by linear response theory in reciprocal space, making the accurate prediction of phonon frequencies for polar materials possible using the direct approach as well as linear response theory. As examples, by using the present approach, we predict the first-principles phonon properties of the polar materials α-Al(2)O(3), MgO, c-SiC, and h-BN, which are in excellent agreement with available experimental data.

Temperature-dependent ideal strength and stacking fault energy of fcc Ni: a first-principles study of shear deformation

Variations of energy, stress, and magnetic moment of fcc Ni as a response to shear deformation and the associated ideal shear strength (τ(IS)), intrinsic (γ(SF)) and unstable (γ(US)) stacking fault energies have been studied in terms of first-principles calculations under both the alias and affine shear regimes within the {111} slip plane along the <112> and <110> directions. It is found that (i) the intrinsic stacking fault energy γ(SF) is nearly independent of the shear deformation regimes used, albeit a slightly smaller value is predicted by pure shear (with relaxation) compared to the one from simple shear (without relaxation); (ii) the minimum ideal shear strength τ(IS) is obtained by pure alias shear of {111}<112>; and (iii) the dissociation of the 1/2[110] dislocation into two partial Shockley dislocations (1/6[211] + 1/6[121]) is observed under pure alias shear of {111}<110>. Based on the quasiharmonic approach from first-principles phonon calculations, the predicted γ(SF) has been extended to finite temperatures. In particular, using a proposed quasistatic approach on the basis of the predicted volume versus temperature relation, the temperature dependence of τ(IS) is also obtained. Both the γ(SF) and the τ(IS) of fcc Ni decrease with increasing temperature. The computed ideal shear strengths as well as the intrinsic and unstable stacking fault energies are in favorable accord with experiments and other predictions in the literature.

Fabrication and Characterization of Beaded SiC Quantum Rings with Anomalous Red Spectral Shift

8 Shapeand size-control of nanocrystals (NCs) is valuable in many aspects of modern science and technology. [ 1–9 ] Wet chemical methods are widely used in the synthesis of metal, metal oxide, and metal sulfi de NCs with various shapes and dimensions in a controllable manner. [ 1–9 ] However, they are unsuitable for the synthesis of carbides and nitrides because of their high melting points (generally > 2000 ° C) as well as a lack of appropriate precursors. Carbide and nitride NCs are of signifi cant interest for biomedical applications and optical devices operated in extreme conditions, because they have high strength and unique optical properties while being chemically inert and biocompatible. [ 10–15 ] The properties of NCs are dependent on not only their constituent materials but also their geometries. Nanorings, for example, are important zero-dimensional nanostructures in many applications and they provide an excellent model to explore quantum-related properties. [ 16–19 ] Their fabrication, however, remains a signifi cant challenge using existing fabrication methodologies. [ 20–22 ]

Effects of Alloying Elements on Stacking Fault Energies and Electronic Structures of Binary Mg Alloys: A First-Principles Study

The growth, deformation, and extrinsic faults in binary Mg–X alloys are investigated via first-principles calculations. Here, the alloying elements X include Al, Ca, Cu, Fe, K, La, Li, Mn, Na, Nd, Pr, Si, Sn, Sr, Y, Zn, and Zr. In addition to stacking fault energies, the effect of the elements on the bond structure of Mg are studied in term of electron localization morphology. It is observed that rod-like directional bonds in non-fault planes transform into tetrahedral morphologies in fault planes and are strengthened by addition of Zn and Al, but weakened by Na.

Insight into structural, elastic, phonon, and thermodynamic properties of α-sulfur and energy-related sulfides: a comprehensive first-principles study

Earth-abundant and nontoxic sulfur (S) is emerging as a key element for developing new materials for sustainable energy. Knowledge gaps, however, still remain regarding the fundamental properties of sulfur especially on a theoretical level. Here, a comprehensive first-principles study has been performed to examine the predicted structural, elastic, phonon, thermodynamic, and optical properties of α-S8 (α-S) as well as energy-related sulfides. A variety of exchange–correlation (X–C) functionals and van der Waals corrections in terms of the D3 method have been tested to probe the capability of first-principles calculations. Comparison of predicted quantities with available experimental data indicates that (i) the structural information of α-S is described very well using an improved generalized gradient approximation of PBEsol; (ii) the band gap and dielectric tensor of α-S are calculated perfectly using a hybrid X–C functional of HSE06; (iii) the phonon and elastic properties of α-S are predicted reasonably well using for example the X–C functionals of LDA and PBEsol, and in particular the PBE + D3 and the PBEsol + D3 method; and (iv) the thermodynamic properties of α-S are computed accurately using the PBEsol + D3 method. Examinations using Li2S, CuS, ZnS, Cu2ZnSnS4 (CZTS), SnS, Sn2S3, SnS2, β-S8 (β-S), and γ-S8 (γ-S) validate further the crucial role of the van der Waals correction, thus suggesting the X–C functionals are PBEsol + D3 and PBE + D3 (and PBEsol in some cases) for sulfur as well as S-containing materials. We also examine the possibility by using the Debye model to predict thermodynamic properties of unusual materials, for example, α-S. In addition, the bonding characteristics and non-polar nature of α-S have been revealed quantitatively from phonon calculations and qualitatively from the differential charge density.

Atomic and electronic basis for the serrations of refractory high-entropy alloys

Refractory high-entropy alloys present attractive mechanical properties, i.e., high yield strength and fracture toughness, making them potential candidates for structural applications. Understandings of atomic and electronic interactions are important to reveal the origins for the formation of high-entropy alloys and their structure−dominated mechanical properties, thus enabling the development of a predictive approach for rapidly designing advanced materials. Here, we report the atomic and electronic basis for the valence−electron-concentration-categorized principles and the observed serration behavior in high-entropy alloys and high-entropy metallic glass, including MoNbTaW, MoNbVW, MoTaVW, HfNbTiZr, and Vitreloy-1 MG (Zr41Ti14Cu12.5Ni10Be22.5). We find that the yield strengths of high-entropy alloys and high-entropy metallic glass are a power-law function of the electron-work function, which is dominated by local atomic arrangements. Further, a reliance on the bonding-charge density provides a groundbreaking insight into the nature of loosely bonded spots in materials. The presence of strongly bonded clusters and weakly bonded glue atoms imply a serrated deformation of high-entropy alloys, resulting in intermittent avalanches of defects movement.High-entropy alloys: cluster-and-glue atoms behind exceptional propertiesA cluster-and-glue model of atomic arrangements explains the yield strength and mechanical response of high entropy alloys. Inspired by metallic glass, a team led by William Yi Wang at China’s Northwestern Polytechnical University and collaborators in the United States of America used molecular dynamics to build different atomic arrangements of refractory high entropy alloys consisting of four or more elements. Depending on atomic size and the periodic table group of each atom, some atoms organized into clusters while others glued the clusters together. Chemical bonds broke and formed with plastic deformation as the alloys went from one atomic arrangement to another via the glue atoms, causing defect avalanches explaining the serrated mechanical response of high entropy alloys. Taking into account atomic arrangement may thus help us predict the properties of high entropy alloys.

Nano-sized Superlattice Clusters Created by Oxygen Ordering in Mechanically Alloyed Fe Alloys

Creating and maintaining precipitates coherent with the host matrix, under service conditions is one of the most effective approaches for successful development of alloys for high temperature applications; prominent examples include Ni- and Co-based superalloys and Al alloys. While ferritic alloys are among the most important structural engineering alloys in our society, no reliable coherent precipitates stable at high temperatures have been found for these alloys. Here we report discovery of a new, nano-sized superlattice (NSS) phase in ball-milled Fe alloys, which maintains coherency with the BCC matrix up to at least 913 °C. Different from other precipitates in ferritic alloys, this NSS phase is created by oxygen-ordering in the BCC Fe matrix. It is proposed that this phase has a chemistry of Fe3O and a D03 crystal structure and becomes more stable with the addition of Zr. These nano-sized coherent precipitates effectively double the strength of the BCC matrix above that provided by grain size reduction alone. This discovery provides a new opportunity for developing high-strength ferritic alloys for high temperature applications.

Low energy structures of lithium-ion battery materials Li(MnxNixCo1−2x)O2 revealed by first-principles calculations

A long-standing issue regarding the low energy structures for the partially disordered cathode materials Li(MnxNixCo1−2x)O2 has been probed by first-principles calculations. It is found that the transitional metals Mn, Ni, and Co in Li(MnxNixCo1−2x)O2 follow the maximum entropy probability distribution (MEPD), instead of the random distribution, according to the distributions of the minimal partial radial distribution functions and the correlation functions. Here, the MEPD is proposed to understand the low energy structures of the partially disordered lithium-ion battery materials.

Bonding charge density from atomic perturbations

Charge transfer among individual atoms is the key concept in modern electronic theory of chemical bonding. In this work, we present a first‐principles approach to calculating the charge transfer. Based on the effects of perturbations of an individual atom or a group of atoms on the electron charge density, we determine unambiguously the amount of electron charge associated with a particular atom or a group of atoms. We computed the topological electron loss versus gain using ethylene, graphene, MgO, and SrTiO3 as examples. Our results verify the nature of chemical bonds in these materials at the atomic level.

Strengthening Mg by self-dispersed nano-lamellar faults

ABSTRACT Here, we show the strategies to strengthen Mg alloys through modifying the matrix by planar faults and optimizing the local lattice strain by solute atoms. The anomalous shifts of the local phonon density of state of stacking faults (SFs) and long periodic stacking-ordered structures (LPSOs) toward the high-frequency mode are revealed by HCP-FCC transformation, resulting in the increase of vibrational entropy and the decrease of free energy to stabilize the SFs and LPSOs. Through integrating bonding charge density and electronic density of states, electronic redistributions are applied to reveal the electronic basis for the ‘strengthening’ of Mg alloys. GRAPHICAL ABSTRACT IMPACT STATEMENT Through integrating the bonding charge density, the phonon and electronic density of states, this work provides an atomic and electronic insight into the strengthening mechanism of Mg alloys.