Born–Oppenheimer Interatomic Forces from Simple, Local Kinetic Energy Density Functionals

Rapid calculation of Born–Oppenheimer (B–O) forces is essential for driving the so-called quantum region of a multi-scale molecular dynamics simulation. The success of density functional theory (DFT) with modern exchange-correlation approximations makes DFT an appealing choice for this role. But conventional Kohn–Sham DFT, even with various linear-scaling implementations, really is not fast enough to meet the challenge of complicated chemo-mechanical phenomena (e.g. stress-induced cracking in the presence of a solvent). Moreover, those schemes involve approximations that are difficult to check practically or to validate formally. A popular alternative, Car-Parrinello dynamics, does not guarantee motion on the B–O surface. Another approach, orbital-free DFT, is appealing but has proven difficult to implement because of the challenge of constructing reliable orbital-free (OF) approximations to the kinetic energy (KE) functional. To be maximally useful for multi-scale simulations, an OF-KE functional must be local (i.e. one-point). This requirement eliminates the two-point functionals designed to have proper linear-response behavior in the weakly inhomogeneous limit. In the face of these difficulties, we demonstrate that there is a way forward. By requiring only that the approximate functional deliver high-quality forces, by exploiting the “conjointness” hypothesis of Lee, Lee, and Parr, by enforcing a basic positivity constraint, and by parameterizing to a carefully selected, small set of molecules we are able to generate a KE functional that does a good job of describing various H _q Si _m O _n clusters as well as CO (providing encouraging evidence of transferability). In addition to that positive result, we discuss several major negative results. First is definitive proof that the conjointness hypothesis is not correct, but nevertheless is useful. The second is the failure of a considerable variety of published KE functionals of the generalized gradient approximation type. Those functionals yield no minimum on the energy surface and give completely incorrect forces. In all cases, the problem can be traced to incorrect behavior of the functionals near the nuclei. Third, the seemingly obvious strategy of direct numerical fitting of OF-KE functional parameters to reproduce the energy surface of selected molecules is unsuccessful. The functionals that result are completely untransferable.     

Local Lattice Dynamics in the Mg0.5Al0.5B2 Superconductor

We have studied temperature evolution of the local as well as the average crystal structure of MgB₂ and Mg_0.5Al_0.5B₂ using real-space atomic pair distribution function (PDF) measured by high resolution neutron powder diffraction in a wide temperature range of T=10–600 K. The mean square relative displacements (MSRD) of atomic B–B, B–Mg (B–Al) pairs are compared with mean-square displacements (MSD) to calculate atomic correlations. In spite of the enhanced atomic disorder in Mg_0.5Al_0.5B₂, where the boron–boron, and boron–magnesium pair motions are found to be small, we find that the same atomic correlations in MgB₂ assume even slightly lower values and remain nearly constant in a wide temperature range of 0–600 K. This anomalous behavior and its physical interpretation provoke new questions on our understanding to the local lattice dynamics in this material.     

Wear resistance of electrodeposited Ni–B and Ni–B–Si<Subscript>3</Subscript>N<Subscript>4</Subscript> composite coatings

The wear resistance of electrodeposited (ED) Ni–B and Ni–B–Si₃N₄ composite coatings is compared. The effect of incorporation of Si₃N₄ particles in the ED Ni–B matrix on the surface morphology, structural characteristics and microhardness has been evaluated to correlate the wear resistance. The wear mechanism of ED Ni–B and Ni–B–Si₃N₄ composite coatings appears to be similar; both involve intensive plastic deformation of the coating due to the ploughing action of the hard counter disc. However, the extent of wear damage is relatively small for ED Ni–B–Si₃N₄ composite coatings.     

Amorphous Li–B–W–O solid-state electrolyte thin film for a solid-state ionic power sources

Li–B–W–O thin film serving as a solid-state electrolyte layer for a solid-state thin film battery has been deposited on a stainless steel (SUS)/Si substrate by thermal evaporation deposition at room temperature. By energy dispersive X-ray spectroscopy and inductively coupled plasma-atomic emission spectrometer measurements, the as-grown thin film showed a stoichiometry of Li_2.99BW_1.8O₉. The as-grown Li–B–W–O solid-state electrolyte thin film possessed an amorphous structure as confirmed by X-ray diffraction. Field emission scanning electron microscopy measurements of the film cross section showed a dense structure that did not have any large defects such as cracks or voids. For a cell structure of SUS/Li–B–W–O/SUS/Si, an impedance measurement conducted at room temperature revealed an ionic conductivity of 2.15 × 10⁻⁷ S cm⁻¹ with activation energy of 0.52 eV, which suggests that Li–B–W–O thin film can possibly be used as an electrolyte in solid-state thin film batteries.     

Structural and magnetic properties of nanocrystalline Mg–Cd ferrites prepared by oxalate co-precipitation method

Polycrystalline spinel ferrites with general formula Mg_1−x Cd _x Fe₂O₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by oxalate co-precipitation method using high purity sulfates. The samples were sintered at 1,050 °C for 5 h. The structural properties of these samples were investigated by XRD, SEM and FTIR techniques. The X-ray diffraction analysis confirms the formation of single phase cubic spinel structure of all the samples. The lattice constant, X-ray density, physical density, porosity, crystallite size, site radii (r _A, r _B), bond length (A–O, B–O) on tetrahedral (A-site) and octahedral (B-site) were calculated for the samples. The lattice constant increases with increase in Cd²⁺ content. The X-ray density increases with increase in Cd²⁺ content. The crystallite size calculated by Scherrer formula is in the range of 27.79–30.40 nm. Physical densities are calculated by Archimedes principle. The SEM study shows that the grain size increases with increasing Cd²⁺ content. The FTIR spectra shows two strong absorption bands around 576 and 431 cm⁻¹ on the tetrahedral and octahedral sites, respectively. The dependence of saturation magnetization on Cd²⁺ content suggests that A–B and B–B super exchange interaction are comparable in strength. Neel’s two sub lattice model is applicable up to x ≤ 0.4, while Y–K three sub lattice models (canted spin) is predominant for x ≥ 0.4.     

Magnetic Studies of Spin Wave in Fe/Ag Multilayer Films

Magnetic properties of Fe/Ag multilayer films are investigated and examined versus Fe layer thickness t _Fe. As a result, spontaneous magnetization M(T) temperature dependence has been revealed to be well described by a T ^3/2 law in all multilayer films (7 ?≤t _Fe≤60 ?). Spin-wave theory based on anisotropic ferromagnetic system has been also used to explain magnetization temperature dependence. For Fe layer thickness, approximate values for J ₀ bulk exchange interaction and J _s surface exchange interaction have been estimated. First principle calculations based on density functional theory (DFT) and Korringa–Kohn–Rostoker (KKR)—coherent potential approximation (CPA) method—combined with Local Spin Density Approximation (LSDA), are performed as well. Magnetic moment, in fcc Ag_1−x Fe _x and bcc Fe_1−x Ag _x systems, versus x is presented and discussed in terms of Fe content on magnetic coupling. Reasonable agreement between experimental data and theoretical calculations is highlighted.     

Processing, crystallization and characterization of polymer derived nano-crystalline Si–B–C–N ceramics

Si–B–C–N ceramics were synthesized from boron modified poly(vinyl)silazanes with the chemical formula (B[C₂H₄–Si(CH₃)NH]₃) _n . The originally amorphous materials are crystallized at temperatures in the order of 1,800–1,900 °C, which results microstructures with grain sizes significantly below 100 nm. Several parameters of the heat treatment, including temperature, holding time and atmosphere, affect the resulting nanostructures. This and the chemical and phase composition were studied via X-ray diffraction (XRD), transmission electron microscopy (TEM), electron spectroscopic imaging (ESI) and spectrochemical analysis in order to gain an understanding of the mechanisms, which control the crystallization behavior. Ceramic samples were also produced using different particle sizes of the precursor polymer in order to quantify the effect of the varying specific surface on the crystallization behavior.     

An adaptive finite element method for second‐order plate theory

We present a discontinuous finite element method for the Kirchhoff plate model with membrane stresses. The method is based on P₂‐approximations on simplices for the out‐of‐plane deformations, using C⁰‐continuous approximations. We derive a posteriori error estimates for linear functionals of the error and give some numerical examples. Copyright © 2009 John Wiley & Sons, Ltd.     

MQSPR modeling in materials informatics: a way to shorten design cycles?

We demonstrate applications of quantitative structure–property relationship (QSPR) modeling to supplement first-principles computations in materials design. We have here focused on the design of polymers with specific electronic properties. We first show that common materials properties such as the glass transition temperature (T _g) can be effectively modeled by QSPR to generate highly predictive models that relate polymer repeat unit structure to T _g. Next, QSPR modeling is shown to supplement and guide first-principles density functional theory (DFT) computations in the design of polymers with specific dielectric properties, thereby leveraging the power of first-principles computations by providing high-throughput capability. Our approach consists of multiple rounds of validated MQSPR modeling and DFT computations to optimize the polymer skeleton as well as functional group substitutions thereof. Rigorous model validation protocols insure that the statistical models are able to make valid predictions on molecules outside the training set. Future work with inverse QSPRs has the potential to further reduce the time to optimize materials properties.     

Magnetic and Superconducting Properties of Doped and Undoped Double Perovskite Sr<Subscript>2</Subscript>YRuO<Subscript>6</Subscript>

We report here SQUID (magnetization) measurements, along with supporting specific heat, Raman, SEM (scanning electron microscope), EDX (energy dispersive X-ray) and XRD (X-ray diffraction) measurements, on Cu-doped and undoped double perovskite Sr₂²⁺Y³⁺Ru⁵⁺O₆^2-\mathrm{Sr}_{2}^{2+}\mathrm{Y}^{3+}\mathrm{Ru}^{5+}\mathrm{O}_{6}^{2-} (abbreviated as SrY2116) system grown as single crystal using high-temperature solution growth technique. These measurements show the undoped system to be a nonmetallic (insulating) spin glass (SG) and the ∼5–30% Cu-doped (i.e. Cu-concentration/(Cu + Ru-concentration) ∼5–30%) system to be a spin glass superconductor (SGSC) with T _c (critical temperature) ∼28–31 K and superconducting volume fraction, f _sc∼2.2–9%. To mention, similar measurements done on undoped and Cu-doped BaY2116 and BaPr2116 systems show for them the same (SG, SGSC) behaviors. However they show a decrease in T _c and f _sc when diamagnetic Y³⁺ ions are replaced by Pr³⁺ spins, presumably due to enhanced internal pair breaking, and also decreased Cu–O–Cu overlap, owing to Pr³⁺ presence; these phenomena are known to exist in the Pr123 compound, PrBa₂Cu₃O_7−δ (δ∼0), due to ∼10% of Pr³⁺ ions having tendency to occupy Ba²⁺ sites. Measurements done on undoped and Cu-doped SrHo2116 show similar SG and SGSC properties. Further, the undoped and Cu-doped SrY2116 crystals grown by hydrothermal growth technique (i.e., grown using lower temperature and high pressure) show same behaviors. From these investigations it can be said that the undoped Ru-double perovskites (A₂BB′O₆, B′=Ru) are SG systems and that Cu-doped Ru-double perovskites (A₂BB′O _δ , δ∼6, B′=Ru_1−x Cu _x , 0<x≲0.3) are SG superconductors (SGSCs). Results are discussed.