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Formation Energies (formation + energy)
Selected AbstractsAtomistic simulation of the self-diffusion in Mg (001) surfaceCRYSTAL RESEARCH AND TECHNOLOGY, Issue 9 2008Jian-Min Zhang Abstract Both the formation energies and the intra- and inter-layer diffuse activation energies of a vacancy in the first six lattice planes of Mg (001) surface have been calculated by combining the modified analytical embedded-atom method (MAEAM) with molecular dynamics (MD). The results show that the effect of the surface on the formation and migration of the vacancy is only down to the third-layer. It is easer for a single vacancy to form and to migrate in the first layer. Furthermore, the vacancy in the second layer is favorable to migrate to the first layer. This is in agreement with the experimental results that the first layer has the highest concentration of the vacancy. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Sc3AlN , A New PerovskiteEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2008Carina Höglund Abstract Sc3AlN with perovskite structure has been synthesized as the first ternary phase in the Sc,Al,N system. Magnetron sputter epitaxy at 650 °C was used to grow single-crystal, stoichiometric Sc3AlN(111) thin films onto MgO(111) substrates with ScN(111) seed layers as shown by elastic recoil detection analysis, X-ray diffraction, and transmission electron microscopy. The Sc3AlN phase has a lattice parameter of 4.40 Å, which is in good agreement with the theoretically predicted 4.42 Å. Comparisons of total formation energies show that Sc3AlN is thermodynamically stable with respect to all known binary compounds. Sc3AlN(111) films of 1.75 µm thickness exhibit a nanoindentation hardness of 14.2 GPa, an elastic modulus of 249 GPa, and a room-temperature electrical resistivity of 41.2 µ, cm. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Ab Initio Guided Design of bcc Ternary Mg,Li,X (X,=,Ca, Al, Si, Zn, Cu) Alloys for Ultra-Lightweight ApplicationsADVANCED ENGINEERING MATERIALS, Issue 7 2010William Art Counts Abstract Ab initio calculations are becoming increasingly important for designing new alloys as these calculations can accurately predict basic structural, mechanical, and functional properties using only the atomic composition as a basis. In this paper, fundamental physical properties (like formation energies and elastic constants) of a set of bcc Mg,Li and Mg,Li-based compounds are calculated using density functional theory (DFT). These DFT-determined properties are in turn used to calculate engineering parameters such as (i) specific Young's modulus (Y/,) or (ii) shear over bulk modulus ratio (G/B) differentiating between brittle and ductile behavior. These parameters are then used to identify those alloys that have optimal mechanical properties for lightweight structural applications. First, in case of the binary Mg,Li system, an Ashby map containing Y/, versus G/B shows that it is not possible to increase Y/, without simultaneously increasing G/B (i.e., brittleness) by changing only the composition of a binary alloy. In an attempt to bypass such a fundamental materials-design limitation, a set of Mg,Li,X ternaries (X,=,Ca, Al, Si, Cu, Zn) based on stoichiometric Mg,Li with CsCl structure was studied. It is shown that none of the studied ternary solutes is able to simultaneously improve both specific Young's modulus and ductility. [source] A theoretical study on the structures and energetics of hypothetical TiM(NCN)3 compounds of the 3d transition metalsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 11 2005Maxence Launay Abstract Quasi-ternary cyanamides and carbodiimides of general formula AB(NCN)3 with A , B have neither been predicted nor synthesized. Thus, hypothetical compounds of that kind containing 3d transition metals were considered (A = Ti, B = Mn, Fe, Co, Ni, Cu) by means of density-functional calculations on 34 structural models, most of which were derived from chemically related phases. After performing structure optimizations based on the local-density approximation, the relative energetic orderings are rationalized in terms of geometrical factors such as molar volumes and polyhedral connections. Total-energy generalized-gradient calculations evidence that the most stable models are enthalpically favored with respect to the elements. Even at ambient temperatures, the ternary phases are predicted as being thermodynamically stable in terms of their Gibbs free formation energies, especially if energetically competing and low-lying binaries (TiC, TiN) can be excluded by a kinetic reaction control. The best models are characterized by low-spin magnetic transition metals found in octahedral coordination, and the TiN6 and MN6 polyhedra either share faces or edges. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1180,1188, 2005 [source] Theoretical Defect Energetics in Calcium Phosphate BioceramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2010Katsuyuki Matsunaga Vacancies, impurities, and foreign ions dissolving in calcium phosphate bioceramics play an important role in the biological properties of the materials. However, little is known about the thermodynamic stability of the defects. In this regard, point defects in hydroxyapatite (HAp) and octacalcium phosphate (OCP) were calculated in a first-principles manner, and the chemical-potential dependence of the defect formation energies was revealed. In particular, because calcium phosphates are usually subjected to an aqueous solution, a methodology to evaluate ionic chemical potentials under chemical equilibrium of the solid,aqueous solution was introduced. In the present article, recent results based on such a methodology (the solution pH dependence of Ca/P molar ratio of HAp and the ion-exchange ability with foreign cations in HAp and OCP) were reviewed. [source] Subgap states, doping and defect formation energies in amorphous oxide semiconductor a-InGaZnO4 studied by density functional theoryPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2010Toshio Kamiya Abstract Amorphous In-Ga-Zn-O (a-IGZO) is expected for channel layers in thin-film transistors (TFTs). It is known that a-IGZO is sensitive to an O/H-containing atmosphere; therefore, it is important to clarify the roles of oxygen and hydrogen in a-IGZO. This paper provides atomic and electronic structures, formation energies of defects and bond energies in a-IGZO calculated by first-principles density functional theory (DFT). It was confirmed that oxygen deficiencies having small formation energies (2,3.6,eV) form either deep fully-occupied localized states near the valence band maximum or donor states, which depend on their local structures. All the hydrogen doping form OH bond and work as a donor. The stable OH bonds have small formation energy of ,0.45,eV and consist of three metal cations coordinated to the O ion. The bond energy of GaO is calculated to be ,2.0,eV, which is the largest among the chemical bonds in a-IGZO (1.7,eV for InO and 1.5,eV for ZnO). This result supports the idea that the incorporation of Ga stabilizes a-IGZO TFTs. [source] Interaction between helium and vacancy in plutonium by embedded atom methodPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2008Bingyun Ao Abstract The formation energies of small Hen Vm clusters (n and m denote the number of He atoms and vacancy, respectively) in Pu have been calculated with molecular dynamics (MD) simulations using the embedded atom method (EAM) potential, the Morse potential and the Lennard,Jones potential for describing the interactions of Pu,Pu, Pu,He and He,He, respectively. The binding energies of an interstitial He atom, an isolated vacancy and a self-interstitial Pu atom to a Hen Vm cluster are also obtained from the calculated formation energies of the clusters. All the binding energies mainly depend on the He-vacancy ratio (n /m) of clusters rather than the clusters size. With the increase of the n /m ratio, the binding energies of a He atom and a Pu atom to a Hen Vm cluster decrease with the ratio, and the binding energy of a vacancy to a Hen Vm cluster increases. He atoms act as a catalyst for the formation of Hen Vm clusters. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic structure and magnetic properties of Fe3C with 3d and 4d impuritiesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2007I. R. Shein Abstract We study the electronic structure, formation energies, and magnetic properties of cementite Fe3C doped by 3d and 4d transition metals (TM) by means of first-principles calculations. All TM elements demonstrate a preference for the substitution of the general iron position in Fe3C. We predict that early elements (Sc, Ti, V, Cr, Zr, and Nb) stabilize the cementite, while the end elements (Ni, Cu, Pd, and Ag) in the TM series sharply destabilize it. The magnetic properties of impurity atoms are found to depend strongly on their atomic numbers and the trends to ferromagnetic or antiferromagnetic coupling of impurities with Fe atoms in Fe3C coincide with the magnetic behavior of these impurities in Fe. The physical reason is a similar location of the Fermi level in the pseudogap of spin-down and almost filled spin-up Fed states in both Fe3C and Fe. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Structural stability of clean GaAs nanowires grown along the [111] directionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010Rita Magri Abstract Using a first-principles approach we have calculated the formation energies of small diameter GaAs nanowires (NWs) with both zinc-blende and wurtzite structure grown along the [111] direction. The section of the wires is hexagonal and the side facets are oriented either {11-20} and {10-10} in the case of the wurtzite structure, and {110} and {112} for the zinc-blende structure. The formation energy of the nanowires as a function of their radius is then interpreted in terms of a model in which the energy contributions from the bulks, the flat surfaces and the ridges are taken explicitly into account. We find that the nanowire stability is mainly explained by the competition between the bulk energy, favoring the zincblende structure and the surface energies favoring the wurtzite structure. We find also that the directly calculated formation energies of some small diameter wurtzite NWs can be reproduced by our model taking into account only the bulk and flat surface contributions. That is, the ridges do not contribute substantially to the nanowire formation energy. Inspection of the ridge structure and band structure reveals that this good agreement occurs when the NWs are semiconducting and the ridges do not add more dangling bonds to the surface with respect to those provided by the sidewalls. Within our model we find the critical diameter for the wurtzite-zinc-blende transition at 6.3 nm. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Structure and energy of partial dislocations in wurtzite-GaNPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 8 2007G. Savini Abstract First-principle calculations have shown that both the partials can be electrically active. In particular we have shown the Ga(g) core partials are a good candidate for the observed absorption peak at 2.4 eV revealed by energy loss spectroscopy measurements. The symmetric and asymmetric reconstructions have relatively close formation energies. Our results have suggested that the asymmetric reconstructions, characterized by strong bonds along the dislocation line are favourable in intrinsic materials. However, in strongly p and n-type materials or in high stress field the symmetric reconstructions can become energetically more stable. These reconstructions are always electrically active with a deep band across the forbidden gap. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] First-principles modelling of defects in advanced nuclear fuelsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2007E. A. Kotomin Abstract In this paper we present and discuss the results of first first-principle modelling of point defects in nitride nuclear fuels. Calculations have been performed using the VASP computer code combined with supercells containing up to 250 atoms. The effective atomic charges, the electronic density redistribution, atomic displacements around U and N vacancies and their formation energies are discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] DFT Study of Effects of Potassium Doping on Band Structure of Crystalline Cuprous AzideCHINESE JOURNAL OF CHEMISTRY, Issue 12 2008Wei-Hua ZHU Abstract The structure and defect formation energies of the K-doped CuN3 were studied using density functional theory within the generalized gradient approximation. The results show that the K-doping breaks the azide symmetry and causes asymmetric atomic displacement. As the K-doping level increases, the band gap of the doped system gradually increases. The K impurity is easily incorporated into the crystal thermodynamically. The Cu vacancy is easily created thermodynamically and the K impurity can serve as nucleation centers for vacancy clustering. Finally the effects of K-doping concentrations on the sensitivity of CuN3 were understood based on electronic structures. [source] First-Principles Calculations of Anion Vacancies in Oxides and NitridesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2002Isao Tanaka The formation energy, structural relaxation, and defect-induced states of neutral anion vacancies of five oxides (i.e., MgO, Al2O3, ZnO, In2O3, and SnO2) and four nitrides (i.e., AlN, Si3N4, Ge3N4, and InN) are systematically discussed, based on first-principles plane-wave pseudopotential calculations. Two types of polymorphs for each compound are compared. The number of atoms included in the supercells ranged from 54 to 96. When a localized vacancy-induced state appears within the band gap, as in a typical ionic crystal, the formation energy can be well scaled by the band gap of the perfect crystal. On the other hand, when an empty and localized vacancy-induced state is located above the highest occupied band or no localized state is formed, the formation energy has a tendency to be smaller. In compounds such as ZnO and SnO2, the formation energy is dependent largely on the crystal structure. This result can be explained by the transition of the vacancy-induced state from occupied to unoccupied, which is caused by the change in atomic arrangement, as represented by the cation coordination number. [source] Subgap states, doping and defect formation energies in amorphous oxide semiconductor a-InGaZnO4 studied by density functional theoryPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2010Toshio Kamiya Abstract Amorphous In-Ga-Zn-O (a-IGZO) is expected for channel layers in thin-film transistors (TFTs). It is known that a-IGZO is sensitive to an O/H-containing atmosphere; therefore, it is important to clarify the roles of oxygen and hydrogen in a-IGZO. This paper provides atomic and electronic structures, formation energies of defects and bond energies in a-IGZO calculated by first-principles density functional theory (DFT). It was confirmed that oxygen deficiencies having small formation energies (2,3.6,eV) form either deep fully-occupied localized states near the valence band maximum or donor states, which depend on their local structures. All the hydrogen doping form OH bond and work as a donor. The stable OH bonds have small formation energy of ,0.45,eV and consist of three metal cations coordinated to the O ion. The bond energy of GaO is calculated to be ,2.0,eV, which is the largest among the chemical bonds in a-IGZO (1.7,eV for InO and 1.5,eV for ZnO). This result supports the idea that the incorporation of Ga stabilizes a-IGZO TFTs. [source] Epitaxial growth of carbon caps on Ni for chiral selectivityPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 13 2006S. Reich Abstract We suggest guiding principles for chirality-selective growth of single-walled carbon nanotubes. The chirality of a tube is determined by the carbon cap that forms during nucleation. Controlling the tube chirality requires controlling the nucleation stage. Certain caps can be favored by their epitaxial relationship to a metal surface. Lattice matched caps require ,0.1 eV/C less formation energy on a Ni surface than non-lattice matched cap structures. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Self-assembled quantum dot formation induced by surface energy change of a strained two-dimensional layerPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2004Frank Tinjod Abstract To account for the occurrence (or not) of the Stranski-Krastanow (SK) transition (two-dimensional to 3D change of surface morphology) during the epitaxial growth of various lattice-mismatched semiconductor systems, we present a simple equilibrium model taking into account not only the lattice mismatch, but also the dislocation formation energy and the surface energy. It demonstrates the importance of these parameters especially for II,VI systems such as CdTe/ZnTe and CdSe/ZnSe. For II,VIs indeed, as misfit dislocations are easier to form than in III,Vs (such as InAs/GaAs) or IV systems (Ge/Si), the 3D elastic transition is short-circuited by the plastic one. Nevertheless, by lowering surface energy, telluride and selenide quantum dots can also be grown as predicted by our model and as evidenced experimentally by reflection high-energy electron diffraction (RHEED), atomic force microscopy and optical measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Structural stability of clean GaAs nanowires grown along the [111] directionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010Rita Magri Abstract Using a first-principles approach we have calculated the formation energies of small diameter GaAs nanowires (NWs) with both zinc-blende and wurtzite structure grown along the [111] direction. The section of the wires is hexagonal and the side facets are oriented either {11-20} and {10-10} in the case of the wurtzite structure, and {110} and {112} for the zinc-blende structure. The formation energy of the nanowires as a function of their radius is then interpreted in terms of a model in which the energy contributions from the bulks, the flat surfaces and the ridges are taken explicitly into account. We find that the nanowire stability is mainly explained by the competition between the bulk energy, favoring the zincblende structure and the surface energies favoring the wurtzite structure. We find also that the directly calculated formation energies of some small diameter wurtzite NWs can be reproduced by our model taking into account only the bulk and flat surface contributions. That is, the ridges do not contribute substantially to the nanowire formation energy. Inspection of the ridge structure and band structure reveals that this good agreement occurs when the NWs are semiconducting and the ridges do not add more dangling bonds to the surface with respect to those provided by the sidewalls. Within our model we find the critical diameter for the wurtzite-zinc-blende transition at 6.3 nm. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] First principles simulations of F centers in cubic SrTiO3PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2005J. Carrasco Abstract Atomic and electronic structure of regular and O-deficient SrTiO3 have been studied. Several types of first principles atomistic simulations: Hartree-Fock method, Density Functional Theory, and hybrid HF-DFT functionals, have been applied to periodic models that consider supercells of different sizes (ranging between 40 and 240 atoms). We confirm the ionic character of the Sr-O bonds and the high covalency of the Ti-O2 substructure. For the stoichiometric cubic crystal; the lattice constant and bulk modulus correctly reproduce the experimental data whereas the band gap is only properly obtained by the B3PW functional. The relaxed geometry around the F center shows a large expansion of the two nearest Ti ions. Moreover, the vacancy formation energy is extremely sensitive to the size and the shape of the supercell as well as the calculation method. The electronic density map indicates the redistribution of two electrons of the missing O atom between the vacancy and 3d atomic orbitals of the two nearest Ti ions, in contrast to the F centers in ionic oxides where the charge centroid does not change. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||