2,468 research outputs found
Automating first-principles phase diagram calculations
Devising a computational tool that assesses the thermodynamic stability of materials is among the most important steps required to build a “virtual laboratory,” where materials could be designed from first principles without relying on experimental input. Although the formalism that allows the calculation of solid-state phase diagrams from first principles is well established, its practical implementation remains a tedious process. The development of a fully automated algorithm to perform such calculations serves two purposes. First, it will make this powerful tool available to a large number of researchers. Second, it frees the calculation process from arbitrary parameters, guaranteeing that the results obtained are truly derived from the underlying first-principles calculations. The proposed algorithm formalizes the most difficult step of phase diagram calculations, namely the determination of the “cluster expanison,” which is a compact representation of the configurational dependence of the alloy’s energy. This is traditionally achieved by a fit of the unknown interaction parameters of the cluster expansion to a set of structural energies calculated from first principles. We present a formal statistical basis for the selection of both the interaction parameters to include in the cluster expansion and the structures to use to determine them. The proposed method relies on the concepts of cross-validation and variance minimization. An application to the calculation of the phase diagram of the Si-Ge, CaO-MgO, Ti-Al, and Cu-Au systems is presented
First-principles computation of the vibrational entropy of ordered and disordered Pd3V
Experimental as well as theoretical work indicates that the relative stability of the ordered and the disordered states of a compound may be significantly affected by their difference in vibrational entropy. The origin of this difference is usually attributed to the fact that disordering reduces the number of stiff bonds between different atomic species in favor of soft bonds between identical atomic species. The results of previous theoretical investigations, however, suggest that this simple mechanism is significantly modified as a result of local atomic relaxations. To gain further insight regarding the importance of relaxations, we employ first-principles calculations to investigate the magnitude of the vibrational entropy difference between the ordered and the disordered state of Pd3V. Our investigation reveals that bond stiffness changes due to relaxation entirely mask the large configurational dependence of vibrational entropy provided by bond stiffness differences. Our analysis also suggests a simple technique to estimate vibrational entropy based on the relationship between bond length and bond stiffness
Role of defects and impurities in doping of GaN
We have calculated formation energies and position of the defect levels for
all native defects and for a variety of donor and acceptor impurities employing
first-principles total-energy calculations. An analysis of the numerical
results gives direct insight into defect concentrations and impurity solubility
with respect to growth parameters (temperature, chemical potentials) and into
the mechanisms limiting the doping levels in GaN. We show how compensation and
passivation by native defects or impurities, solubility issues, and
incorporation of dopants on other sites influence the acceptor doping levels.Comment: 8 pages, 3 figures, to appear in "The Physics of Semiconductors
Conditions for resistivity from electron-electron scattering
Many complex oxides (including titanates, nickelates and cuprates) show a
regime in which resistivity follows a power law in temperature (). By analogy to a similar phenomenon observed in some metals at low
temperature, this has often been attributed to electron-electron (Baber)
scattering. We show that Baber scattering results in a power law only
under several crucial assumptions which may not hold for complex oxides. We
illustrate this with sodium metal () and
strontium titanate (). We conclude that an
observation of is not sufficient evidence for
electron-electron scattering.Comment: 5 pages, 4 figure
Theory of Doping and Defects in III-V Nitrides
Doping problems in GaN and in AlGaN alloys are addressed on the basis of
state-of-the-art first-principles calculations. For n-type doping we find that
nitrogen vacancies are too high in energy to be incorporated during growth, but
silicon and oxygen readily form donors. The properties of oxygen, including
DX-center formation, support it as the main cause of unintentional n-type
conductivity. For p-type doping we find that the solubility of Mg is the main
factor limiting the hole concentration in GaN. We discuss the beneficial
effects of hydrogen during acceptor doping. Compensation of acceptors by
nitrogen vacancies may occur, becoming increasingly severe as x increases in
Al_x Ga_(1-x)N alloys.Comment: 7 pages, 2 figures. Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
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