276 research outputs found
Atomistic models of hydrogenated amorphous silicon nitride from first principles
We present a theoretical study of hydrogenated amorphous silicon nitride (a-SiNx:H), with equal concentrations of Si and N atoms (x=1), for two considerably different densities (2.0 and 3.0 g/cm3). Densities and hydrogen concentration were chosen according to experimental data. Using first-principles molecular-dynamics within density-functional theory the models were generated by cooling from the liquid. Where both models have a short-range order resembling that of crystalline Si3N4 because of their different densities and hydrogen concentrations they show marked differences at longer length scales. The low-density nitride forms a percolating network of voids with the internal surfaces passivated by hydrogen. Although some voids are still present for the high-density nitride, this material has a much denser and uniform space filling. The structure factors reveal some tendency for the nonstoichiometric high-density nitride to phase separate into nitrogen rich and poor areas. For our slowest cooling rate (0.023 K/fs) we obtain models with a modest number of defect states, where the low (high) density nitride favors undercoordinated (overcoordinated) defects. Analysis of the structural defects and electronic density of states shows that there is no direct one-to-one correspondence between the structural defects and states in the gap. There are several structural defects that do not contribute to in-gap states and there are in-gap states that do only have little to no contributions from (atoms in) structural defects. Finally an estimation of the size and cooling rate effects on the amorphous network is reported.
Ab initio study on the effects of transition metal doping of Mg2NiH4
Mg2NiH4 is a promising hydrogen storage material with fast (de)hydrogenation
kinetics. Its hydrogen desorption enthalpy, however, is too large for practical
applications. In this paper we study the effects of transition metal doping by
first-principles density functional theory calculations. We show that the
hydrogen desorption enthalpy can be reduced by ~0.1 eV/H2 if one in eight Ni
atoms is replaced by Cu or Fe. Replacing Ni by Co atoms, however, increases the
hydrogen desorption enthalpy. We study the thermodynamic stability of the
dopants in the hydrogenated and dehydrogenated phases. Doping with Co or Cu
leads to marginally stable compounds, whereas doping with Fe leads to an
unstable compound. The optical response of Mg2NiH4 is also substantially
affected by doping. The optical gap in Mg2NiH4 is ~1.7 eV. Doping with Co, Fe
or Cu leads to impurity bands that reduce the optical gap by up to 0.5 eV.Comment: 8 pages, 4 figure
Interactions of adsorbed CO on water ice at low temperatures
We present a computational study into the adsorption properties of CO on
amorphous and crystalline water surfaces under astrophysically relevant
conditions. Water and carbon dioxide are two of the most dominant species in
the icy mantles of interstellar dust grains and a thorough understanding of
their solid phase interactions at low temperatures is crucial for understanding
the structural evolution of the ices due to thermal segregation. In this paper,
a new HO-CO interaction potential is proposed and used to model the
ballistic deposition of CO layers on water ice surfaces, and to study the
individual binding sites at low coverages. Contrary to recent experimental
results, we do not observe CO island formation on any type of water
substrate. Additionally, density functional theory calculations are performed
to assess the importance of induced electrostatic interactions.Comment: Accepted for publication in Physical Chemistry Chemical Physic
Geometric, electronic and magnetic structure of FeO clusters
Correlation between geometry, electronic structure and magnetism of solids is
both intriguing and elusive. This is particularly strongly manifested in small
clusters, where a vast number of unusual structures appear. Here, we employ
density functional theory in combination with a genetic search algorithm,
GGA and a hybrid functional to determine the structure of gas phase
FeO clusters. For FeO cation clusters we also
calculate the corresponding vibration spectra and compare them with
experiments. We successfully identify FeO, FeO,
FeO, FeO and propose structures for
FeO. Within the triangular geometric structure of
FeO a non-collinear, ferrimagnetic and ferromagnetic state are
comparable in energy. FeO and FeO are
ferrimagnetic with a residual magnetic moment of 1~\muB{} due to ionization.
FeO is ferrimagnetic due to the odd number of Fe atoms. We
compare the electronic structure with bulk magnetite and find
FeO, FeO, FeO to be mixed
valence clusters. In contrast, in FeO and FeO
all Fe are found to be trivalent.Comment: 14 pages, 21 figure
Low work function of the (1000) Ca2N surface
Polymer diodes require cathodes that do not corrode the polymer but do have
low work function to minimize the electron injection barrier. First-principles
calculations demonstrate that the work function of the (1000) surface of the
compound Ca2N is half an eV lower than that of the elemental metal Ca (2.35 vs.
2.87 eV). Moreover its reactivity is expected to be smaller. This makes Ca2N an
interesting candidate to replace calcium as cathode material for polymer light
emitting diode devices.Comment: 3 pages, 4 figures, accepted by J. Appl. Phy
Interrelation of work function and surface stability: the case of BaAl4
The relationship between the work function (Phi) and the surface stability of
compounds is, to our knowledge, unknown, but very important for applications
such as organic light-emitting diodes. This relation is studied using
first-principles calculations on various surfaces of BaAl4. The most stable
surface [Ba terminated (001)] has the lowest Phi (1.95 eV), which is lower than
that of any elemental metal including Ba. Adding barium to this surface neither
increases its stability nor lowers its work function. BaAl4 is also strongly
bound. These results run counter to the common perception that stability and a
low Phi are incompatible. Furthermore, a large anisotropy and a stable
low-work-function surface are predicted for intermetallic compounds with polar
surfaces.Comment: 4 pages, 5 figures, to be published in Chem. Ma
NMR shieldings from density functional perturbation theory: GIPAW versus all-electron calculations
We present a benchmark of the density functional linear response calculation
of NMR shieldings within the Gauge-Including Projector-Augmented-Wave method
against all-electron Augmented-Plane-Wavelocal-orbital and uncontracted
Gaussian basis set results for NMR shieldings in molecular and solid state
systems. In general, excellent agreement between the aforementioned methods is
obtained. Scalar relativistic effects are shown to be quite large for nuclei in
molecules in the deshielded limit. The small component makes up a substantial
part of the relativistic corrections.Comment: 3 figures, supplementary material include
Thermodynamic stability of Fe/O solid solution at inner-core conditions
We present a new technique which allows the fully {\em ab initio} calculation
of the chemical potential of a substitutional impurity in a high-temperature
crystal, including harmonic and anharmonic lattice vibrations. The technique
uses the combination of thermodynamic integration and reference models
developed recently for the {\em ab initio} calculation of the free energy of
liquids and anharmonic solids. We apply the technique to the case of the
substitutional oxygen impurity in h.c.p. iron under Earth's core conditions,
which earlier static {\em ab initio} calculations indicated to be
thermodynamically very unstable. Our results show that entropic effects arising
from the large vibrational amplitude of the oxygen impurity give a major
reduction of the oxygen chemical potential, so that oxygen dissolved in h.c.p.
iron may be stabilised at concentrations up a few mol % under core conditions
Can the Earth's dynamo run on heat alone?
The power required to drive the geodynamo places significant constraints on the heat passing across the core-mantle boundary and the Earth's thermal history. Calculations to date have been limited by inaccuracies in the properties of liquid iron mixtures at core pressures and temperatures. Here we re-examine the problem of core energetics in the light of new first-principles calculations for the properties of liquid iron.
There is disagreement on the fate of gravitational energy released by contraction on cooling. We show that only a small fraction of this energy, that associated with heating resulting from changes in pressure, is available to drive convection and the dynamo. This leaves two very simple equations in the cooling rate and radioactive heating, one yielding the heat flux out of the core and the other the entropy gain of electrical and thermal dissipation, the two main dissipative processes.
This paper is restricted to thermal convection in a pure iron core; compositional convection in a liquid iron mixture is considered in a companion paper. We show that heat sources alone are unlikely to be adequate to power the geodynamo because they require a rapid secular cooling rate, which implies a very young inner core, or a combination of cooling and substantial radioactive heating, which requires a very large heat flux across the core-mantle boundary. A simple calculation with no inner core shows even higher heat fluxes are required in the absence of latent heat before the inner core formed
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