7,118 research outputs found
Thermodynamically stable lithium silicides and germanides from density-functional theory calculations
Density-functional-theory (DFT) calculations have been performed on the Li-Si
and Li-Ge systems. Lithiated Si and Ge, including their metastable phases, play
an important technological r\^ole as Li-ion battery (LIB) anodes. The
calculations comprise structural optimisations on crystal structures obtained
by swapping atomic species to Li-Si and Li-Ge from the X-Y structures in the
International Crystal Structure Database, where X={Li,Na,K,Rb,Cs} and
Y={Si,Ge,Sn,Pb}. To complement this at various Li-Si and Li-Ge stoichiometries,
ab initio random structure searching (AIRSS) was also performed. Between the
ground-state stoichiometries, including the recently found LiSi
phase, the average voltages were calculated, indicating that germanium may be a
safer alternative to silicon anodes in LIB, due to its higher lithium insertion
voltage. Calculations predict high-density LiSi and LiGe
layered phases which become the ground state above 2.5 and 5 GPa
respectively and reveal silicon and germanium's propensity to form dumbbells in
the LiSi, stoichiometry range. DFT predicts the stability of
the LiGe , LiGe and LiGe
phases and several new Li-Ge compounds, with stoichiometries LiGe,
LiGe, LiGe and LiGe.Comment: 10 pages, 5 figure
Ultracold atoms at unitarity within quantum Monte Carlo
Variational and diffusion quantum Monte Carlo (VMC and DMC) calculations of
the properties of the zero-temperature fermionic gas at unitarity are reported.
The ratio of the energy of the interacting to the non-interacting gas for a
system of 128 particles is calculated to be 0.4517(3) in VMC and 0.4339(1) in
the more accurate DMC method. The spherically-averaged pair-correlation
functions, momentum densities, and one-body density matrices are very similar
in VMC and DMC, but the two-body density matrices and condensate fractions show
some differences. Our best estimate of the condensate fraction of 0.51 is a
little smaller than values from other quantum Monte Carlo calculations
Hydrogen/nitrogen/oxygen defect complexes in silicon from computational searches
Point defect complexes in crystalline silicon composed of hydrogen, nitrogen,
and oxygen atoms are studied within density-functional theory (DFT). Ab initio
Random Structure Searching (AIRSS) is used to find low-energy defect
structures. We find new lowest-energy structures for several defects: the
triple-oxygen defect, {3O}, triple oxygen with a nitrogen atom, {N, 3O}, triple
nitrogen with an oxygen atom, {3N,O}, double hydrogen and an oxygen atom,
{2H,O}, double hydrogen and oxygen atoms, {2H,2O} and four
hydrogen/nitrogen/oxygen complexes, {H,N,O}, {2H,N,O}, {H,2N,O} and {H,N,2O}.
We find that some defects form analogous structures when an oxygen atom is
replaced by a NH group, for example, {H,N,2O} and {3O}, and {H,N} and {O}. We
compare defect formation energies obtained using different oxygen chemical
potentials and investigate the relative abundances of the defects.Comment: 9 pages, 13 figure
When a Bailout is a Taking: Can Takings Solve the Problem of the Government as Controlling Shareholder?
When a Bailout is a Taking: Can Takings Solve the Problem of the Government as Controlling Shareholder?
Energetics of hydrogen/lithium complexes in silicon analyzed using the Maxwell construction
We have studied hydrogen/lithium complexes in crystalline silicon using
density-functional-theory methods and the ab initio random structure searching
(AIRSS) method for predicting structures. A method based on the Maxwell
construction and convex hull diagrams is introduced which gives a graphical
representation of the relative stabilities of point defects in a crystal and
enables visualization of the changes in stability when the chemical potentials
are altered. We have used this approach to study lithium and hydrogen
impurities in silicon, which models aspects of the anode material in the
recently-suggested lithium-ion batteries. We show that hydrogen may play a role
in these anodes, finding that hydrogen atoms bind to three-atom lithium
clusters in silicon, forming stable {H,3Li} and {2H,3Li} complexes, while the
{H,2Li} complex is almost stable.Comment: (5 pages, 4 figures
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