6,867 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
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
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