A. Mujica et Rj. Needs, 1ST-PRINCIPLES CALCULATIONS OF THE STRUCTURAL-PROPERTIES, STABILITY, AND BAND-STRUCTURE OF COMPLEX TETRAHEDRAL PHASES OF GERMANIUM - ST12 AND BC8, Physical review. B, Condensed matter, 48(23), 1993, pp. 17010-17017
We report the results of a first-principles study of the structural pr
operties, stability, and band structure of Ge in two complex tetrahedr
al phases: the ST12 phase (simple tetragonal with 12 atoms per unit ce
ll) and the BC8 phase (body-centered cubic with 8 atoms per cell). We
focus mainly on the ST12 phase, which is well established as being for
med in Ge after release of pressure from the beta-tin phase. Local-den
sity-functional calculations of total energies, forces, and stresses w
ere carried out using the plane-wave-pseudopotential method. At each f
ixed volume considered, the structures were fully relaxed with respect
to their structural degrees of freedom (five for ST12, one for BC8).
A comparison with the beta-tin and diamond phases allows us to study t
he phase stability and pressure-induced phase transformations of these
structures. For the ST12 and BC8 phases we find zero pressure volumes
in good agreement with the experimental values. The ST12 phase turns
out to be metastable, which is consistent with its formation on releas
e of pressure from the beta-tin phase. The BC8 phase is found to be le
ss stable than the ST12 phase over the entire pressure range investiga
ted (0-200 kbar). ST12 Ge is found to have a direct band gap, whereas
BC8 is found to be semimetallic in our calculations.