We have performed systematical ab initio studies of the electronic and stru
ctural properties of the disordered phase of the Si1-xGex alloy. For the de
fect free alloy, we find that the Si-Si bond length has a tendency to maint
ain its natural value (x = 0) all the way up to x > 0.5, similarly to what
is found experimentally. We also analyze the vacancy, and note, as expected
for a structurally as well as chemically disordered solid. that there is n
o local symmetry around the vacancy site. Finally, we perform some studies
of the alloy under pressure all the way up to 30 GPa, and find that the str
ucture is highly distorted in some cases. indicating that an amorphous phas
e may be found in the pressure region of the diamond-beta -tin phase transi
tion. (C) 2001 Elsevier Science B.V. All rights reserved.