The crystal structures of new forms of silicon and germanium

Abstract

The structures of new dense forms of silicon and germanium, recovered from high pressure experiments, have been determined from their Debye-Scherrer patterns.:For silicon, the space group is Ia3 (T~), a = 6.636 _+ 0.005 A, Z = 16, and there is one structural parameter corresponding to occupancy of 16(c) positions, with x = 0.1003 _+ 0"0008.The measured and calculated density is 2.55 g.cm -s.The space group of germanium is P43212 (D4 s) with a=5.93 _+0.01, c=6-98_+0-01 A, and Z=12.Two kinds of germanium atom occupy the 4(a), xxO, etc., and 8(b) xyz, etc., positions; x for 4(a) is 0.0912_+0.0060,and for 8(b) x = 0.1730 _+ 0.0037, y--0.3784_+0-0051,z=0.2486_+0-0048.The measured density is 5-88 g.cm -a, the calculated 5.91 g.cm -3.The two structures represent two different ways of achieving higher density relative to the diamond structure without change of coordination number and with little effect on interatomic distance.The angular distortions, particularly for germanium, are, however, appreciable.:New dense forms of silicon and germanium, recovered after subjecting the ordinary forms to high pressures, have been reported recently (Bundy& Kasper, 1963;Wentorf & Kasper, 1963).Only a brief account of the approximate crystal structures could be given in these reports and it is the purpose of this article to give the details of the structural investigation and the resulting refined values of structural parameters.