It is now possible to make some comment about the trends in behaviour
down group IV. In Si, Ge and Sn the stable phase at low pressure
and temperature is diamond whilst in C this phase is metastable with
respect to graphite. Under pressure the transformation to the
metallic 
-Sn structure occurs at progressively lower pressures
in Si - Ge - Sn (in tin itself, this transition occurs at ambient
pressure due to temperature alone). Likewise, metastable phases exist
which have intermediate density and maintain fourfold coordination.
These are easier to form as one goes down group IV, apparently because
bond-bending is easier for 
hybrids with larger n (and therefore
smaller overlap under distortions). These concepts are used
in the development of empirical potentials, and are developed
further in Chapter 4. Carbon is different. Not only does
it exhibit the graphite phase under ambient pressure but it cannot form
a metastable phase with BC8 or ST12 symmetry without a significant
departure from fourfold coordination.