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.