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. Likewise, metastable phases exist which have intermediate density and maintain fourfold coordination. 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.