Negative thermal expansion of diamond and zinc-blende semiconductors.

Experimentally it is well known that diamond and zinc-blend semiconductors show an "unusual" (i.e. , negative) thermal expansion at about 100 K. We performed density-functional-theory calculations of thermodynamic potentials (i.e. , total energies and entropies) for perfect crystals, to study the temperature dependence of the lattice parameter. The origin of the negative expansion eff'ect is traced back to the entropy contribution of the Gibbs free energy.