AB-INITIO CALCULATION OF THERMODYNAMIC PROPERTIES OF SILICON

We present a fully ab initio calculation of the thermodynamic properti es far silicon within the quasiharmonic approximation, making use of v olume-dependent phonon frequencies obtained from. pseudopotential loca l-density calculations. The temperature dependence of the thermal-expa nsion coefficient, specific heat (at constant volume), and other relat ed quantities are studied. We confirm that the thermal-expansion coeff icient behaves differently in three temperature regions: positive for temperature below 15 K, negative between 15 and 125 K, and positive ag ain above 125 K. This finding agrees with experiment. The abnormal (ne gative) thermal-expansion coefficient at low temperatures is explained through a detailed study of mode Gruneisen parameters. Both specific- heat and thermal-expansion-coefficient values calculated are in excell ent agreement with experiment up to a few hundred kelvin.