On the basis of empirical electron theory of solid and molecules（EET）

the valence electron structure（VES）of AlP and Si unite cell were calculated.Through comparing with the parameters of valence electron structure

it is concluded that the reaction of pure P atom or phosphide with Al atom in Al-Si molten can generate AlP particle which exhibits f.c.c structure and distribution of electron space similar to primary Si phase and low mismatching degree between AlP and Si unit as well as smaller nA value

so the AlP phase is prior to precipitating during the solidification to as heterogeneous nuclei of primary Si phase

increasing nucleation rate of primary Si phase.Calculated results confirm the theory of AlP phase as heterogeneous nuclei.