First-principles Study of Piezoelectricity of Si_3N_4 Crystal

Si3N4 is a popular material with many excellent physicochemical properties.This present work make a comparative study on its low and high temperature phases(α-and β-Si3N4) by first-principles calculation based on density functional theory(DFT).As for α phase,the calculated lattice parameters are a=0.7678nm,c=0.5566nm;elasticstiffness coefficients are c11=4.232×1011N/m2,c33=4.615×1011N/m2;piezoelectric strain coefficients are d33=0.402pC/N.While for β phase,they are a =0.7536nm,c = 0.2874nm,c11=4.241×1011N/m2,c33=5.599×1011N/m2 and the piezoelectric strain coefficients are nearly zero.The analysis shows that both α and β phases are high strength and high hardness,relating to their network structures of tetrahedron.The piezoelectricity is poor,especially for β-Si3N4.This can be attributed to the structural symmetry,higher symmetry and lower piezoelectricity.