First―principlesStudy of Piezoelectricity of Si3N4 Crystal

Si3N4 is apopular material with many excellent physicochemical properties. This presentwork 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). Asfor α phase, the calculated lattice parameters are <>a=0.7678nm, <>c=0.5566nm;elastic― stiffness 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 area=0.7536nm, c = 0.2874nm, c11=4.241×1011N/m2, c33=5.599×1011N/m2 and the piezoelectric strain coefficients are nearlyzero. The analysis shows that both α and β phases are high strength and highhardness, relating to their network structures of tetrahedron. Thepiezoelectricity is poor, especially for β―Si3N4. Thiscan be attributed to the structural symmetry, higher symmetry and lowerpiezoelectricity.