Comment on the Nonlinear Susceptibilities of AlN

Abstract

Recently observed nonlinear susceptibilities of AlN |d₃₃₃|=23.3±35% |d₃₁₁(Quartz)| and |d₃₁₁|\lesssim0.04|d₃₃₃| lead us to the conclusion that the bond-ionicity difference Δf_i=f_ix−f_iz is −0.20 in AlN, where f_ix and f_iz are the bond ionicities in the x directional (perpendicular to c axis) bond and z directional (parallel to c axis), respectively. The proportionality between the bond-ionicity difference and the distortion Δ(c⁄a) is discussed in the distorted wurtzite type crystals.