ANALYTICAL MODEL OF CRITICAL IONICITY FOR A(N)B(8-N) BINARY COMPOUNDS

By using the pseudopotential method, the transverse effective charges Z(T)ZB and Z(T)RS of zincblende and rock-salt structures, respectively, are presented as a function of the Phillips ionicity f(i). The critical ionicity F(c) is analytically determined from the critical point where Z(T)ZB is equal to Z(T)RS. Its average value for various tetrahedral compounds F(c)BAR is estimated to be F(c)BAR=0.78, which is very close to the Phillips critical ionicity F(i) 0.785. The relationship between F(c) and F(i) is discussed with respect to the structural instability caused by the large effective charge Z(T).