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 criti cal ionicity F(c) is analytically determined from the critical point w here Z(T)ZB is equal to Z(T)RS. Its average value for various tetrahed ral compounds F(c)BAR is estimated to be F(c)BAR=0.78, which is very c lose to the Phillips critical ionicity F(i) 0.785. The relationship be tween F(c) and F(i) is discussed with respect to the structural instab ility caused by the large effective charge Z(T).