LATTICE-CONSTANT DEPENDENCE OF THE DYNAMICAL EFFECTIVE CHARGE IN ALASAND GAAS

We have used Raman spectroscopy to study the lattice-constant dependen ce of the optical-phonon frequencies in AlAs and GaAs. From these stud ies, we deduce the dynamical effective charge for AlAs to be e(T) = 2 .20 +/- 0.01-(5.8 +/- 0.1)(-Delta a/a(0)). For GaAs we find a dynamica l effective charge e(T) = 2.18 +/- 0.01-(4.8 +/- 0.7)(-Delta a/a(0))- (28 +/- 16)(-Delta a/a(0))(2). First-principles calculations, recently confirmed by experiment, indicate that the interatomic force constant s (related to the second derivative of the interatomic potential) are virtually identical for GaAs and AlAs. The results reported in this pa per indicate that higher-order derivatives of the interatomic potentia l are also remarkably similar for these two materials.