The existence of anharmonic localization of lattice vibrations in a perfect 3-D diatomic ionic crystal is established for the rigid-ion model by molecular dynamics simulations. For a realistic set of NaI potential parameters, intrinsic localized gap modes vibrating in the [111] and [110] directions are observed for the fcc and zinc blende lattices. An axial elastic distortion is an integral feature of this mode which forms more readily for the zinc blende than for the fcc structure. Molecular dynamics simulations verify that in each structure this localized mode may be stable for at least 200 cycles. The interchange of the IGM vibration between [111] and [110] directions is expected, for example, with hindered rotational motion of the excitation about the lattice site. This concomitant low frequency component of the IGM may provide new experimental ways to excite and identify these nonlinear excitations.

In order to identify the eigenvectors of these strongly anharmonic localized modes, an artificial dynamical simulated annealing technique of the Car-Parrinello-type has been developed. First, the nonlinear difference equations for the particles' vibrational amplitudes up to second harmonic in frequency are obtained self-consistently. Then, the resulting system of up to 9,000 nonlinear equations is solved successfully using this method. This numerical technique is general enough to apply to any nonlinear system which allows a classical molecular dynamics treatment, including the rigid-ion and shell models.

Last modified: April 12, 1997