ANHARMONIC GAP MODE IN A ONE-DIMENSIONAL DIATOMIC LATTICE WITH NEAREST-NEIGHBOR BORN-MAYER-COULOMB POTENTIALS AND ITS INTERACTION WITH A MASS-DEFECT IMPURITY

Both stationary and moving intrinsic anharmonic gap modes are generate d in a perfect one-dimensional diatomic chain. Within the rotating-wav e approximation, the eigenfrequency, eigenvector, and energy of such a localized packet can be found from differential-difference equations. A connection between the anharmonic system treated here and the harmo nic one is that since the effective force constants are determined by the eigenvector of the particular localized mode, they can be viewed a s renormalized force constants in a harmonic lattice. For the diatomic chain the even-parity anharmonic mode is unstable against conversion to an odd-parity mode while the odd-parity mode shows long term stabil ity, in contrast with the result found earlier for a monatomic chain. Part of the mean energy of the odd-parity gap mode is associated with kinetic and potential terms of the ac vibration while the rest resides in a localized dc distortion of the lattice. Strongly localized gap m odes can be approximated by the dynamics of a triatomic molecule. For larger vibrational amplitudes and associated dc distortions, the poten tial for the gap mode becomes double valued and the rotating-wave appr oximation fails. When the interaction of intrinsic gap modes with stat ionary anharmonic mass defect impurity modes is examined in numerical simulation studies, a variety of scattering results are found dependin g on the mass defect magnitude and the site in the diatomic chain. Two important features of the trajectories are that the gap mode is trapp ed at the mass defect when the vibrational frequencies of the moving m ode and the anharmonic defect mode are near resonance and that the sca ttering is elastic when the frequencies are far apart.