NONLINEAR GALLERY EXPANSION OF RANDOMLY INTERCALATED ANHARMONIC BILAYERS

We study the combined effects of local anharmonicity and transverse la yer rigidity on the nonlinear gallery expansion in ternary intercalate d systems. Numerical simulations are performed using both Lennard-Jone s potentials and cubic anharmonic potentials between the intercalant a toms and the host-layer atoms. The harmonic approximation is used for the host-layer deformation energy. Simulation results are compared wit h analytic calculations within an effective-medium approximation. We f ind that the gallery expansion is determined by a competition between the compressibilities of the intercalants and the transverse rigidity of the host layer. A single small impurity limit clearly brings out th e role of anharmonicity. The effective-medium solution reduces to exac t results in three limiting cases; for perfectly floppy and perfectly rigid host layers, and for the harmonic potential with arbitrary host- layer rigidity. While it is well known that Vegard's law (linear expan sion of the gallery spacing) is observed in the harmonic limit when th e two intercalants have the same compressibility, we find that the inc lusion of anharmonicity gives rise to deviations from Vegard's law whi ch increases with the increase in host-layer rigidity and the degree o f anharmonicity.