Neutron and X-Ray Scattering Studies of Layered Solids.

We report inelastic and quasielastic neutron scattering studies of graphite intercalation compounds and x-ray scattering studies of semiconductor films and superlattices which were grown by molecular beam epitaxy. For graphite intercalation compounds, these include measurements of the (001) longitudinal phonons in the ternary compounds {rm K_{1-x}Rb_{x}C_8 } and {rm KC_{24 }(ND_3)_{x}}. The results for the K-Rb compound show an anomalous softening of the compressional elastic constant C_ {33}. This stage 1 system also displays one optic mode behavior which can be described by a simple virtual crystal model. No local or gap modes were observed. The acoustic branch of the stage 1 K-ammonia ternary shows a splitting due to coupling of these phonons to a dispersionless librational excitation of the ammonia molecules. In addition, the interlayer force constants in this compound are appreciably softer than in its binary counterpart. We then go on to describe quasielastic neutron scattering results for a hydrogenated K-ammonia sample, which show the existence of two separate molecular rotations and also translational diffusion of the ammonia. The final study of graphite intercalation compounds shows that the Lyddane-Sachs-Teller (LST) splitting for excitations polarized in the (001) direction is quenched by free carrier screening in the stage 1 binary CsC_ 8. The x-ray scattering results involve two types of systems: GaAs grown on Si and Ge substrates and terraced superlattices. First, we show that the polar semiconductor GaAs can be grown free from antiphase domains on non-polar Si and Ge substrates. This is done by carefully measuring the widths of various Bragg peaks and showing that the broadening can be completely described in terms of residual strain and finite crystallite size. We then conclude by describing the phenomena of terracing for both unstrained and strained-layer superlattices. A model is presented which reasonably accounts for the peak positions, widths, and intensities for the strained-layer system GaAs/ {rm GaAs_{1-x}Sb_{x }.}.