Structure and dynamics of a monolayer KCN film grown onto a KBr(001) substrate

Helium atom scattering was used to investigate the surface structure and dy namics of a monolayer film of KCN grown onto the nearly lattice-matched KBr (001) substrate. Although previous studies had shown that the cleaved (001) surface of KCN is disordered, the film appeared stabilized on this substra te with an ordered (001) surface. At about T-c approximate to 97 K, the fil m appears to undergo a second-order phase transition from a high-temperatur e (1 X 1) phase to a low-temperature (1 X 2)/(2 X 1) phase with an antiferr oelectric alignment of the CN- ions. For a monolayer of mixed KBr0.17(CN)(0 .83). this transition temperature is reduced to 78 K. In addition, for the pure KCN films, diffraction features appear for the low-temperature phase, which corresponds to a 2 root 2 X 6 root 2R45 degrees superstructure. Final ly, measurements of the surface dispersion curves in the [100] and [110] di rections provide evidence for a Rayleigh mode and a dispersionless mode at 11.5 meV, which are essentially identical above and below T-c.