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

Abstract

Helium atom scattering was used to investigate the surface structure and dynamics 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 substrate with an ordered (001) surface. At about T{sub c}{approx}97 K, the film appears to undergo a second-order phase transition from a high-temperature (1{times}1) phase to a low-temperature (1{times}2)/(2{times}1) phase with an antiferroelectric alignment of the CN{sup {minus}} ions. For a monolayer of mixed KBr{sub 0.17}(CN){sub 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{radical} (2) {times}6{radical} (2) R45{degree} superstructure. Finally, measurements of the surface dispersion curves in the [100] and [110] directions provide evidence for a Rayleigh mode and a dispersionless mode at 11.5 meV, which are essentially identical above and below T{sub c}. {copyright} {ital 1999} {ital The American Physical Society}