Neutron scattering study of /sup 36/Ar monolayer films adsorbed on graphite

Journal Article · Tue Nov 15 04:00:00 EST 1977 · Phys. Rev., B; (United States)

DOI:https://doi.org/10.1103/PhysRevB.16.4551· OSTI ID:5346224

Taub, H; Carneiro, K; Kjems, J K; Passell, L; McTague, J P

Diffraction of neutrons from /sup 36/Ar monolayers adsorbed on graphite basal planes indicates that an ordered, two-dimensional (2D) triangular lattice is formed at low temperature. The lattice constant is found to be slightly larger than that of the bulk 3D solid but significantly smaller than that of a registered ..sqrt.. 3 x ..sqrt.. 3 overlayer. Thermal expansion of the monolayer is anomalously large; up to 60 K the linear expansion is 4.5 times greater than in the 3D solid. There is no evidence of a sharp melting transition. Instead, the positional correlations (which extend to the full dimensions of the crystallite surfaces at low temperatures) are observed to decrease smoothly above 40 K, falling from 100 to 15 A at 80 K. Little if any positional order remains at temperatures where the nearest-neighbor distance in the monolayer matches that of a registered ..sqrt.. 3 x ..sqrt.. 3 phase. The spectrum of neutrons inelastically scattered from /sup 36/Ar monolayers in the nominally in-plane configuration can be reasonably well described at low temperatures by a 2D harmonic-phonon model while the scattering in the out-of-plane configuration seems to be best represented in terms of a resonant coupling of the monolayer film to out-of-plane collective motions of the graphite substrate. There is some evidence of renormalization of the in-plane transverse modes of the monolayer at higher temperatures; however, the in-plane longitudinal modes and the out-of-plane response do not appear to be similarly affected.

• Research Organization: Brookhaven National Laboratory, Upton, New York 11973
• OSTI ID: 5346224
• Journal Information: Phys. Rev., B; (United States), Journal Name: Phys. Rev., B; (United States) Vol. 16:10; ISSN PLRBA
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360602* -- Other Materials-- Structure & Phase Studies
ADSORPTION
ARGON
COHERENT SCATTERING
CRYSTAL LATTICES
CRYSTAL STRUCTURE
DIFFRACTION
ELEMENTS
EXPANSION
FILMS
LATTICE PARAMETERS
MELTING
NEUTRON DIFFRACTION
NEUTRON SPECTRA
NONMETALS
PHASE TRANSFORMATIONS
RARE GASES
SCATTERING
SORPTION
SPECTRA
THERMAL EXPANSION