K. Knorr

Solid Ar, N2, CO, and O2 in Nanopores

Molecular solids enclosed in pores with diameters in the nm-range are a research field which attracts growing interest. The major questions asked in this context are: what is the structure and the dynamics? How are phase and glass transitions modified by the geometrical constraint? In this contribution we present structural information, by x-ray diffraction measurements, as well as thermodynamic characterization, by vapor pressure and heat capacity measurements, on Ar, which is usually considered the simplest of all condensates, and on the small diatomic molecules N2, C0, O2 in porous glasses with average pore diameters from 50 to 130Å.

MELTING OF MONOLAYERS OF SOME HALOMETHANES AND HALOETHANES PHYSISORBED ON GRAPHITE

The melting of monolayers of nine halomethanes and haloethanes physisorbed on exfoliated graphite is discussed on the basis of heat capacity and X-ray diffraction results. It is shown that the melting transition of incommensurate, triangular, orientationally disordered phase is continuous, whereas other two–dimensional solid phases melt discontinuously.

Optical Transmission Measurements on Phase Transitions of O2 and CO in Mesoporous Glass

The optical transmission of O2 and CO condensates embedded in porous Vycor glass has been studied as function of the filling fraction and of the thermal history of the samples. The freezing transition as well as the solid-solid transitions (β-α of CO and γ-β of O2) induce a coarsening of the separation into empty and filled regions which results from the hysteretic behavior of the transitions in the presence of a pore size distribution. In the birefringent β phase of O2 domains with a fixed crystallographic orientation extend over distances much larger than the pore diameter (10 nm).

Solid Nitrogen Confined in Porous Glass

N2condensed into nanoporous glass has been investigated by X-ray diffraction and adsorption/filling isotherms. The material in the pores consists of an amorphous layer at the pore walls and quasi–bulk material in the center of the pores. The freezing temperature of the bulk component is reduced. Its structure is hcp down to lowest temperatures with a stacking fault probability as large as 0.3. Thus the transition hcp–Pa3 of the bulk solid is suppressed.

Monolayers of C2Cl2F4 and C2ClF5 physisorbed on graphite

Abstract Mono- and submonolayers of the haloethanes C 2 Cl 2 F 4 and C 2 ClF 5 physisorbed on exfoliated graphite have been investigated by X-ray diffraction and by some complementary heat capacity and ellipsometric measurements. Both adsorbates have an orientationally disordered “plastic” mesophase with an incommensurate triangular center-of-mass lattice at intermediate temperatures which gradually melts into the 2D liquid. At low temperatures they freeze into a 2D orientational glass. For C 2 ClF 5 the glassy state is in competition with a 4 × 4 phase with a 120 “antiferromagnetic”-type orientational order.

Growth kinetics of Ar monolayers physisorbed on graphite (001)

Monolayers of Ar physisorbed on graphite have been evaporated by intense laser pulses and the readsorption transients monitored in time with ellipsometry. The transients have been studied for a wide range of supersaturations and temperatures (from below the 2D triple point to above the 2D critical point). In the region of the solid phase and in the liquid phase at high supersaturation, the adsorption kinetics can be described by a rate-balance model. For the liquid phase at low supersaturation a regime of slower kinetics has been observed, for which an explanation in terms of growth induced by steps of the substrate is proposed.

X-ray diffraction study of C2F3Cl physisorbed on graphite

Monolayers of the planar molecule C2F3Cl, a polar haloethen, physisorbed on exfoliated graphite have been investigated by X-ray diffraction. At a lower coverage, the molecules lie flat on the surface, and at a higher coverage, they stand on the FCl dipod. The coverage–temperature phase diagram has been established and the structures of the three 2D crystalline phases determined. One of the two crystalline phases of the low coverage regime is commensurate 7×7 and orientationally disordered. The second phase of this regime is incommensurate and goes through a sequence oblique–rectangular–triangular with increasing temperature.