The melting line for ethylene on graphite

Larese, J. Z.; Rollefson, R.

doi:10.1016/0039-6028(83)90033-x

Cited by 23 publications

(9 citation statements)

References 8 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…As it happens, nothing is known about rotational motions in this dense, 2D monolayer because the NMR measurements of Ref. 2 were not extended into the temperature regime where it is stable. Unfortunately, we are not able to provide any reliable insights into the structures of the ID monolayer and bilayer phases because our zerotemperature calculations of minimum-energy configurations cannot be expected to give meaningful results at higher temperatures where dynamics begins to play an important role.…”

mentioning

confidence: 96%

Neutron Diffraction from Ethylene Adsorbed on Graphite

et al. 1983

View full text Add to dashboard Cite

A neutron-diffraction study of deuterated-ethylene overlayers on graphite basal planes reveals the existence of three distinct adsorbed solid phases as well as the coexistence of adsorbed and bulk phases at higher coverages. Evidence of a surface wetting transition involving transfer of molecules from the bulk to adsorbed phase is also obtained.PACS numbers: 68.10. Gw, 64.60.Cn, 68.40.+ e, 68.45.Da In striking contrast to other gases studied to date, ethylene layer formation on graphite basalplane surfaces exhibits a pronounced dependence on temperature. Adsorption isotherms 1 show evidence of an increasing number of layers as the temperature is raised, exactly opposite to what is usually observed. Low-coverage NMR 2 scans made at low temperatures first indicate an adsorbed solid monolayer; then, when the coverage is increased, signs of a second, bulklike component appear. This, on heating, reverts to an adsorbed phase. Evidently raising the temperature switches the system from a configuration in which a second adsorbed layer cannot form to one in which it can. What appears to be taking place is an unusual type of bulk-to-surface phase transition known as the "prewetting" transition. 3 ' 4 We are currently in the process of investigating ethylene overlayers on graphite with neutron diffraction and would like to report here some of what we have learned about the structural relationships underlying this curious competition between two-and three-dimensional ordering. Concurrent with our measurements, Sutton, Mochrie, and Birgeneau 5 have made an x-ray study of the same system. Comparison of the experiments has been of invaluable help in interpreting both sets of data.All of our work was done with a triple-axis neutron spectrometer operated at a fixed energy of 14.4 meV and with the analyzer section of the instrument set to accept only elastically scattered neutrons. A pyrolytic graphite filter was used to remove higher-order contamination. Our sample was deuterated ethylene (C 2 D 4 ) on either Grafoil or slightly recompressed, exfoliated graphite. Substrate surface areas were determined by a separate nitrogen-adsorption isotherm measurement. Thus we define coverage on a scale in which a complete V~3x/~3 commensurate monolayer [nearest-neighbor (nn) distance 4.26 A, area 0.0636 molecule/A 2 ] is assigned unit value even though ethylene seems not to form a commensurate overlayer in any coverage regime. As is usual in this type of experiment, the diffracted signal was obtained by taking the difference between counts with and without the adsorbate in the sample cell.Plotted in Fig. 1 are the low-temperature diffraction patterns observed at a number of repre-2000 C2D4 ON GRAFOIL \H 1000 2.85 FIG. 1. Low-temperature diffraction patterns from ethylene overlayers at various coverages. Warren lineshape fits to the data are represented by the solid lines. The arrows indicate positions of the diffraction peaks of 3D, monoclinic-solid ethylene.

show abstract

mentioning

confidence: 96%

Neutron Diffraction from Ethylene Adsorbed on Graphite

et al. 1983

View full text Add to dashboard Cite

show abstract

“…Such an effect has been observed by NMR in the cases of CH 4 and C 2 H 2 at 17 and 35 K, respectively, where these molecules execute isotropic rotation. 27 Since the transition around 39 K is seen at all coverages investigated in this work, the molecular motion, responsible for this effect, should be possible in all three solid phases. In this connection, it is important to realize that the ethane molecule can undergo several types of rotational motion.…”

Section: Ethane Coverage ‫51؍‬mentioning

confidence: 78%

“…NMR has been employed to study rotational transitions in methane and ethylene. 27 In both of these systems substantial rotational mobility is observed below the freezing transition. Some or all of this mobility is lost in rotational transition for ethylene at about 35 K and for methane at about 17 K. Due to an asymmetric shape, the ethane molecule can exhibit several types of mobilities.…”

Section: Introductionmentioning

confidence: 99%

Phase transitions in physisorbed ethane investigated by positron-annihilation spectroscopy

et al. 1999

View full text Add to dashboard Cite

Data are presented for the lifetime and Doppler broadening parameters for positrons annihilating in ethane physisorbed on grafoil. The coverage dependence of these parameters has been investigated up to about 2.5 monolayers at 108 K. Both the positron lifetimes and Doppler broadening parameters reflect significant variations in the surface electron density as a function of adsorbate coverage, with clear evidence for minimum electron density occurring at coverages of ϳ0.8 and 2. The temperature dependencies of the lifetime and Doppler broadening data, studied for coverages of 0.4, 0.9, and 1.5 monolayers, show structural phase transitions in the adsorbate. We propose a mechanism for this structural phase transition at 39 K.

show abstract

“…Both steric arguments and configuration energy calculations suggest that the C=C axes of the molecules in the LD phase are aligned parallel to the substrate surface (8,9). It is also known from Larese and Rollefson's nmr study of the spin-lattice relaxation time, T I , that some type of orientational ordering occurs near 37 K, although the structure of the ordered phase could not be identified (10). Additional evidence of ordering is seen in the high resolution heat capacity measurements of Kim et al (1 1) who find a weak heat capacity signal near 40 K which they conjecture to be associated with orientational ordering.…”

Section: Experimental Details Spherical Outer Electron Contours Form mentioning

confidence: 97%

Orientational ordering of ethylene on graphite

Larese¹,

Passell²,

Ravel³

1988

Can. J. Chem.

View full text Add to dashboard Cite

A neutron diffraction study of the low temperature structure and loss of orientational order was performed for the low density phase of ethylene adsorbed on graphite. At temperatures below 30 K the ethylene molecules form a centered-rectangular herringbone structure with their C=C bond parallel to the surface. At T > 30 K we observe a gradual loss of orientational order and a simultaneous increase in the near-neighbor spacing. This correlates well with the onset of rotational motions about an axis perpendicular to the C = C bond which remains predominately parallel to the surface plane. We speculate that the transition to this rotator phase is accompanied by a structural change to a triangular lattice.

show abstract

The melting line for ethylene on graphite

Cited by 23 publications

References 8 publications

Neutron Diffraction from Ethylene Adsorbed on Graphite

Neutron Diffraction from Ethylene Adsorbed on Graphite

Phase transitions in physisorbed ethane investigated by positron-annihilation spectroscopy

Orientational ordering of ethylene on graphite

Contact Info

Product

Resources

About