Mobility measurements of two kinds of two-dimensional fluids. Methane adsorbed on graphite

Coulomb, J.P.; Bienfait, M.; Thorel, P.

doi:10.1051/jphys:01981004202029300

Cited by 86 publications

(24 citation statements)

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“…l x (2T) compared with the ideal case is due partly to the misorientation of the graphite platelets in the Papyex sheets which consist of a mixture of well-oriented platelets and dust (approximately 25%). 24 The best fit for both field orientations was obtained for a Gaussian spread P{£l) of platelet orientations with a half-width at half maximum of 18° and a fraction a = 0.31 of sites with p -Q. The value of the order parameter inferred from the fits is /> 0 = 0.83 ±0.05, but the precise value depends on the as- A slightly better fit can be obtained if one allows for a small width in the distributions P(p) of order parameters rather than the delta function form assumed above.…”

Section: ' 15 H £>I '=-(Kd/3)p 2 (6 I )(3i 2 2 -2) mentioning

confidence: 99%

NMR Study of the Orientational Ordering of Monolayers ofN215on Graphite

Sullivan

Vaissière

1983

Phys. Rev. Lett.

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NMR pulse techniques have been used to study the orientational ordering of 15 N 2 molecules physisorbed on graphite. A small hysteresis (~2.5%) and small discontinuity in the evolution of the NMR echo amplitudes which depend directly on the local order parameters indicates that the order-disorder transition is first order.PACS numbers: 64.70. Kb, 76.60.Lz We report results of an NMR study of the orientational ordering of monolayers of 15 N 2 molecules physisorbed on exfoliated graphite. This system is a physical realization of the two-dimensional (2D) ordering of electric quadrupoles on a triangular lattice for which topological frustration and critical fluctuations are expected to play major roles. 1 ' 2 The transition from the orientationally disordered phase has stimulated considerable interest since it is believed to belong to the same universality class as the n = 3 component Heisenberg model with face-oriented anisotropy, and it is predicted that critical fluctuations will drive the transition first order. 1,3 ' 4 Although recent low-energy-electron diffraction measurements 5 * 6 have shown that the orientationally ordered phase is a (2x1) herringbone configuration, the nature of the phase transition has not been elucidated. In order to determine whether or not the transition is first order we have used NMR techniques to follow the evolution of the local order parameters. The results indicate that the transition is first order with a small discontinuity in the order parameter and a hysteresis ^ 2.5% on thermal cycling. This agrees qualitatively with recent Monte Carlo simulations. 3 Monolayers of N 2 form a commensurate 2D solid on graphite below 47 K with the molecules occupying the center of every third carbon ring, i.e., a {/ZxV~3)R3Q° triangular net. 7 At high temperatures, the molecular axes are disordered as a result of thermal agitation, but at low temperatures this rotational symmetry is broken by anisotropic forces acting on the molecules. There are two contributions: (1) the strong anisotropic potential of the graphite substrate 8 for which the molecules tend to lie parallel to the graphite, and (2) the anisotropic interaction between the molecules (principally electric quadrupole-quadrupole) which leads to a collective orientational ordering of the axes. 9 The results of en-ergy minimization, 10 Monte Carlo simulations, 11 and mean-field-approximation calculations 1 show that the ground state is a two-sublattice herringbone array.The orientational ordering transition was first seen in heat-capacity studies 12 and later confirmed by neutron scattering. 13 The herringbone structure was identified by low-energy electrondiffraction measurements 5 ' 6 which were able to detect several superlattice reflections. The intensity of the superlattice peaks varied continuously on warming with an apparently smooth transition at approximately 30 K although a weak intensity persisted up to 39 K. It was not possible to conclude from these studies whether the transition was smooth or first order.More detailed...

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Section: ' 15 H £>I '=-(Kd/3)p 2 (6 I )(3i 2 2 -2) mentioning

confidence: 99%

NMR Study of the Orientational Ordering of Monolayers ofN215on Graphite

Sullivan

Vaissière

1983

Phys. Rev. Lett.

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“…Ainsi, la R.M.N. et la diffusion quasi 61astique des neutrons se completent pour constituer un puissant moyen d'etude des mouvements moleculaires au sein des fluides adsorbes [3,4].…”

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“…Pour des molecules portant des spins nucleaires un tel choix peut se fonder sur la relaxation par couplage dipolaire intermoleculaire. 4. Relaxation intermoléculaire de spins nucléaires identiques excentres.…”

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Quelques résultats théoriques sur l'étude par R.M.N. des fluides polyatomiques plans

Fries

1981

J. Phyique Lett.

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“…A more rigorous treatment for preferred orientation has been accomplished by Ruland and Tompa [43]. H(y) (called £«(1, ç) in [43]) is actually the integrated distribution of the crystallite surfaces (more details will be published elsewhere [44]). In the present work, H(y) is 03C0/2 normalized by requiring that H(y) dy = 1 and has 0 been measured in our case by recording a rocking curve on the 01 peak of CF4 adsorbed on papyex.…”

Section: Expérimentât Procédures and Characterization -mentioning

confidence: 99%

Two-dimensional polymorphism and melting in a monolayer of nitric oxide adsorbed on graphite

Coulomb¹,

Suzanne²,

Bienfait³

et al. 1980

J. Phys. France

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Three two-dimensional (2D) solids γ, δ and β have been observed at low temperature by neutron scattering in the first monolayer of nitric oxide adsorbed on graphite. The structure of two of these solids, y and β, has been determined. In both cases, nitric oxide molecules are dimerized, but in the β phase N2O2 molecules stand up whereas in the y solid they lie down on the substrate. The β and y configurations are analogous to the packing of two planes of the nitric oxide bulk crystal namely the (001) plane for the β phase and the (101) plane for the y solid. The intermediate δ structure is not resolved but a possible unit cell is given. Increasing the temperature results in a melting of the 2D solids. A particular emphasis is made on the β-liquid transition which is shown to be first order. Other first order transitions are also observed like the β solid ⇄ δ solid, liquid ⇄ gas and y solid ⇄ gas transformations. All these results combined with previous adsorption isotherm measurements are put together in a proposed 2D phase diagram displaying three triple points, one critical temperature and one tricritical point

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Mobility measurements of two kinds of two-dimensional fluids. Methane adsorbed on graphite

Cited by 86 publications

References 50 publications

NMR Study of the Orientational Ordering of Monolayers ofN215on Graphite

NMR Study of the Orientational Ordering of Monolayers ofN215on Graphite

Quelques résultats théoriques sur l'étude par R.M.N. des fluides polyatomiques plans

Two-dimensional polymorphism and melting in a monolayer of nitric oxide adsorbed on graphite

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