Graphite commensurate herringbone ordering

Figure 4.14. Phase diagram, coverage vs. temperature, of N2 physisorbed on graphite. Symbols used fluid without any positional or orientational order (F), reentrant fluid (RF), commensurate orientationally disordered solid (CD), commensurate herringbone ordered solid (HB), uniaxial incommensurate orientation-ally ordered (UlO) and disordered (UID) solid, triangular incommensurate orientationally ordered (lO) and disordered (ID) solid, second-layer liquid (2L), second-layer vapour (2V), second-layer fluid (2F), bilayer orientationally ordered (2SO) and disordered (2SD) solid. Solid lines are based on experimental results whereas the dashed lines are speculative. Adapted from Marx Wiechert, 1996.

Figure 38. Striped domain wall in a model of uniaxially compressed N2 monolayers on graphite (a) observed in Monte Carlo simulations of 52 X 12 molecules at 10 K and a coverage of 1.026 monolayers. Dots denote the centers of the honeycomb hexagons of the graphite basal plane, and crosses mark the mean positions of the molecular centers of mass. The inset of (a) shows the herringbone order in the commensurate region at the left and right boundaries of (a). The center-of-mass distribution in the region of the domain wall of (a) sampled from the Monte Carlo trajectory is magnified in (b). (Adapted from Fig. 1 of Ref. 283.)...

It is found [138] that the increase of the corrugation due to the inclusion of axially symmetric (experimentally determined bulk) quadrupole moments located at the carbon sites [361] which model the aspheiical charge distribution in the graphite substrate [see (3.9) and (3.10) in Section III.D.l] stabilizes the commensurate herringbone structure. This structure is head-tail-ordered as in Ref. 17 (see Fig. 53a or Fig. 54Z>, where the molecular axes have a systematic out-of-plane tilt) the unit cell is deformed because of the displacement of the molecular centers on the two sublattices. The Brillouin-zone-center frequency gap in the phonon spectrum is estimated [138] to amount to about 10 K in the ground state,... 

Figure 4.15. Na/graphite. 2 x 1 in-plane herringbone orientational ordered superstructure within the positionally ordered commensurate (V3 x /3)R30° phase.

Figure 26. LEED [93] (open circles and dashed line) and x-ray [242] (filled circles and solid line) intensity as a function of temperature for commensurate N2 on graphite. The intensity plotted on a linear scale is the herringbone superlattice peak intensity divided by the peak intensity of the (s/j x V3)JJ30° center-of-mass structure present in both the orienta-tionally ordered and disordered phases the data are arbitrarily normalized to the same intensity at 18 K. (Adapted from Fig. 6 of Ref. 108.)...

Figure 54. Possible head-tail ordered herringbone structures for a complete commensurate CO monolayer on the basal plane of graphite only the projection on the surface plane is shown and the directions of the molecular dipole moments are indicated by arrows, (a) Ferrielectric stmcture with the net dipole perpendicular to the herringbone symmetry axis, (b) Ferrielectiic structure with the net dipole parallel to the herringbone symmetry axis, (c) An-tiferroelectric structure with no net dipole. The principal axes of the ellipses correspond to the 95% electronic charge density contour given in Table I. Note that each type of head-tail ordering can be combined with any of the six herringbone ground states from Fig. 6.

---