Graphite commensurate head-tail ordering

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.

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 49. Phase diagram of CO physisorbed on graphite with the following phases commensurate orientationally disordered (>/3 x -s/3)/J30° solid phase (CD), commensurate orientationally ordered herringbone phase (HB), head-tail (dipolar) ordered commensurate herringbone phase (DHB), fluid (F), reentrant fluid (RF), incommensurate orientationally disordered solid (ID), incommensurate orientationally ordered pinwheel phase (PW), second-layer orientationally disordered solid (2SD), second-layer liquid (2L), second-layer vapor (2V), second-layer fluid (2F), bilayer orientationally ordered solid (BO) the parentheses are included for clarity only. The solid lines are based on experimental results, whereas the dashed lines are speculative. Note that the phase boundary (ID + CD) RF and the orientational disordering temperamre of the BO phase are unknown. Coverage unity corresponds to a coverage of CO forming a complete ( /3 x Vs) commensurate monolayer. (The phase diagram is based on Fig. 1 of Ref. 112, Fig. 2 of Ref. 113, and Refs. 380 and 381.

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