Herringbone phase

S. Chains in the S phase are also oriented normal to the surface, yet the unit cell is rectangular possibly because of restricted rotation. This structure is characterized as the smectic E or herringbone phase. Schofield and Rice [204] applied a lattice density functional theory to describe the second-order rotator (LS)-heiTingbone (S) phase transition. 

Figure 12.9 Schematic representation of structural changes of PTCDA on ordering in the herringbone phase [35]. Left disordered low temperature phase. Right herringbone phase.

For large negative crystal field at low temperatures the stable stmcture is the ideal two-sublattice in-plane herringbone phase—that is, the 2-in phase in Fig. 4a. The two sublattices which can be oriented in three different ways relative to the triangular lattice lead to six equivalent ground states. The excitation spectrum of this phase in general has a gap. In this phase the molecular wave functions are localized in the substrate plane, and classically the molecular axes are parallel to the surface see Appendix A of Ref. 141 for an interpretation of the order parameters. Thus, the orientational degeneracy of the pararotational phase is broken by the quadrupolar interactions. A closely related structure was already proposed based on atomistic Lennard-... 

Figure 7. Setting angle of the herringbone phase of the quadrupolar-dipolar model (2.8) and the same model including a sixfold-modulated in-plane crystal-field (2.10) as a function of the reduced temperature T = TIT i at X = 0.9 for jSj = 2, where X = 1 note that 02 only sets the temperature scale. Solid line no crystal field. Dashed line K3 = 0.1. (Adapted from Fig. 7 of Ref. 71.)...

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.

Fig. 12 Configurations of a hexabenzocoronene dodecyl derivative in the columnar herringbone phase at 300 K (a) and the hexagonal one at 400 K (b). Reprinted with permission from [151], Copyright 2008, American Institute of Physics...

Fig. 25.9 Schematic 2D model showing the various degenerate channel sites and unit cell structures for various threefold and fourfold channel packing motifs that are appropriate for alkali metal intercalation, (a)-(c) Sequential progression of high symmetry structures from the pristine herringbone phase, HB (a) to the intermediate x (or 120° [74]) phase (b) to the distorted-120° phase (c). (d)-(f) Sequential progression of high symmetry structures from the pristine herringbone phase, HB (d) to the intermediate stage 2 phase (e) to the final stage 1 phase.

---