Herringbone orientational ordering

An orientational transition of the molecular axes is observed near 28 K independent of coverage up to the full ML and slightly beyond. This type of low-temperature orientational ordering is the 2 x 1 in-plane herringbone orientationally ordered... 

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

The ( /3 X y/3)R30° in-plane (2 x 1) herringbone monolayer was found to be the most stable configuration for all types of modeling [298]. The uniform uniaxial incommensurate monolayer has also in-plane herringbone orientational order even up to higher compressions, and a search for a stable out-of-plane herringbone monolayer of the type shown in Fig. 4c led to no viable candidates. However, pinwheel stmcmres exist for certain potential... 

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.

Fig. 24. Herringbone (a) and pinwiiccl (b) orientational ordering of uniaxial diatomic molecules on a triangular lattice. The heavy bars represent planar rotators and the circles denote vacancies, p is the degeneracy of the ordering. From Mourilsen (1985).

Figure 28. X-ray intensities as a function of temperature for commensurate N2 (filled circles and solid line) and CO (open circles and dashed line) on graphite (Papyex) at a coverage of about 0.83 + 10% monolayers. The intensity plotted on a linear scale is the herringbone superlattice peak intensity divided by the peak intensity of the (V3 x V3)/ 30° center-of-mass structure present in both the orientationally ordered and disordered phases. (Adapted from Fig. 4(a) of Ref. 242.)...

In the NMR study [335] a hysteresis of about 2.5% and a small discontinuity at 28.1 K in the temperature dependence of the order parameter was interpreted in terms of an evidence for a first-order herringbone transition. However, this assignment is based on NMR echo amplitudes which define only local order parameters. For the authors of the x-ray study [242] it seemed also probable that the herringbone orientational transition is a first-order transition. 

Figure 33. Herringbone orientational correlation functions F (3.15) in the inset and the logarithmic derivatives 62 In (3.18) as a function of distance I in units of the lattice constant a - 4.26 A in the disordered phase of the anisotropic-planar-rotor model (2.5) from Monte Carlo simulations at 7" = 25.5 K and a linear system size of L = 180. The different symbols distinguish the three symmetty axes a, and the dashed line marks the plateau 2/. In the inset all three F fall on top of each other and the different symbols denote here the two oscillating parts of the antiferromagnetic-like ordering pattern. (Adapted from Fig. 1 of Ref. 273.)...

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