Uniaxial incommensurate solids

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.

The transition from the commensurate solid to the uniaxial incommensurate phase, as well as that from the uniaxial to the triangular orientationally... 

Dynamical properties of the commensurate and uniaxial incommensurate phases according to the model of Refs. 232, 340, and 342 could also be explored by the molecular dynamics technique used [203, 352]. It is found that in-plane and out-of-plane motions can be analyzed separately for ori-entationally ordered N2 on graphite [203]. The 40-ps simulations below the orientational ordering transition (see Ref. 342) show [203] that the amplitude of reorientation is small and the out-of-plane motion nearly harmonic in both phases, whereas the in-plane motion is more complex, because it is anhar-monic and collective. The out-of-plane motion in the disordered phases is still harmonic, but more strongly damped, and the in-plane dynamics cannot be analyzed any more in terms of a cumulant expansion. Thus, there is little qualitative difference between the reorientational motion observed in the commensurate and uniaxially compressed solids. Only the out-of-plane motion is slightly less damped in the uniaxial phase, and the fluctuations from the planar configuration are more pronounced. 

F re 36. Compressed-monolayer and low-temperature phase diagram of N2 on graphite (solid lines) based on heat capacity measurements. Filled circles represent positions of heat capacity peaks. Hatched regions denote two-phase coexistence, and dashed lines as well as boundaries separating UIO -I- TI, UID, and TI phases are speculative. The following abbreviations have been used Commensurate -73 solid (C), incommensurate Vs solid (I), uniaxial (U), triangular (T), orientationally disordered (D), orientationally ordered (O). (Adapted from Fig. 2 of Ref. 394.)... 

Fig. 7a - d. Models of commensurate structures observed for I adsorption on W(110). The partially transparent circles represent the I atoms and the black quadrilaterals highlight the unit cells. The solid lines represent the commensurate surface unit cell while the dashed lines the adsorbate unit cell, (a) The structure observed at 0.25 ML. Compressing this structure uniaxially as indicated by the arrows creates incommensurate structures with coverages up to 0.33 ML. (b) The (3x2) structure observed at 0.5 ML. The arrows show how expanding this structure leads to lower coverage structures, (c) The (2x1) structure also observed at 0.5 ML. Compression of the (2x1) unit cell as indicated by the arrows ultimately leads to the c(2x6) structure shown in (d) observed at the saturation coverage of 0.58 ML. 

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