Orientational phase transition

Fabbri U and Zannoni C 1986 A Monte Carlo investigation of the Lebwohl-Lasher lattice model in the vicinity of its orientational phase transition Mol. Phys. 58 763-88... 

Thus, in a study on the properties of dipole systems, most promise is shown by the representation of chain interactions, which, first, reflects the tendency toward ordering of dipole moments along the axes of chains with a small interchain to intrachain interaction ratio. Second, this type of representation makes it possible to use, with great accuracy, analytical equations summing the interactions of all the dipoles on the lattice. Third, there are grounds for the use of the generalized approximation of an interchain self-consistent field presented in Refs. 62 and 63 to describe the orientational phase transitions. 

An inference of fundamental importance follows from Eqs. (2.3.9) and (2.3.11) When long axes of nonpolar molecules deviate from the surface-normal direction slightly enough, their azimuthal orientational behavior is accounted for by much the same Hamiltonian as that for a two-dimensional dipole system. Indeed, at sin<9 1 the main nonlocal contribution to Eq. (2.3.9) is provided by a term quadratic in which contains the interaction tensor V 2 (r) of much the same structure as dipole-dipole interaction tensor 2B3 > 0, B4 < 0, only differing in values 2B3 and B4. For dipole-dipole interactions, 2B3 = D = flic (p is the dipole moment) and B4 = -3D, whereas, e.g., purely quadrupole-quadrupole interactions are characterized by 2B3 = 3U, B4 = - SU (see Table 2.2). Evidently, it is for this reason that the dipole model applied to the system CO/NaCl(100), with rather small values 0(6 25°), provided an adequate picture for the ground-state orientational structure.81 A contradiction arose only in the estimation of the temperature Tc of the observed orientational phase transition For the experimental value Tc = 25 K to be reproduced, the dipole moment should have been set n = 1.3D, which is ten times as large as the corresponding value n in a gas phase. Section 2.4 will be devoted to a detailed consideration of orientational states and excitation spectra of a model system on a square lattice described by relations (2.3.9)-(2.3.11). 

Orientational phase transitions in planar systems of nonpolar molecules... 

A switch to double-angle vectors given by Eq. (2.3.12) not only significantly simplifies the treatment of orientation phase transitions in planar systems of nonpolar molecules but also leads to a number of substantial inferences on the transition nature. First of all note that the long-range-order parameter t] (vanishing in a disordered phase and equal to unity at T = 0) in a -dimensional space (specified by the orientations of long molecular axes) can be defined as ... 

Recently, in the theoretical studies on the simulation for N2 adsorption in micropore, some researchers102-104 reported that the monolayer adsorption occurs even in the micropore whose pore width is greater than the bilayer thickness of N2 (about 0.7 nm). In addition, Kaneko et al. showed the presence of the orientational phase transition of N2 on the graphitic micropore wall, which is the same as the phase transition of the monolayer on the flat graphite surface,105 and gave an effective method for the surface area determination in the microporous system.106 Therefore, even for micropores whose width is greater than 0.7 nm, dV MS can be... 

Rusakov 107 108) recently proposed a simple model of a nematic network in which the chains between crosslinks are approximated by persistent threads. Orientional intermolecular interactions are taken into account using the mean field approximation and the deformation behaviour of the network is described in terms of the Gaussian statistical theory of rubber elasticity. Making use of the methods of statistical physics, the stress-strain equations of the network with its macroscopic orientation are obtained. The theory predicts a number of effects which should accompany deformation of nematic networks such as the temperature-induced orientational phase transitions. The transition is affected by the intermolecular interaction, the rigidity of macromolecules and the degree of crosslinking of the network. The transition into the liquid crystalline state is accompanied by appearence of internal stresses at constant strain or spontaneous elongation at constant force. 

N.V. Gapon, K.L. Dudko, Orientation phase transition and intermediate state in monoklinic antiferromagnet MWO4 in longitudinal external field.. Sov. Phys.-JETP, 77, No 4(10), 1528-1543 (1979)... 

Under pressure, C70 shows three phase transitions. The highest transition temperature increases with pressure at a rate of 6.8 K kbar-1, the lowest at 8.4 K kbar-1 and the intermediate one at 5.3 K kbar-1 [21] (Fig. 11). A recent DSC study [26], also shows indications of a transition at 330 K besides the previously reported ones at 280 K and 337 K. It appears that application of pressure delineates like phases of similar energies in both Cgo and C70 giving rise to two or three orientational phase transitions respectively. 

Mohwald, H., Erdle, E., and Thaer, A. Orientational phase transition in a charge-transfer crystal triplet excitons as probes for lattice dynamics. Chemical Physics 27, 79-87 (1978). 

Moving from structure to dynamics, we recall the NMR results, [Ty91, Ya91a] which indicate that the molecules in solid Ceo tumble rapidly at room temperature but freeze at lower temperatures. Considering the structural data reviewed above, one would expect a more-or-less continuous rotational diffusion in the room temperature fee phase, and either small oscillations or molecular jumps between favorable orientations in the low temperature phase. This difference was very clearly seen in the overall spin-lattice relaxation time near the orientational phase transition.[Ty91b] It was also found that the... 

G. B. M. Vaughan et al., The Orientational Phase Transition in Solid Buckminsterfullerene Epoxide (CeoO), Chemical Physics 168, 185-193 (1992). 

Nagano et al. [22] measured the CO2 uptake in Cgg in a study of the effects of supercritical fluid treatment, the aim of which was to remove solvent molecules from Cgg. Carbon dioxide was found to interact strongly with Cgg, and to have a remarkable effect on the orientational phase transition of Cgg crystals at 250 K. The kinetic features of the process suggested that CO2 absorbs inside the C g... 

The Si02 hydroxyl coating formed by groups with a substantial dipole moment has local and collective properties. The local characteristics of stmctural hydroxyls define their sorptive activity and reaction capacity. The collective properties manifest themselves in such phenomena as variation of the spectral line shape in the IR range, orientation phase transitions in the lattice of OH groups. The theory of rotation mobility of such groups developed earlier [3] reveals a significant role of their collective properties in adsorption and chemisorption processes. 

The spin-orientation transitions can be distinguished by their unique features from the large family of magneto-structural phase transitions. Unfortunately, the thermal conductivity of RECs has not been investigated for all groups of magnetic phase transitions. Reliable data are available for the MPT of the order-disorder type and for the spin-orientation phase transition (SOPT). 

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