Trimer phase transition

High pressure Raman and IR spectroscopy on (NPCl2)j4 show second order phases transitions at 22 and lOkbar for the trimer and tetramer respectively . In a comparison of the isoelectronic (Cl2SiO)j4 and (Cl2PN)3 systems, the fragmentation patterns in the mass spectra are similar as are the vibrational spectra of the trimers. Differences are observed between tetramers due to the planarity of (CljSiO). ... 

Up to now the model has been applied with monomeric, dimeric and trimeric solute molecules. Although the study of these cases is not complete, possibly due to computational difficulties, it seems that some of the adsorption features are satisfactorily predicted only in the case of non-polar monomeric and polar dimeric solute molecules, provided that the latter exhibit certain orientations on the electrode surface. " In the case of polar monomeric and dimeric molecules that may adsorb either vertically or flat, the model does not give satisfactory predictions. This is shown in Figure 3 where the solid lines represent adsorption isotherms predicted by the model and the dotted lines represent the best Frumkin s isotherms that describe them. In the case of the trimeric solutes, the model predicts the existence of a surface phase transition. However, the transition properties, due to the use of an inappropriate statistical mechanical treatment, contradict thermodynamic and experimental data. Thus, despite its novelty the three-dimensional lattice approach has not given the expected results yet. 

On this issue Guidelli et al. expressed the view that phase transitions take plaee when the shape of the solute molecules hinders H-bond formation between water (solvent) molecules. In this case the water molecules are squeezed out of the adsorbed layer, leaving behind a compact film of solute molecules. This view seems to be verified by the three-dimensional lattice model, which in the presence of non-polar trimeric solute molecules does predict the occurrence of a phase transition. However, due to an inappropriate statistical mechanical approach based on the use of the grand ensemble H instead of the generalized ensemble A, it is not possible to know whether this model predicts correctly or not the properties of the phase transitions. ... 

Table 3 Thermal pressure coefficients y, volume changes A V, and volume-dependent transition entropies ASy of dimer and trimer LCs for the crystal-nematic (CN) phase transitions... 

We will discuss the case where the motion of heavy atoms is confined to two dimensions, while the motion of light atoms can be either two- or three-dimensional. It will be shown that the Hamiltonian 10.76 with Ues in 10.44 supports the first-order quantum gas-crystal transition at T = 0 [68], This phase transition resembles the one for the flux lattice melting in superconductors, where the flux lines are mapped onto a system of bosons interacting via a two-dimensional Yukawa potential [73]. In this case Monte Carlo studies [74,75] identified the first-order liquid-crystal transition at zero and finite temperatures. Aside from the difference in the interaction potentials, a distinguished feature of our system is related to its stability. The molecules can undergo collisional relaxation into deeply bound states, or form weakly bound trimers. Another subtle question is how dilute the system should be to enable the use of the binary approximation for the molecule-molecule interaction, leading to Equations 10.76 and 10.44. 

Here, we address the more general question of the relative stability of monomers, dimers and triangular trimers on the (111) surface of FCC transition metals of the same chemical species as a function of the d band filling Nd. All possible atomic configurations of the systems are considered monomers and dimers at sites N and F, triangles with A and B borders at sites N and F (Fig. 4). The d band-filling includes the range of stability of the FCC phase (Nd > 7.5e /atom). The densities of states are obtained from... 

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