Commensurate-incommensurate transition compressed monolayers

Orientational ordering transitions [394] clearly occur in the commensurate phase (possibly in coexistence with the fluid) up to 1.101 monolayers at 28.1 K, as well as in the uniaxial compressed phase from 1.142 to 1.178 monolayers at 29-30 K based on adiabatic calorimetry [156] a heat capacity peak at 28.7 K was reported at a coverage of 1.109 which was interpreted as the transition to the uniaxially compressed incommensurate phase, whereas this peak is located near 27.5 K in the submonolayer regime. The ac heat capacity scans [394] in the triangular incommensurate phase at 1.200 and 1.302 monolayers show only very weak and broad features in the relevant temperature range from 22 K to 32 K. Thus, it is concluded [394] that their orientational order is only gradually lost as the temperature is increased... 

Figure 48. Semilogarithmic plot of the isothermal compressibility of N2 on boron nitride at 60.8 K as a function of the coverage in units of the complete /3 monolayer. The peak sequence starting at low coverages is attributed to the fluid to commensurate solid F-C and commensurate solid to reentrant fluid C-RF transitions and finally to second-layer growth RF-B (instead of a transition from the reentrant fluid to an incommensurate solid phase). (Adapted from Fig. 5 of Ref. 1.)...

A main field of activities is focused on structure and reactivity in two-dimensional adlayers at electrode surfaces. Significant new insights were obtained into the specific adsorption and phase formation of anions and organic monolayers as well as into the underpotential deposition of metal ions on foreign substrates. The in situ application of structure-sensitive methods with an atomic-scale spatial resolution, and a time resolution up to a few microseconds revealed rich, potential-dependent phase behavior. Randomly disordered phases, lattice gas adsorption, commensurate and incommensurate (compressible and/or rotated) stmctures were observed. Attempts have been developed, often on the basis of concepts of 2D surface physics, to rationalize the observed phase changes and transitions by competing lateral adsorbate-adsorbate and adsorbate-substrate interactions. 

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