Neutron-diffraction study of the commensurate-incommensurate phase transition of deuterium monolayers physisorbed on graphite

Neutron-diffraction measurements have been performed to investigate in detail the commensurate-incommensurate (C-IC) phase transition of deuterium monolayers physisorbed on the basal planes of graphite. Several novel features have been observed. Above completion of the commensurate ({}3 ×{} {}3 )R30^∘{} phase a continuous phase transition to a striped superheavy domain-wall phase ({α} phase) occurs at temperatures below 7.3 K and densities between 1.05 and 1.16 (densities {ρ} are given in units of the complete C {}3 phase). With increasing density the system undergoes a first-order phase transition to the {γ} phase. This phase (1.19{≤}{ρ}{≤}1.32) can be described as a strongly density modulated phase, which is equivalent to a hexagonal IC phase with heavy domain walls. Due to its special modulation and rotational epitaxy, the {γ} phase locks into higher-order commensurate phases, a (5 {}3 ×{}5 {}3 ) phase ({δ} phase) and a (4×{}4) phase ({ε} phase). A detailed description of domain-wall lattice models is presented. At densities beyond {ρ}=1.33 a uniformly compressed IC phase was found. At temperatures above {}7 K a reentrant fluid phase ({β} phase) is squeezed in between the C and IC phases, which is interpreted to be a domain-wall fluid. It evolves continuously to an isotropic fluid at temperatures above 18 K. Due to this unique sequence of phases occurring in a relatively wide density range (1{≤}{ρ}{≤}1.33), D₂ on graphite can be regarded as a model system for the study of the C-IC phase transition. A phase diagram is proposed based on neutron-diffraction and specific-heat measurements. The results are compared to those of H₂/graphite and discussed in the light of current theories of the C-IC transition.