The synchrotron radiation X-ray diffraction study of the liquid-solid phase transition of the quasi-two-dimensional potassium layer in graphite intercalation compound is presented. Some critical properties show the behaviors of the continuous transition : (1) The wave number of the first in-plane diffraction peak, q0, shows no discontinuous gap at the transition point. (2) The liquid layer has the same symmetry as the solid. (3) The intra- and interplane positional correlations of potassium atoms grow up in liquid state as approaching the critical point. However, the correlations show a discontinuous change at the transition. These results indicate that the transition is of the weakly first order. The phase transition can be understood in terms of the lattice-gas model.
