By using in situ X-ray diffraction methods, a study was made of the order-disorder phase transition on the (001) surface by measuring the temperature dependence of a superlattice reflection which was specific to the c(4 x 2) low-temperature phase. The results indicated that the transition corresponded to a 2-dimensional phase transition; with anisotropic interaction energies for directions which were parallel to, and perpendicular to, the dimer rows that formed the (2 x 1) surface. Due to pinning of the c(4 x 2) domains by defects, it was not possible to observe any universal critical behavior. The results indicated that the number of buckled dimers which was involved in the c(4 x 2) reconstruction was conserved throughout the transition. This implied that, at temperatures above ambient, the (2 x 1) surface consisted of a random array of buckled dimers.

C.A.Lucas, C.S.Dower, D.F.McMorrow, G.C.L.Wong, F.J.Lamelas, P.H.Fuoss: Physical Review B, 1993, 47[16], 10375-82