We present high-q resolution low-energy electron diffraction results of the approach to thermal roughening of the Ni(110) surface. The process is found to occur in two reversible steps. At temperatures above 900 K vibrations become a significant fraction of the bond length and a large anharmonicity in the vibrational amplitude appears. A large asymmetry between in- and out-of-plane vibrations exists, causing the off-specular diffraction peaks to decay rapidly to zero. Finally, near 1200 K, a significant density of vacancies begins to form. Above 1300 K the surface roughens.

• Received 24 August 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.447