Thermal expansion at a metal surface: A study of Mg(0001) and Be(10(1)over-bar0)

Quantitative low-energy electron diffraction current-voltage measurements have been utilized to determine the thermal expansion of the Mg(0001) and Be(10 (1) over bar0) surfaces. The close-packed Mg(0001) surface exhibits a small thermal expansion while the more open Be(10 (1) over bar0) surface has a dramatic thermal contraction in the first interlayer spacing, accompanied by an expansion in the second interlayer spacing. A comparison of this data with all other measurements of the low-temperature thermal expansion reveals a quite striking difference for open surfaces of different metals. Significant negative thermal contraction at the surface occurs only on open faces of light mass metals. A simple force constant model indicates that this behavior correlates with the ratio of thermal motion parallel and perpendicular to the surface.