Thermal expansion at a metal surface: A study of Mg(0001) and Be(101¯0)

Quantitative low-energy electron diffraction current-voltage measurements have been utilized to determine the thermal expansion of the Mg(0001) and $\mathrm{Be}\left(101\mathrm{}¯0\right)$ surfaces. The close-packed Mg(0001) surface exhibits a small thermal expansion while the more open $\mathrm{Be}\left(101\mathrm{}¯0\right)$ 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.