ANOMALOUSLY LARGE THERMAL-EXPANSION AT THE (0001)-SURFACE OF BERYLLIUM WITHOUT OBSERVABLE INTERLAYER ANHARMONICITY

We have measured a large thermal surface expansion, 6 times larger tha n the bulk, on Be(0001) using low-energy electron diffraction. This ob servation seems to be inconsistent with previous measurements reportin g negligible anharmonicity in the surface phonon modes normal to the s urface. Density-functional theory calculations for the thermal expansi on from the minimum in the free energy within the quasiharmonic approx imation agree with the experimental observations and demonstrate that the enhanced thermal expansion is caused largely by a softening of the in-plane vibrations.