Thermal contraction and disordering of the Al(110) surface

Al(110) has been studied for temperatures up to 900 K via ensemble density- functional molecular dynamics. The strong anharmonicity displayed by this s urface results in a negative coefficient of thermal expansion, where the fi rst interlayer distance decreases with increasing temperature. Very shallow channels of oscillation for the second-layer atoms in the direction perpen dicular to the surface support this anomalous contraction, and provide a no vel mechanism for the formation of adatom-vacancy pairs, preliminary to the disordering and premelting transition. Such characteristic behaviour origi nates in the free-electron-gas bonding at a loosely packed surface. [S0031- 9007(99)08925-5].