Anomalous thermal expansion of Ni(001)

We show that the high-temperature Ni(001) surface undergoes a large vertical expansion. Low-energy electron diffraction experiments indicate that the surface thermal-expansion coefficient ${\mathrm{\alpha }}_{\mathrm{surf}}$ increases rapidly between 900 and 1300 K, reaching a value nearly 20 times larger than in the bulk. These results directly confirm that a large anharmonicity in the interatomic potential develops on metal surfaces and that it may be a precursor to various surface disordering transitions.