Bulk and surface premelting phenomena in alpha-gallium

We present a detailed x-ray study of premelting phenomena in the bulk and at the (100) surface of alpha-gallium single crystals using conventional and grazing angle diffraction. In the bulk we find anisotropic anomalies of the lattice vibrations and the thermal expansion above onset temperatures of about 20degreesC along the [010] direction and 27degreesC along the [001] direction. At the melting temperature T-m=29.76 degreesC the vibrational amplitude along [010] approaches the difference of covalent and metallic bond lengths, which is likely to induce a charge transfer between these bonds. We believe that this is the crucial effect that eventually destabilizes the stacking of metallic (010) bilayers characteristic for alpha-gallium and initiates the transition to the isotropic liquid phase. We observed no anomalies of the lattice vibrations or the thermal expansion along the [100] direction. At the Ga(100) surface isotropic blocked surface melting sets in simultaneously with the bulk anomalies and appears to be initiated by the bulk disordering. The liquidlike surface layer is found to grow logarithmically with temperature up to a final thickness of about 11 Angstrom. In the presence of this layer, we observe anomalous thermal expansion in an adjacent relaxed subsurface layer.