Direct structure determination of a composite double-layer surface alloy by x-ray photoelectron diffraction: <i>p</i> (2×2)-Na/Al(111)

The local structure of p(2×2)-Na/Al(111) has been investigated using angle-scanned full hemispherical x-ray photoelectron diffraction. The resulting diffraction patterns allow a <i>direct</i> and <i>unequivocal</i> determination of the surface atomic structure. Even though Na and Al are completely immiscible in the bulk, the p(2×2)-Na/Al(111) phase consists of a composite <i>double-layer surface</i> alloy where Na atoms take substitutional and fcc hollow sites and Al atoms removed from the surface occupy hcp hollow sites.