Surface-sandwich segregation and multilayer relaxation on ${\mathrm{Pt}}_{0.5}$${\mathrm{Ni}}_{0.5}$(110) measured by low-energy electron diffraction: An observation of face-related segregation reversal

Low-energy electron diffraction (LEED) is used for measuring the surface segregation and multilayer relaxation on the (110) surface of the ${\mathrm{Pt}}_{0.5}$ ${\mathrm{Ni}}_{0.5}$ alloy. The electron scattering in the substitutionally disordered metallic alloy is described by a layer-by-layer version of the averaged-t-matrix approximation, LEED is calculated by the layer-doubling method (whose convergence is verified), and the measured and calculated spectra are compared by means of metric distances. We find that surface segregation creates a three-layer surface sandwich which has, from the crystal face and inwards, the compositions 100 at. % Ni, 95 at. % Pt, and 83 at. % Ni and the respective multilayer relaxations 19% contraction, 11% expansion, and 1% contraction. The fact that the surface segregation on the (110) face is reversed relative to that which was observed earlier on the (111) face (from the surface and inwards, 88 at. % Pt and 91 at. % Ni) presents a challenge to the theory of alloys. We also discuss the possibility to measure surface vacancy defects by LEED.