  For most applications, zirconia (ZrO2) is doped with yttria. Doping leads to
the stabilization of the tetragonal or cubic phase, and increased oxygen ion
conductivity. Most previous surface studies of yttria-doped zirconia were
plagued by impurities, however. We have studied doping of pure, 5-monolayer
ZrO2 films on Rh(111) by x-ray photoelectron spectroscopy (XPS), scanning
tunneling microscopy (STM), and low-energy electron diffraction (LEED). STM and
LEED show that the tetragonal phase is stabilized by unexpectedly low dopant
concentrations, 0.5 mol% Y2O3, even when the films are essentially fully
oxidized (as evidenced by XPS core level shifts). XPS also shows Y segregation
to the surface with an estimated segregation enthalpy of $-23 \pm 4$\,kJ/mol.
