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We have studied the surface of pure and oxidized Pt$_3$Zr(0001) by scanningtunneling microscopy (STM), Auger electron microscopy, and density functionaltheory (DFT). The well-annealed alloy surface shows perfect long-range chemicalorder. Occasional domain boundaries are probably caused by nonstoichiometry.Pt$_3$Zr exhibits ABAC stacking along [0001]; only the A-terminated surfacesare seen by STM, in agreement with DFT results showing a lower surface energyfor the A termination. DFT further predicts a stronger inward relaxation of thesurface Zr than for Pt, in spite of the larger atomic size of Zr. A closedZrO$_2$ film is obtained by oxidation in $10^{-7}$ mbar O$_2$ at 400 $^\circ$Cand post-annealing at $\approx 800 ^\circ$C. The oxide consists of an O-Zr-Otrilayer, equivalent to a (111) trilayer of the fluorite structure of cubicZrO$_2$, but contracted laterally. The oxide forms a $(\sqrt{19} \times\sqrt{19})$R23$^\circ$ superstructure. The first monolayer of the substrateconsists of Pt and contracts, similar to the metastable reconstruction of purePt(111). DFT calculations show that the oxide trilayer binds rather weakly tothe substrate. In spite of the O-terminated oxide, bonding to the substratemainly occurs via the Zr atoms in the oxide, which strongly buckle down towardsthe Pt substrate atoms, if near a Pt position. According to DFT, the oxide hasa bandgap; STM indicates that the conduction band minimum lies $\approx 2.3$ eVabove $E_\mathrm{F}$.

Pt$_3$Zr(0001): A substrate for growing well-ordered ultrathin zirconia films by oxidation