Fig. 1: Hydrogen-adsorption induced formation of a nonperiodic tiling phase on the Pd terminated surface of delafossite PdCrO₂.

a Side-view crystal structure of delafossite oxide PdCrO₂. Inside the bulk, Pd layers possess electrical charge of  +1 per Pd atom, layers of CrO₂ contain charge of  −1 per CrO₂ octahedra. Upon cleaving, the vacuum-exposed Pd surface layer possesses charge of  +0.5 per Pd atom. b As (a), following dissociative adsorption of hydrogen, leading to a nonperiodic tiling phase consisting of (1 × 1)-H clusters formed at the surface. c Topographic STM image of the Pd-terminated surface of PdCrO₂ [V = −20 mV, I = 500 fA; image size: (20 nm)²]. d As (c), overlaid with an array of hexagons of different colours representing clusters of different types distributed on the surface. e Fourier transformation from a (50 nm)² area of the nonperiodic tiling phase. Blue circles mark the Bragg peaks associated with the lattice of Pd atoms. A red arrow indicates the wave-vector, q_avg, associated with the spatial arrangement of the hydrogen clusters. The overlaid red dashed circle has a radius (~0.61 Å ⁻¹) equal to ∣q_avg∣. f Structural model of the Pd surface layer observed in this study. The surface is chemi-sorbed by hydrogen, leading to a tiling phase that lacks any periodicity. In the model, raspberry-red spheres represent atomic H adsorbed on each Pd atom in the surface layer.