Structures of FeTiO₃ (0001) surfaces observed by scanning tunneling microscopy

Scanning tunneling microscopy (STM) was used to investigate the (0001) surface structure of a natural single crystal of FeTiO₃, following argon-ion sputtering and annealing in O₂. Low energy electron diffraction (LEED) of the FeTiO₃ (0001) surface shows two different diffraction patterns that depend on preparation. We examined surfaces with a hexagonal pattern that was interpreted as a (1 x 1) bulk termination. Wide-scale images of the (1 x 1) bulk termination show two distinct co-existing areas: large atomically rough terraces and small, smoother, atomically resolved areas within. The observed single step height of 2.2 ± 0.3 Å (doubled values are also found) plus data from the surface orientation implies that two termination types are seen on the (0001) surface after initial stages of preparation, and that these are either cation-terminated surface planes (Fe²⁺ or Ti⁴⁺) or close-packed oxygen terminations. Atomic-resolution STM images of smooth terrace areas show features arranged in a hexagonal array, with a separation of 4.8 ± 0.2 A. Å model is proposed that identifies this termination as an unreconstructed (0001) termination of FeTiO₃ that exposes half a layer of either Fe²⁺ or Ti⁴⁺ cations over a close-packed O layer, with each feature arising from a trimer of O atoms capped by a single cation (Fe²⁺ or Ti⁴⁺).