The atomic and electronic structure of the (001) surface of monoclinic pyrrhotite (Fe₇S₈) as studied using STM, LEED and quantum mechanical calculations

The properties of monoclinic Fe₇S₈(001) surfaces have been examined using LEED and STM following sputtering and annealing of the surface at ∼ 300 °C. A phase transition is observed in LEED patterns taken at elevated temperatures. At temperatures above ∼300 °C, the LEED patterns show only the periodicity of the roughly hexagonal closest-packed S atoms, whereas at lower temperatures, additional LEED spots appear which reflect the ordering of Fe vacancies. This transition is reversible. STM images taken at negative bias voltages exhibit triangular terraces separated by steps. The measured step heights are integer multiples of 2.9 Å, which is one quarter of the 4C Fe₇S₈ unit cell size in the c direction, and corresponds to the distance between two consecutive Fe or S layers. Although the STM images of single terraces appear to have an atomic arrangement corresponding to the ordering within those Fe layers which contain vacancies, bright spots in the images are most likely to represent S atoms, with a vacancy ordering which is induced by the Fe vacancies. This conclusion is supported by experimental STM images, which show a reversed orientation of the surface geometry on successive terraces when separated by steps of 2.9 Å, and by quantum mechanical calculations of STM images which show S 3p-like states at the top of the valence band. Pyrrhotite (001) surfaces contain triangular etch pits with dimensions ranging from the atomic scale to more than 100 Å across. These are formed by the successive removal of Fe₃S₃ units from the surface. Images taken following exposure of the surface to 6000 L O₂ did not alter the flat terraces, but the formation of adsorptive structures near steps, especially at corners, was observed.