Figure 1

(Color) Illustrations of hematite (001) surface structures. Blue spheres represent oxygen; in (c), they represent the uppermost oxygen layer. Red spheres represent Fe; in (c) they represent Fe in "B" and "C" octahedral sites below the uppermost oxygen layer. Every third octahedral site is vacant. Green circles represent Fe in surface sites above the uppermost oxygen layer. (a) Illustration of the Fe termination; Fe resides in trigonal surface sites over vacancies in the underlying octahedral layer [see the 3V sites in panel (c)]. (b) The O termination; the uppermost Fe ions in (a) have simply been removed (relaxations, as well as adsorbed H₂O and its dissociation products are not shown). (c) Illustration of some possible "adatom" or "adsorbed" Fe sites. The 3V site is an Fe over a vacancy (V) in the underlying octahedral layer; this is also called the "A" site and can be part of a bulk termination [identical to the uppermost Fe in panel (a)]. The F site shares an octahedral face with an underlying octahedral Fe, and although it could be part of a bulk termination, such sites have not been observed. The 2V and 1V sites represent Fe in or near a 3V site but with one and two bonds to the surface severed. The 2E and 1E sites are Fe in sites sharing 2 and 1 edge with underlying octahedra. The 2V, 1V, 2E, and 1E sites (yellow boxes) do not occur as part of a termination of the hematite bulk structure.