Cobalt oxide (CoO) films epitaxially grown on Ir(100) in (111) orientation were investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. We find with high crystallographic precision that in the bulk of the films the rocksalt structure prevails while near the surface there is a switch towards the wurtzite structure. As a consequence, nanosized CoO cannot be considered as a single structural phase. The film surfaces prove to be metallic, apparently connected with polarity compensation.

• Received 25 April 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.016103