X-ray structure analysis of the Cr2O3(0001)-(1 x 1) surface:: evidence for Cr interstitial

The geometric structure of the unreconstructed Cr2O3(0001) surface has been investigated by surface X-ray diffraction. Our analysis provides evidence that at room temperature the surface is terminated by a disordered surface Cr3+ arrangement. It is characterized by a 2/3 occupation probability of the top layer site and a 1/3 occupation probability of the interstitial site between the first and second O2- layer. This site is unoccupied in the bulk. The total occupation of one Cr3+ ion per (1 x 1) unit cell is in agreement with the requirement for electrostatic stability. The surface Cr3+ ions relax vertically by -6%. The coordination polyhedra around the top- and second-layer Cr3+ ions are almost identical with the bulk polyhedra. The interstitial site model provides an explanation for the order-disorder [(root 3 x root 3)-->(1 x 1)] and order-order [(1 x 1)-->(root 3 x root 3)] phase transitions at 150 K and 90 K respectively. (C) 1999 Elsevier Science B.V. All rights reserved.