The initial stages of Cr and Ti growth on SiO2(0001)

The initial stages of titanium growth on the 1 x 1 and (root 84 x root 84)R 11 degrees terminations and chromium growth on the (root 84 x root 84)R11 d egrees termination of alpha-quartz SiO2(0001) have been studied using polar ization-dependent SEXAFS. Three metal coverages have been examined: nominal ly 0.25, 0.5 and 1.0 ML. The Ti K-edge results evidence reduction of the se lvedge through the formation of a titanium oxide; with metallic Ti clusters formed at higher coverages. Ti-O distances of 1.96-2.00 +/- 0.03 Angstrom are observed along with Ti-Ti distances of 2.89-2.92 +/- 0.03 Angstrom. The former range is close to the Ti-O bond distances in rutile, whereas the la tter is within the experimental error of that of bulk titanium. The Cr K-ed ge results are consistent with the formation of 15 atom clusters having an average nearest-neighbour Cr-Cr distance of 2.36 +/- 0.03 Angstrom. This re presents a contraction of about 6% from the bulk bcc lattice spacing. Such a contraction is consistent with the prediction of recent calculations. (C) 1999 Elsevier Science B.V. All rights reserved.