SEGREGATION OF ORDERED CRN AND CRC SURFACE PHASES ON A FE-15-PERCENT-CR(100) CRYSTAL

Angle-resolved photoemission of Cr2p(3/2) N1s and C1s core level photo electrons was used to determine the structure of segregated surface ph ases an an Fe-15%Cr(100) single crystal containing dissolved N and C. The surface phases of CrN and CrC were generated at elevated temperatu re by a controlled annealing procedure. Polar angle intensity distribu tions of photoelectrons (X-ray photoelectron diffraction) were recorde d in the [001] and [011] azimuths, respectively. LEED showed a sharp 1 X1 pattern for both surface phases. For CrN the angular intensity dist ributions exhibited forward scattering peaks for Cr2p(3/2) but not for N1s. For CrC, on the other hand, Cr2p(3/2) and C Is showed both stron g forward scattering features in all distributions. It follows that th e segregated surface phases are about two to four layers thick and epi taxed to the substrate (100) surface. The CrN phase consists of two Cr layers with an adsorbed N layer, while the CrC phase consists of at l east three complete planes and has a NaCl structure, with CrC(100)para llel to FeCr(100) and CrC[010]parallel to FeCr[011]. The positions of forward scattering peaks for CrN indicate a lattice expansion between the first and second layer of about 16%.