The 'clock' reconstruction of the Ni(100)(2 x 2)p4g-N system has been measu
red using surface X-ray diffraction. An in-plane displacement of the surfac
e layer Ni atoms of d(xy) = 0.30 +/- 0.01 Angstrom was found. This value is
smaller than that measured for this system using photoelectron diffraction
(PED) and surface extended X-ray absorption fine structure (SEXAFS). Possi
ble reasons for this are discussed. (C) 1999 Elsevier Science B.V. All righ
ts reserved.