STRUCTURE AND DISORDERING OF THE c(4X2) RECONSTRUCTION OF Ge(001) SURFACES. S. Ferrer, X. Torrelles, J. Alvarez, V. H. Etgens*, H. A. van der Vegt** and P. Fajardo, ESRF, BP 220, 38043 Grenoble Cedex, France, *Philips Semiconductors Nijmegen, Gerstweg 2, 6534 AE Nijmegen, The Netherlands **CNRS-LMC, Universite, Paris VI, 4 Place Jussieu,75005 Paris Cedex, France

The structure of the low temperature reconstruction of Ge(001) has been investigated by X ray diffraction. It consists in an array of buckled surface dimers with antiferromagnetic ordering. Upon increasing temperatures the surface disorders via a phase transition that may be described by the critical exponents of a mean field transition. The structure of the disordered phase has also been studied in detail.