Surface phase transition close to a bulk tricritical point: An x-ray studyof ND4Cl

Investigations of the near-surface critical behavior close to the bulk tric ritical point of ND4Cl single crystals are presented. The order parameter i s expected to follow universal power laws with a temperature-dependent cros sover from the bulk to the surface exponent. In contrast to the theory the measurements show an evidence for a surface layer with a constant nonvanish ing order parameter. The thickness of this layer increases in temperature a nd seems to diverge at the critical temperature. The results disagree with the expected universal behavior of surface order parameters. [S0163-1829(99 )07505-0].