NEAR-SURFACE TRICRITICAL BEHAVIOR OF V2H(010) AT THE BETA(1)-BETA(2) PHASE-TRANSITION

We present results of an x-ray grazing-incidence diffraction experimen t on a V2H(010) surface. The temperature dependence of the in-plane (h /2 0 -h/2) superlattice reflections discloses the surface critical beh avior across the beta(1)-beta(2) phase boundary. The influence of a ne arby bulk tricritical point on the expected values for the critical ex ponents is briefly discussed. Using the depth sensitivity of evanescen t scattering and by a proper subtraction of critical diffuse intensity , we determined the surface critical exponent beta(surf). A remnant Br agg contribution above the critical temperature, however, complicates the evaluation of other surface exponents associated with the critical diffuse scattering. We speculate that the origin of this effect is as sociated with the surface oxide layer necessarily present on the (hydr ogen-loaded) metallic crystal.