SURFACE-INDUCED CHANGES IN THE CRITICAL-BEHAVIOR OF THE BETA(1)-BETA(2) PHASE-TRANSITION IN V2H

The influence of the surface on the beta(1)-beta(2) phase transition o f V2H was studied using bulk and grazing incidence (GID) X-ray diffrac tion, This second-order transition is characterized by the change of t he interstitial occupancy of the hydrogen atoms from ordered to random between (1 0 1) planes, which can be determined from the hydrogen-ind uced distortions of the host lattice. An effective bulk critical expon ent beta was determined to be beta = 0.34 +/- 0.04 at the specific con centration of 49% (H/V), whereas at a slightly higher concentration pr ior data [1] yielded beta = 0.17. The present crystal traverses a two- phase region which in turn may complicate the study of the critical be havior, Our GID results indicate an increase of beta in the near-surfa ce region of the crystal and an ordered phase persisting up to 8 K abo ve the bulk critical temperature, as opposed to a bulk ordered phase t hat persists 1.0 K above the phase boundary. Preliminary data on the o rdering kinetics indicate a somewhat sluggish behavior at the surface which is driven by a more rapid bulk ordering.