X-ray study of bulk and surface order of vanadium(2) hydrogen near its tricritical point

We present results of an X-ray scattering study of the bulk and surface critical behavior of V{dollar}sb2{dollar}H close to the tricritical point of its {dollar}betasb1{lcub}-{rcub}betasb2{dollar} phase transition. The critical behavior of VH{dollar}sb{lcub}rm x{rcub}{dollar} (x = 0.48-0.51) was analyzed as a function of hydrogen concentration. The bulk critical behavior was studied in detail at the atomic concentration of 0.51 (H/V) using in-house bulk x-ray scattering, from which the long-range order and the pair-correlation function critical exponents were determined. The bulk critical diffuse scattering is shown to be highly anisotropic with a very unusual (and yet unexplained) temperature dependence. At the concentration of 0.48 this system crosses a two-phase coexistence region which limits the observable critical region, evidencing the proximity of the tricritical point. The critical behavior of the (010) surface was studied for 2 different concentrations using depth-resolved x-ray grazing incidence diffraction. From the determination of the surface long-range order exponent {dollar}betasb{lcub}rm surf{rcub},{dollar} we demonstrate the existence of a surface ordinary transition for both compositions, even with the presence of a protective oxide layer which introduces fluctuating interaction constants at the surface (quenched surface disorder). We do observe, however, a "surviving" surface Bragg scattering above the bulk critical temperature which we associate with the stress imposed by the disordered cap layer. This remaining long-range order results in a clear damping of surface critical diffuse scattering compared to the bulk where critical fluctuations are shown to be present.