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Structural relaxation induced by hydrostatic pressure is experimentally observed for the metallic glass Pd 40 Ni 40 P 20 . The pressure dependence of the structural state is displayed in the shape of the glasstransition peak that is measured by means of differential scanning calorimetry after annealing treatments at 563 K and pressures up to 0.75 GPa. Quantification of the effect using the free-volume theory yields a formation volume of 5.9 6 0.5 Å 3 for defects in this glass. In combination with the previously determined pressure dependence for the Au diffusivity, a value of 5 6 4 Å 3 is derived for the migration volume for Au diffusion. [S0031-9007(97) PACS numbers: 61.43. Dq, 61.72.Hh, 62.50. + p, Structural relaxation in metallic glasses under the influence of an elevated temperature is a well-known phenomenon that has been widely studied (e.g., [1]). Within the amorphous structure, changes occur that lead to significant changes in physical properties, which are particularly pronounced in the atomic-transport properties viscosity and diffusivity [2]. Although pressure, similar to temperature, is liable to induce structural relaxation as well, experimental studies of pressure-induced structural relaxation are very few [3]. The pressure dependence of the structural state of metallic glasses is particularly of interest when regarding the diffusion process, since it can give insight into the formation volume for defects. Combined with the pressure dependence of the diffusivity, which yields the so-called activation volume, the migration volume for diffusion can also be derived. A separate determination of these two quantities adds to the understanding of the diffusion process.In the present Letter the phenomenon of structural relaxation induced by pressure will be investigated for the metallic glass Pd 40 Ni 40 P 20 . Experimental evidence obtained by means of differential scanning calorimetry (DSC) after annealing samples at hydrostatic pressures up to 0.75 GPa directly shows the occurrence of pressureinduced structural relaxation. The state of relaxation that is reached during these treatments is quantified by means of the free-volume theory [2,4]. This results in the determination of the formation volume for defects in amorphous Pd 40 Ni 40 P 20 . Combining the result with the pressure dependence of the diffusivity of gold determined previously on the same samples [5] yields a quantitative estimate for the migration volume for diffusion of Au in amorphous Pd 40 Ni 40 P 20 as well.Experiments were performed on a set of amorphous Pd 40 Ni 40 P 20 ribbons, being the very ribbons on which previously the pressure effect of the diffusivity has been measured [5]. The ribbons were first brought in metastable equilibrium, without inducing crystallization, by annealing at ambient pressure and a temperature of 563 K during 10 000 s. Under these conditions equilibrium is reached in approximately 5000 s [6]. Subsequently, the samples were annealed for 43 200 s at 563.0 6 0.1 K and hydrostatic pressures up to 0.75 GPa ͑60...

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