The mechanism responsible for the stability of the delta-phase in plutonium alloys remains poorly understood despite previous extensive studies. In the present experiments, three alloys (Pu-8at%Am, Pu-15at%Am and Pu-2at%Ga) were studied under pressure by X-ray diffraction in diamond anvil cells and revealed i) a pressure-induced softening in the compressibility of the delta-phase and ii) the presence of an intermediate γ’-phase across the δ → α’ phase transition. The softening, an anomalous behaviour, is shown to be coherent with other known unusual properties found for these alloys like NTE (Negative Thermal Expansion). Furthermore, it has been observed that the cell volume for the α’-phase formed under pressure depends on the rate of the pressure increase. This behaviour may be related to the diffusion of solute atoms to specific sites within the α’-phase unit cell. Such diffusion may also explain the α’-phase unit cell’s volume shrinkage versus time in a Pu-2at%Ga sample kept at a constant pressure.
These XRD results, obtained under pressure, may be used to test first-principle calculations or other general modelling which aim at predicting the solute’s diffusion and/or self-irradiation effects in plutonium alloys.