Clarification of origin of positive excess volume of Pd–Fe binary alloys by using first-principles calculations and HAXPES

Disordered Pd–Fe alloys show a positive excess-volume effect, contrary to the predictions of the Iida–Guthrie model. The effect becomes larger at a composition of approximately Pd_0.5Fe_0.5. The origin of the positive excess-volume effect was investigated by using first-principles calculations to study the electronic structures in the ordered and disordered states of Pd–Fe alloys with two different compositions, i.e., Pd_0.53Fe_0.47 and Pd_0.75Fe_0.25. The calculated partial density of states for the disordered state in Pd_0.53Fe_0.47 indicates weakening of Pd metallic bonds because of fully occupied 4d majority-spin states with enhanced anti-bonding characteristics, which cause an increase in the volume. In addition, the magneto-volume effect, which minimizes the increase in the electronic kinetic energy caused by exchange splitting, contributes to the positive excess volume. The calculated electronic states and those obtained from hard X-ray photoelectron spectra were consistent with each other.