Phonon DOS and pDOS of ordered FeV (green) are clearly softened compared to the disordered phase (black).
When alloy systems undergo chemical ordering, the stiffness of bonds is increased due to the ordering transition and thus in general an ordered phase has less vibrational entropy than a disordered phase. However, in recent HPCAT experiments, Munoz and coworkers have measured phonon spectra of FeV as a B2 ordered compound and as a bcc solid solution. The data showed that phonons in the B2 ordered phase are softer than in the bcc solid solution. This result is contrary to the behavior of ordering alloys to date. The unexpected experimental results were explained from first-principles calculations which showed that electronic screening is enhanced during ordering process and thus the average energy of phonons was reduced. [Munoz et al, PRL 107, 115501, 2011]