Dynamics of iron atoms across the pressure-induced Invar transition in Pd3Fe

Abstract

The $^{57}\mathrm{Fe}$ phonon partial density of states (PDOS) in $L{1}_{2}$-ordered Pd${}_{3}$Fe was studied at high pressures by nuclear resonant inelastic x-ray scattering (NRIXS) measurements and density functional theory (DFT) calculations. The NRIXS spectra showed that the stiffening of the $^{57}\mathrm{Fe}$ PDOS with decreasing volume was slower from 12 to 24$ $GPa owing to the pressure-induced Invar transition in Pd${}_{3}$Fe, with a change from a high-moment ferromagnetic (FM) state to a low-moment (LM) state observed by nuclear forward scattering. Force constants obtained from fitting to a Born--von K\'arm\'an model showed a relative softening of the first-nearest-neighbor (1NN) Fe-Pd longitudinal force constants at the magnetic transition. For the FM low-pressure state, the DFT calculations gave a PDOS and 1NN longitudinal force constants in good agreement with experiment, but discrepancies for the high-pressure LM state suggest the presence of short-range magnetic order.