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

The Fe-57 phonon partial density of states (PDOS) in L1(2)-ordered Pd3Fe 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 57Fe PDOS with decreasing volume was slower from 12 to 24GPa owing to the pressure-induced Invar transition in Pd3Fe, 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 Karman 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.