Influence of atomic order on TA₁[110] phonon softening [4] and displacive phase transition in Fe₇₂Pt₂₈ Invar alloys

Abstract:

The acoustic phonon dispersions of two Invar crystals Fe₇₂Pt₂₈, one ordered with the L1₁ (Cu₃Au) structure, the other disordered fcc, have been investigated between 3.4 K and 470 K by inelastic and elastic neutron scattering. For the ordered crystal, pronounced softening of the whole TA₁ $[\xi \xi 0]$ phonon branch is observed on cooling below the Curie temperature. Particularly strong phonon softening at the M-point zone boundary of the L1₂ structure leads to a displacive, antiferrodistortive phase transition at low temperatures. For the disordered crystal, much weaker softening of the TA₁ $[\xi \xi 0]$ phonons is observed and restricted to the region near the Brillouin zone center, where increasing elastic scattering with decreasing temperature indicates the growth of local tetragonal strain. This strain is considered as a typical precursor of the transformation to bct martensite. Specific heat measurements, performed at low temperatures on both crystals confirm the neutron scattering results and reveal considerable enhancement of the low energy phonon density of states in the ordered crystal.