Influence of atomic order on TA(1)[110] phonon softening and displacive phase transition in Fe72Pt28 Invar alloys

The acoustic phonon dispersions of two Invar crystals Fe72Pt28, one ordered with the L1(2) (Cu3Au) structure, the other disordered fee, have been inve stigated between 3.4 K and 470 K by inelastic and elastic neutron scatterin g. For the ordered crystal, pronounced softening of the whole TA(1)[xi xi 0 ] phonon branch is observed on cooling below the Curie temperature. Particu larly strong phonon softening at the M-point zone boundary of the L1(2) str ucture leads to a displacive, antiferrodistortive phase transi tion at low temperatures. For the disordered crystal, much weaker softening of the TA(1 )[xi xi 0] phonons is observed and restricted to the region near title Bril louin zone center, where increasing elastic scattering with decreasing temp erature indicates the growth of local tetragonal strain. This strain is con sidered as a typical precursor of the transformation to bet martensite. Spe cific heat measurements, performed at low temperatures on both crystals con firm the neutron scattering results and reveal considerable enhancement of the low energy phonon density of states in the ordered crystal.