TA(1)[110] phonon dispersion and martensitic phase transition in ordered alloys Fe3Pt

In a previous neutron scattering study, we had observed that the TA(1)[xi x i 0] phonon softening in Ll(2)-ordered ferromagnetic Fe72Pt28 Invar is pron ounced at the zone boundary M-point and leads to an antiferrodistortive pha se transition at low temperatures. Here, we report on similar neutron scatt ering investigations on two ordered crystals with higher Fe content to inve stigate the relation between the TA(1)[xi xi 0] phonon softening and the ma rtensitic transformation, which occurs in Fe-rich ordered Fe-Pt. We find th at the TA(1)[xi xi 0] phonon softening, especially at the M-point zone boun dary, does not depend on the composition of the investigated crystals. In F e74.5Pt25.5, however, the antiferrodistortive phase transition temperature is enhanced due to tetragonal strain preceding the martensitic transition. In Fe77Pt23 a precursor driven premartensitic phase transition is not obser ved. The structure of the martensite is, however, influenced by the soft mo de lattice instability of the austenite. This would explain the origin of s tructural details found previously for Fe3Pt thermoelastic martensite.