THE PRESSURE-DEPENDENCE OF THE TA(1)[110] PHONON FREQUENCIES IN THE ORDERED INVAR ALLOY FE72PT28 AT PRESSURES UP TO 7 GPA

The pressure dependence of the TA(1) [110] phonon frequencies in order ed Fe72Pt28 was studied by inelastic neutron scattering up to 7 GPa. V ery large frequency shifts (similar to 50%) were observed in this pres sure range. We found that the mode Gruneisen parameters remain very la rge (similar to 8) for pressures exceeding the critical pressure of ab out 3 GPa which drives ordered Fe72Pt28 from a high-spin to a low-spin state. This indicates that the unusually large Gruneisen parameters a re probably not a magnetoelastic effect, but rather have to be associa ted with the martensitic instability.