Lattice Dynamics of Fe_(0.65)Ni_(0.35) Classical Invar

摘要

The lattice dynamics of the invar Fe_(0.65)Ni_(0.35) single crystal was investigated at room temperature. The photon dispersion relations along the [100], [110], and [111] directions are measured by inelastic neutron scattering. The Born-von Karman analysis, using the seven-neighbour model, is carried out and the force constants and elastic constants are calculated. The phonon frequency shifts triangle open #omega#(q) and the self-energy #SIGMA#_j(q, #omega#_j(q)) functions are determined for Fe_(0.65)Ni_(0.35) from the experimental phonon frequencies for the alloy and Ni crystals. The dips observed on these functions made it possible to disclose the same phonon softening on longitudinal L [100] branch as is on transverse T_1 [110] branch. The softening of the longitudinal branches disclosed by these functions has been observed for the first time in these investigations. The rich and coherent structure of these functions seems to contain a lot of essential information on interatomic coupling and will enable testing of various models proposed to explain the invar effect.