Compositional variation of the phonon dispersion curves of bcc Fe-Ga alloys

Inelastic neutron scattering techniques have been used to measure the phonon dispersion curves of bcc Fe_1-xGaₓ ($x=10.8$, 13.3, 16.0, 22.5) alloys as a function of Ga concentration. The phonon frequencies of every branch were found to decrease significantly with increasing Ga concentration. The softening was most pronounced for the T₂[ξξ0] branch and, to a lesser extent, the L[ξξξ] branch in the vicinity of ξ=2/3. The concentration dependence of the shear elastic constant C^'=1∕2(C₁₁-C₁₂), calculated from the slope of the T₂[ξξ0] branch, was found to agree with the results of sound velocity measurements. For the higher concentration sample measured, 22.5 at. % Ga, new branches appeared, an effect associated with the increase in the number of atoms per unit cell.