Phonon dispersion relations in the disorderedNi1−xPtxsystem

The phonon dispersion relations of the substitutional disordered alloys ${\mathrm{Ni}}_{1-x}{\mathrm{Pt}}_x$ ($x=0.05, 0.30, 0.50, 0.75, \mathrm{and} 0.95$) were studied by neutron inelastic scattering. The resonance mode for the sample with $x=0.05\mathrm{}$ is well described by the mass-defect coherent-potential-approximation (MCPA) theory. Experimental results deviate from the MCPA calculations for the samples with $x\ge 0.30$ and the difference is most conspicuous for the maximum phonon frequency which decrease faster with increasing Pt concentration than the theory predicts. The energy splitting of the phonon dispersion curves at the resonancemode frequency has a branch dependence in contrast to the MCPA prediction; that of the $L$ branch always takes place at a higher frequency than that for the $T$ branch.