Anomalous phonon behavior in the high-temperature shape-memory alloy Ti₅₀Pd_50−xCr_x

Ti₅₀Pd_50−xCr_x is a high-temperature shape-memory alloy with a martensitic transformation temperature strongly dependent on the Cr composition. Prior to the transformation, a premartensitic phase is present with an incommensurate modulated cubic lattice with wave vector of q₀=(0.22,0.22,0). The temperature dependence of the diffuse scattering in the cubic phase is measured as a function temperature for x=6.5, 8.5, and 10 at. %. The lattice dynamics has been studied and reveals anomalous temperature and q dependences of the [110]-TA₂ transverse phonon branch. The phonon linewidth is broad over the entire Brillouin zone and increases with decreasing temperature, contrary to the behavior expected for anharmonicity. No anomaly is observed at q₀. The results are compared with first principles calculation of the phonon structure.