Anomalous phonon damping in the high temperature shape memory alloy Ti₅₀Pd₄₂Cr₈

Neutron-scattering measurements of the high temperature shape memory alloy Ti₅₀Pd_50-xCr_x were performed to probe the phonon precursors to the martensitic transformation. For x = 0, the transformation temperature, T_M, is ∼ 800 K [1] and, for the composition studied, x = 8%, it is reduced to ∼ 400 K. Above this temperature the crystal possesses a simple CsCl-type cubic structure. Transverse acoustic phonons propagating along the [ζ ζ 0] direction with atomic displacements along [-ζ ζ 0] were studied at temperatures up to 873 K. The phonons are well defined near the zone center but, for ζ > 0.15, they are strongly over-damped near the transition temperature, and become better defined at higher temperatures. An elastic peak develops in the cubic phase at ζ = 0.22 and increases in intensity as T_M is approached. However, the phonons do not show any characteristic anomaly at this particular ζ.