Nb-Ru is a high temperature shape memory alloy that undergoes a Martensitic transformation from a parent cubic b-phase into a tetragonal b phase at TM 900 C. Measurements of the phonon dispersion curves show that the [110]-TA2 phonon branch, corresponding in the q=0 limit to the elastic constant C=1/2(C11-C12) has an anomalous temperature dependence. Nearly the entire branch softens with decreasing temperature as TM is approached. The temperature dependence of the low-q phonon energies suggests that the elastic constants would approach 0 as T approaches TM, indicating a second order transition. No additional lattice modulation is observed in the cubic phase.
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