  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.
