Lattice dynamics of the high-temperature shape-memory alloy Nb-Ru

Nb-Ru is a high-temperature shape-memory alloy that undergoes a martensitic transformation from a parent cubic β -phase into a tetragonal β′ phase at TM ∼900°C. Measurements of the phonon dispersion curves on a single crystal show that the [110]- T A2 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.