  Nitinol (NiTi), the most widely used shape-memory alloy, exhibits an
austenite phase that has yet to be identified. The usually assumed austenite
structure is cubic B2, which has imaginary phonon modes, hence it is unstable.
We suggest a stable austenite structure that on average has B2 symmetry
(observed by X-ray and neutron diffraction), but exhibits finite atomic
displacements from the ideal B2 sites. The proposed structure has a phonon
spectrum that agrees with that from neutron scattering, has diffraction spectra
in agreement with XRD, and has an energy relative to the ground state that
agrees with calorimetry data.
