Measurements of neutron inelastic scattering, thermal expansion and low-tem
perature heat capacity were used to study the phonon entropy of the martens
ite-to-austenite transformation in NiTi, which is found to be (0.5 +/- 0.05
) kB atom(-1). The inelastic scattering spectra were corrected for the diff
ering scattering amplitudes of Ni and Ti through a lattice dynamics simulat
ion. The phonon density-of-states (DOS) curves so obtained account for all
the transformation entropy measured by calorimetry. From simulations and th
e measured DOS, the vibrational entropy of austenite was found to be larger
than that of martensite because the transverse acoustic (TA(2)) and longit
udinal acoustic modes of austenite are softer than those of martensite. Sim
ulations suggest that this originates with a large and negative first-neare
st-neighbour transverse force constant. This Phi (1nn)(T) may also be respo
nsible for the soft modes involved in the mechanism of the martensitic tran
sformation.