PHONON DENSITIES OF STATES AND VIBRATIONAL ENTROPIES OF ORDERED AND DISORDERED NI3AL

We performed inelastic neutron-scattering measurements on powdered Ni3Al. The alloy was prepared in two states of chemical order: (1) with equilibrium L1(2) order, and (2) with disorder (the material was a fee solid solution prepared by high-energy ball milling). Procedures to convert the energy loss spectra into approximate phonon density of states (DOS) curves for Ni3Al in the two states of chemical order were guided by Born-von Karman analyses with force constants obtained from previous single-crystal experiments on L Ii-ordered Ni,AI and fee Ni metal. The main difference in the phonon DOS of the ordered and disordered alloys occurs near 39 meV, the energy of a peak arising from optical modes in the ordered alloy. These high-frequency optical modes involve primarily the vibrations of the aluminum-rich sublattice. The disordered alloy, which does not have such a sublattice, shows much less intensity at this energy. This difference in the phonon DOS around 39 meV is the main contributor to the difference in vibrational entropy of disordered and ordered Ni3Al, which we estimate to be S-vib(dis)-S-vib(ord) = (+0.2+/-0.1)k(B)/atom at high temperatures.