Phonons in NaO2near the order-disorder transition

The acoustic phonons in cubic Na${\mathrm{O}}_2$ have been studied above and below the orderdisorder transition by inelastic neutron scattering. Above the transition the ${\mathrm{O}}_2^{-}$ ions are disordered and undergoing rapid reorientation about the body diagonals. This reorientation couples with and affects certain acoustic modes in the crystal. The results are described in terms of a phonon-pseudospin coupling theory.