Phonon dispersion relations in PrBa2CU3O6+x(x0.2)

We report measurements of the phonon dispersion relations in nonsuperconducting, oxygen-deficient PrBa 2 CU 3 O 6 + x (x0.2) by inelastic neutron scattering. The data are compared with a model of the lattice dynamics based on a common interatomic potential. Good agreement is achieved for all but two phonon branches, which are significantly softer than predicted. These modes are found to arise predominantly from motion of the oxygen ions in the CuO 2 planes. Analogous modes in YBa 2 Cu 3 O 6 are well described by the common interatomic potential model.