Onset of dielectric modes at 110 K and 60 K due to local lattice distortions in nonsuperconducting YBa

We report the observation of two dielectric transitions at 110 and 60 K in the microwave response of nonsuperconducting YBa2Cu3O6.0 crystals. The transitions are characterized by a change in polarizability and presence of loss peaks, associated with overdamped dielectric modes. An explanation is presented in terms of changes in polarizability of the apical O atoms in the Ba–O layer, affected by lattice softening at 110 K, due to change in buckling of the Cu–O layer. The onset of another mode at 60 K strongly suggests an additional local lattice change at this temperature. Thus microwave dielectric measurements are sensitive indicators of lattice softening which may be relevant to superconductivity.