c-Axis Intra-Layer Couplings in the CuO_2 Planes of High-T_c Cuprates
  We discuss the static deformations of the doped CuO_2 lattices in YBa_2Cu_3O_x. Intrinsic lattice effects driven by the strong correlations of the electron system are separated from extrinsic chemical and bandstructure effects. c-axis displacements of the planar copper atoms seem to be a generic property of the metallic CuO_2 lattices. 
J. R\"ohler