The underdoped-overdoped transition in YBa_2Cu_3O_x
  Oxygen doping in metallic YBa_2Cu_3O_x induces quadrupolar "alpha-ortho'', and breathing "beta-ortho" deformations of the CuO_2 planes. Breathing beta-ortho deformations favour hybridizations of the pd sigma Cu3d_x^2-y^2 - O2p_x,y with the pd pi Cu3d_x,z, 3d_y,z-O2p_z bands relaxing the confinement of the carriers in the overdoped regime, x>6.95. 
J. R\"ohler (Universit\"at zu K\"oln)