PSEUDOROTATIONAL AVERAGING OF EPR-SPECTRUM OF CU(II)O5 COMPLEX IN YBA2CU3O7-DELTA IN LOW-TEMPERATURES

In low temperatures the condensation of oxygen was found to occur at 05 site in an elementary cell of YBa2Cu3O7-delta being the fifth ligand forming the CuO5 complex around Cu1 copper in chain. This change of coordination from CuO4 to CuO5 is the origin of a pseudorotation related to a strong vibronic coupling of two distorted configurations: a tetragonal pyramid C4v and a trigonal bipyramid D3h, which yields a pseudocubic EPR spectrum in low temperatures. The averaged spectroscopic splitting coefficient is related to a superposition of vibronically coupled orbital states \x2-y2> and \3z2-r2>. The averaged spectrum was for the first time observed in low temperatures since the oxygen condensation in YBa2Cu3O7-delta at 05 site of the chain occur only when oxygen undergoing fast diffusion among the chains, gets localized with decreasing temperature. The activation energy of oxygen desorption from the 05 site is 36 K.