Add to ORKGThe relaxation of Yb3+ in YBa2 Cu3 Ox (6<x<7) was studied using electron paramagnetic resonance. It was found that both electronic and phononic processes contribute to the Yb3+ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy (∼500 K) optical phonons or an Orbach-type process via the excited vibronic levels of the Cu2+ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping x=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below Tc. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible fo...