Rare-earth valency and electric field gradient in Eu, Gd and Yb based high Tc superconductors

From Moessbauer measurements on 155Gd and 151Eu in REBa2 Cu3O7-δ, we have obtained an estimation of the second order crystal electric field (CEF) parameters at the rare earth site. A comparison of the temperature dependence of the measured electric field gradient (EFG) in EuBa2 Cu3O7 and EuBa2 Cu3O6 shows that the spin-orbit coupling is much stronger in the O6 than in the O7 compound. 170Yb Moessbauer measurements give some information on the EFG due to the 4f electrons. 10% substitution of Ba by Eu almost completely destroy superconductivity. A 151Eu measurement on this compound shows that the valency of Eu is 3+ at the Ba site and that the EFG is about the same at the Ba and the RE site. This is confirmed by a 133Ba perturbed angular correlation (PAC) measurement. From the three Moessbauer probes we get a Debye temperature sum massD ≅ 270 (15) K for the O7 compounds and EuBa2Cu3O6