Abstract
From Mo¨ssbauer measurements on 155Gd and 151Eu in REBa 2Cu 3O 7−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 EuBa 2Cu 3O 7 and EuBa 2Cu 3O 6 shows that the contribution of the excited electronic state of Eu is much stronger in the O 7 than in the O 6 compound. 170Yb Mo¨ssbauer 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 Mo¨ssbauer probes we get a Debye temperature ⊚ D ≊ 270 (15) K for the O 7 compounds and EuBa 2Cu 3O 6.
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