The valence of Ru, Ce and Eu ions in the magneto-superconductor Eu1.5Ce0.5RuSr2Cu2O10

The superconducting (T-c similar to 32K) Eu1.5Ce0.5RuSr2Cu2O10 (Ru-2122) ma terial is also magnetically ordered (T-M similar to 122 K) with T-M much gr eater than T-c. Superconductivity (SC) is confined to the CuO2 planes, wher eas magnetism is due to the Ru sublattice. Mossbauer spectroscopy performed at 90 and 300 K on (EU)-E-151 shows a single narrow line with an isomer sh ift = 0.69(2) and a quadrupole splitting of 1.84 mm/s, indicating that the Eu ions are trivalent with a nonmagnetic J = 0 ground state. This is in agr eement with X-ray-absorption spectroscopy (XAS) taken at L-III edges of Eu, Ce which shows that Eu is trivalent and Ce is tetravalent. XAS experiments at the K edge of Ru indicate that Ru is pentavalent. This indicates, that in the M-2122 system, SC exists only for pentavalent M ions such as Ta, Nb and Ru. (C) 1999 Elsevier Science B.V. All rights reserved.