Rare-earth valence and doping in T-, T, and T*-phase R2CuO4 (R=rare earths)

Charge-carrier doping strongly affects the magnetic, transport, and superconducting properties of the T-, T-, and T*-phase R2CuO4 compounds (R=rare earths). The nature of doping and doping level depend not only on the heterovalent cation substitution and oxygen stoichiometry but also on the rare-earth valence. We have studied the valence of various rare earths in the R2CuO4 materials using x-ray-absorption spectroscopy. We found that Ce is tetravalent in both T-phase Nd2-xCexCuO4 and T*-phase Pr2-x-yCexSryCuO4. Pr is trivalent in the T and T structures. In the T* structure, both Pr and Tb have an average valence greater than 3+, very likely arising from an inhomogeneous mixture of trivalent and tetravalent ions, and the latter reside at the eightfold-oxygen-coordinated position. Other rare earths are trivalent in the T, T, and T* phases.