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 Nd_2-xCe_xCuO₄ and T^*-phase Pr_2-x-yCe_xSr_yCuO₄. 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.