Carrier doping in Pr_{1+x}Ba_{2-x}Cu_3O_7 studied by NMR
  We investigated the type of the carriers doped in the CuO$_2$-planes of non-superconducting Pr$_{1+x}$Ba$_{2-x}$Cu$_3$O$_7$ single crystals by nuclear magnetic resonance of Pr and chain-site Cu. The different spectra in the solid solution system show that the holes in Pr-rich crystals are localized in states similar to the ones in stoichiometric samples, that is in the $4f^2-2p\pi$-hybridization band proposed by Fehrenbacher and Rice. In contrast, the spectra of crystals prepared under conditions favoring Ba-rich phases indicate the presence of holes in Zhang-Rice singlets in the CuO$_2$-plane. Our results strongly support the model that superconductivity reported recently for Pr123-crystals is due to Ba-rich regions in the inhomogeneous samples. 
M.W. Pieper, F. Wiekhorst, T. Wolf