Nominal doping and partition of doped holes between planar and apical orbitals in La_2−xSr_xCuO₄

By considering both the average structural parameters obtained from Rietveld refinement of neutron powder diffraction data, and the local structural parameters obtained from the atomic pair distribution function, we have tested the recent hypothesis of Perry et al. Phys. Rev. B 65 144501 (2002) that doping in a La_2−xSr_xCuO₄ system occurs as localized defects of predominantly Cud_3z²−r²-Op_z character associated with the Sr dopants accompanied by a local destruction of the Jahn-Teller distortion. While the structural parameters behave qualitatively according to the prediction of this model, a quantitative analysis indicates that doped holes predominantly appear in the planar Cud_x²−y²-Op_x,y band as is normally assumed. However, a small amount of the doped charge does enter the Cud_3z²−r²-Op_z orbitals and this should be taken into account when theoretical phase diagrams are compared to experiment. We present a calibration curve, p=x[1.00(1)−0.45(7)x], for the planar charge doping, p, vs strontium content, x, for the La_2−xSr_xCuO₄ system.