Structural and electronic properties of technologically important lead-zirconate-titanate (PZT) crystals produced by Nb-doping are reported. The results are obtained using semi-empirical quantum-chemical simulations applied to a tetragonal PbZr0.53Ti0.47O3 monocrystal, i.e. the morphotropic phase boundary of the PZT. The proposed concentration of Nb impurity is 1.5%, which, according to some experimental data, gives better piezoelectric properties of the material at this concentration. Analysis of the displacements of atoms surrounding the defect, changes in atomic charges and electronic properties explaining the augmentation of electric conductivity are given in light of the available experimental data. The results suggest the occurrence of free electrons in the conduction band of the material. The former observation might explain the formation of conducting Jahn-Teller (JT) electron polarons previously found in the other titanates.