Pb1-xGdxTe single crystals, doped with x=0.001-0.04, were grown by using a Bridgeman method. The influence of doping element Gd on microstructure and creating of defects into materials were investigated. The experimental results from recent studies of microhardness and structural measurements (X-ray, SEM), are shown. Powder X-ray diffractograms have shown that the cubic-phase NaCl, with a lattice parameter a = 6.46 angstrom, was obtained for all samples with different compositions. To determine the sample's microhardness, the Vickers indentor has been used consecutively loaded by 50 g. The substitutional site of the Gd3+ ions and the influence of the concentration of defects, as vacancies and impurity atoms, on structural parameters are discussed. Here we investigate the concentration dependence of impurity ion sites into host lattice of PbTe, and a model of formation of a three-dimensional superlattice of ionized impurities is presented. (c) 2007 Elsevier B.V. All rights reserved.