1 Structural Properties of PbTiO 3 and PbZr x Ti 1-x O 3 : a Quantum-Chemical Study ARVIDS STASHANS, a, b CESAR ZAMBRANO, b, a ALFREDO SANCHEZ, b, a LUIS MIGUEL PROCEL b, a a Centro de Investigación en Física de la Materia Condensada, Corporación de Física Fundamental y Aplicada, Apartado 17-12-637, Quito, Ecuador b Departamento de Química, Colegio Politécnico, Universidad San Francisco de Quito, Apartado 17-12-841, Quito, Ecuador ABSTRACT: We investigate the structural and electronic properties of a pure and Zr-doped PbTiO 3 crystals. Nature of atomic relaxation around the Zr impurity is studied through quantum-chemical simulations based on the Hartree-Fock theory and a periodic large unit cell (LUC) model adopted within the so-called intermediate neglect of differential overlap (INDO) approximation. The most stable defect configurations are predicted for different impurity concentrations. The obtained results are compared with another theoretical studies and a number of experimental measurements carried out on this technologically important perovskite-type crystal. Key words: PbTiO 3 ; PbZrTiO 3 ; LUC; impurity-doping; structural and electronic properties