Temperature evolution of the crystal structure in SrTiO3 doped by W6+, Ni3+, Fe3+ and La3+

Profile analysis of powder diffraction lines was used to determine the changes of the lattice parameters versus temperature in perovskite SrTiO3 doped by W6+, Ni3+, Fe3+ and La3+ cations. The experiments were conducted in the temperature range from 12 K up to 300 K using the X-ray diffraction method. The temperatures of the structural phase transitions (SPT) as well as the type of distortion were determined from the splitting of some cubic lines such as (200), (220), (330), (400), (420). Our experiments revealed a small distortion from the cubic to the tetragonal symmetry. From our studies it also resulted that the tilting of oxygen octahedra is the main mechanism responsible for the SPT. The scheme of these transitions (according to Glazer's notation) may be written as follows: tetragonal (a(0)a(0)c(-)) --> cubic(a(0)a(0)a(0)). For this model the crystal structure at low and high temperature symmetry was determined with the use of the Rietveld profile method.