Characteristic length scale for strain fields around impurity cations in perovskites

Lattice parameters have been determined from high-resolution neutron-powder diffraction data collected as a function of temperature for perovskites in the Pr_1-xLaₓAlO₃ solid solution in order to determine the influence of La doping on the Imma↔C2/m transition temperature, Tc^. A plateau in Tc^ at ~150 K extends to x≈0.016±0.002, followed by a linear reduction with increasing LaAlO₃ content. The plateau is interpreted in terms of a saturation effect such that there is a composition limit below which the strain fields surrounding individual La³⁺ ions are isolated in the PrAlO₃ matrix and above which they overlap. On this basis the strain fields are estimated to have diameters of ~16--18, which appears to be a characteristic length scale for structural relaxation around dopant atoms and other types of local clusters in oxide perovskites.