Structural study of Pr1?xCaxMnO3 and Y1?xCaxMnO3 perovskites

The structural properties of Prx_1?xCa_xMnO₃ and Y_1?xCa_xMnO₃ perovskite systems were studied by X-ray and magnetic susceptibility measurements. The differences in the structural properties of both systems are attributed to the differences of tolerance factors, i.e. of ionic radii of the ions present and the results are interpreted as being due to simultaneous action of steric effect, orbit-lattice interactions of Mn³⁺ ions and Coulomb interactions among 3d electrons. Their individual contributions depending on the compositions are analyzed. Besides the usual cooperative Jahn-Teller effect existing in PrMnO₃ a new type of transformation was found which is connected, under condition of Mn³⁺ ? Mn⁴⁺ valency migration, with the formation of local Jahn-Teller distortions. It is characterized by the equality of v_hop and v_phon at a critical temperature T_crit; it has been found that T_crit depends only very weakly on the composition. A correpsonding O'→O change in the yttrium system is, however, rather continuous which is explained by a strong influence of the cooperative buckling effect. An analysis of the magnetic data points to the presence of double exchange in the praseodymium system at high temperatures, while at low temperatures this mechanism applies only in the region, where cooperative Jahn-Teller effect exists, i.e. at x0.4.