The effect of a cation radii on physical properties of doped manganites

The effect of divalent substitution into lanthanide sites in La0.7Sr0.3-xCa xMnO3 oxides has been studied. In this system the average size (r(A)) Of th e A site cations is systematically varied while keeping the Mn3+/Mn4+ ratio fixed at 7/3. Polycrystalline samples of La0.7Sr0.3-xCaxMnO3 were prepared using solid-state reaction. X-ray diffraction patterns show that these com pounds exist in a single-phase having a perovskite-like structure at room t emperature. The paramagnetic to ferromagnetic transition temperature Te inc reases from 260 to 370 K when (r(A)) varies from 1.205 to 1.244 Angstrom. T he ionic radii variance sigma(2) increases with the increase of (r(A)). The double-exchange energy is sensitive to the average ionic radius of the A s ite (r(A)) and to the variance sigma(2) of the A cation radii distribution. The structural, magnetic and electrical properties are correlated with the change of Mn-O-Mn bond angle. The variation of the activation energy E-a i n our samples can be related to the existence of two structures, cubic and rhombohedral.