Phase stability, structural and magnetic properties of some CMR manganites

Discoveries of colossal magneto-resistance (CMR) in the perovskite manganites, Ln(1-x)A(x)MnO(3) (Ln = La, rare earth; A = divalent ions), as well as in the layered manganites, (La,Sr)(3)Mn2O7, has created a renewed interest in them. Their physical characteristics point to the existence of a fine interplay between spin, charge and lattice interactions. Substitutional studies, particularly at the rare earth and Mn sites in these compounds, can throw light on important crystal chemical aspects. Through systematic site-selective substitutions we have tried to evaluate the phase stability of some of these layered and perovskite manganites. In particular, our study on the layered manganites, La-1.2(Sr1-xCax)(1.8)Mn2O7, and Ca3-xLaxMn2O7, establishes that the compositions Ca3-xLaxMn2O7 (1.2 less than or equal to x less than or equal to 2.3) do not form layered compounds with Sr3Ti2O7 - structure, in contrast to literature reports. Instead, they form multiphase mixtures with La1-xCaxMnO3 as the majority phases which, in fact, determine their magnetic and transport behavior, By careful structural and magnetic studies on La0.7Ce0.3MnO3 and La0.7MnO3 we have also proved that the compositions La1-xCexMnO3 do not form electron-doped manganites, but actually yield multiphase mixtures comprising La1-xMnO3 and CeO2. Lastly, we demonstrate that both Co and Cr as dopant ions destabilize the double-exchange mechanism driven ferromagnetism and metallic characters in La0.67Ca0.33Mn1-xCoxO3 and La0.67Ca0.33Mn1-yCryO3, and highlight the differences in the nature of the magnetic exchange interaction of Mn ions with Co, and with Cr ions in these hole-doped perovskite manganites.