Structural, transport, and magnetic properties of A-site substituted perovskite manganites

Perovskite manganites, AMnO3, have been an interesting avenue for studying magnetic, resistive, and thermoelectric properties since they are strongly correlated to the crystal structure tunable by selection of charges and sizes of the A-site ions. In this study several unique systems with tolerance factor, t > 1 has been successfully obtained for A = La, Nd, Ba and Sr by development of novel synthesis procedure. These systems predicted to be cubic have a unique behavior since properties are predominately controlled by the change of the [Mn-O] bond length, which is stretched beyond the equilibrium value. X-ray, neutron diffraction, and thermogravimetric measurements confirmed achievement of the desired compounds with A-site cationic disorder and order, respectively. Unexpectedly, compositions with the largest t have been found non-cubic, which may indicate noteworthy distortions of the MnO6 octahedra that warrants detailed future studies of structural and ferroelectric properties. Physical property measurements have confirmed the expected suppression of the magnetic transition temperatures and the insulating properties. Comparison of the A-site ordered with disordered materials of the same chemical compositions showed remarkable increase of the Curie transition temperatures.