Transport, magnetic, and structural properties of La_1−xM_xMnO₃ (M=Ba,Sr, Ca) for0<∼x<∼0.20

Abstract

At high temperatures both the resistivity ( ρ ) and thermopower of La_1−xM_xMnO₃ ( M = Ba , Sr, Ca) single crystals can be described well by a small-polaron hopping model. The polaron binding energy decreases while the orbital ordering transition ( T_JT ) to a state with a static Jahn-Teller distortion increases as the ionic size of M reduces from Ba to Ca. With doping ( x ) , T_JT decreases and Curie temperature T_C increases. Orthorhombic to rhombohedral transition ( T_OR ) decreases almost linearly with x up to 0.125 and 0.20 for Ba- and Sr-doped samples, respectively. For Ca-doped system, T_OR decreases at a much slower rate with x. Unlike Sr-doped sample, both Ca- and Ba-doped systems do not show large drop in ρ at T_C followed by an abrupt increase below T_C for x = 1 / 8 doping. Both distortion and disorder due to the difference in size between La and M affect transport, magnetic, and structural properties of manganites. A phase diagram has been constructed from this study.