Synthesis and properties of the vacancy-free La_1-xBa_xMnO₃

Thermogravimetric analysis was used to determine the synthesis conditions for obtaining samples of the Ba-substituted LaMnO₃ with stoichiometric oxygen content. Single-phase and vacancy-free samples of La_1-xBa_xMnO₃ have been prepared for x=0.12–0.24 in air by quenching from 1100–1450 °C. The x<0.12 compositions require synthesis under reduced oxygen pressure. The structure was studied using neutron powder diffraction and is discussed in the framework of the tolerance factor concept. At room temperature, the structural transition from orthorhombic to rhombohedral symmetry occurs around x=0.13. The structural, magnetic, and resistive measurements show that properties are similar to the Sr-substituted system for slightly smaller substitution level. Curie temperatures are higher than for the Sr-substituted samples for small x, x=0.10–0.14, due to a better match of the Mn-O and La(Ba)-O bond lengths, and are slightly lower for larger x, x=0.20–0.24 due to an increased local distortion. The largest magnetoresistive effect at room temperature was found for x=0.22. By comparing the Ba- and Sr-substituted systems it is shown that structural properties are governed by the ionic sizes while physical properties are controlled by both the ionic sizes and the hole doping.