SYNTHESIS AND PROPERTIES OF THE VACANCY-FREE LA1-XBAXMNO3

Thermogravimetric analysis was used to determine the synthesis conditi ons for obtaining samples of the Ba-substituted LaMnO3 with stoichiome tric oxygen content. Single-phase and vacancy-free samples of La1-xBax MnO3 have been prepared for x=0.12-0.24 in air by quenching from 1100- 1450 degrees C. The x <0.12 compositions require synthesis under reduc ed oxygen pressure. The structure was studied using neutron powder dif fraction and is discussed in the framework of the tolerance factor con cept. At room temperature, the structural transition from orthorhombic to rhombohedral symmetry occurs around x=0.13. The structural, magnet ic, and resistive measurements show that properties are similar to the Sr-substituted system for slightly smaller substitution level. Curie temperatures an 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 l engths, and are slightly lower for larger x, a = 0.20-0.24 due to an i ncreased local distortion. The largest magnetoresistive effect at room temperature was found for x=0.22. By comparing the Ba- and Sr-substit uted systems it is shown that structural properties are governed by th e ionic sizes while physical properties an controlled by both the ioni c sizes and the hole doping. [S0163-1829(98)01529-X].