Thermogravimetric methods were used to determine the conditions which were required in order to obtain samples of Ba-substituted LaMnO3 with a stoichiometric O content. Single-phase and vacancy-free samples of La1-xBaxMnO3, where x ranged from 0.12 to 0.24, were prepared in air by quenching from 1100 to 1450C. Compositions with x-values of less than 0.12 required synthesis under a reduced O pressure. The structure was studied by using neutron and powder diffraction methods, and was analyzed in terms of the tolerance factor concept. At room temperature, the transition from orthorhombic to rhombohedral symmetry occurred at an x-value of about 0.13. Structural, magnetic and resistive measurements showed that the properties were similar to those of the Sr-substituted system, at slightly lower levels of substitution. The Curie temperatures were higher than those for the Sr-substituted samples, for x-values of 0.10 to 0.14, due to a better match of the Mn-O and La(Ba)-O bond lengths. They were slightly lower, for x-values of 0.20 to 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 was shown that the structural properties were governed by the ionic sizes whereas the physical properties were controlled by ionic size and hole doping.

B.Dabrowski, K.Rogacki, X.Xiong, P.W.Klamut, R.Dybzinski, J.Shaffer, J.D.Jorgensen: Physical Review B, 1998, 58[5], 2716-23