Crystal structure and giant magnetoresistance(GMR) of Ba substituted $LaMnO_3$ have been investigated by x-ray diffraction (XRD), transmission electron microscopy (TEM) and superconducting quantum interference device (SQUID). Especially, the crystal structure of Ba substituted $LaMnO_3$ has been tentatively determined with the Rietveld analysis technique.
In the composition range between 0 and 33 mol.% of $Ba^{2+}$ ions at the A-site($La^{3+}$-site), {½ ½ ½} superlattice reflections are observed in the electron difrraction patterns, which may be interpreted as arising from the anti-phase tilting of oxygen octahedra. Based on the x-ray and electron difrraction results, the space group of the present oxide system can be determined to be $R_3c$, after Rietveld analysis of the x-ray by diffraction data. By refining the parameters used in Rietveld method, it has been demonstrated in the x-ray diffraction that the rhombohedral distortion decreases with the increasing Ba content. It has also been observed from the SQUID measurements that as the Ba content increases, resistivity is lowered whereas the MR value and Curie temperature($T_c$) is raised. These phenomena can be acounted to the change of Mn-O-Mn bonding angle due to the oxygen octahedra tilting.
