A TRANSMISSION ELECTRON-MICROSCOPY STUDY OF ACCOMMODATION TWINS IN BA-NI-MN OXIDES

An intergranular Ba-Ni-Mn oxide precipitate is identified in Ba-doped Ni manganites. The structure of this oxide, as determined by X-ray and electron diffraction, is isomorphous with monoclinic hollandite. The observed domain microstructure (accomodation twins) suggests that a hi gh-temperature ferroelastic phase transformation, tetragonal to monocl inic, has occurred. A theoretical analysis indicates that these twins should be of the 'third-kind' type, that is the four twinning elements are irrational and could be considered as approximately type I twins. Medium-resolution electron microscopy observations confirm these pred ictions, the domain walls lying along (1(p) over bar0$) and (p10) plan es with p similar or equal to 2. However, high-resolution electron mic roscopy observations suggest that these twins are made of short ration al sections connected by ledges.