This intergranular oxide precipitate, in Ba-doped Ni manganites, was found to have a structure (as determined by using X-ray and electron diffraction techniques) which was isomorphic with monoclinic hollandite. The observed domain microstructure (accommodation twins) suggested that a high-temperature ferro-elastic phase transformation (tetragonal-to-monoclinic) had occurred. A theoretical analysis indicated that these twins were of the so-called third kind. That is, the 4 twinning elements were irrational and could be considered to be approximately type-I twins. Medium-resolution electron microscopic observations confirmed these predictions; with the domain walls lying along (1¯p0) and (p10) planes, where p was approximately equal to 2. High-resolution electron microscopic observations suggested that these twins were composed of short irrational sections which were connected by ledges.

J.J.Couderc, S.Fritsch, M.Brieu, G.Vanderschaeve: Philosophical Magazine A, 1996, 74[6], 1351-65