The use of  in situ  transmission electron microscopy showed that phase transformation from the rhombohedral to the cubic form during heating was gradual, and that the crystal became isotropic at temperatures of between 400 and 420C. It was also found that many of the ubiquitous twins reappeared again, with the same morphology, when the crystal passed through the phase transformation during cooling. This memory effect was more marked in crystals which had been annealed at high temperatures before study. It was suggested that annealing reduced the number of structural defects in the crystal and at thin edges. Thus, any remaining defects were expected to have a marked effect upon the nucleation of twin boundaries during cooling of the crystal through the transition temperature.

M.G.Norton, R.R.Biggers: Scripta Metallurgica et Materialia, 1995, 32[4], 481-5