No abstract

(18) Kazansky, V. B.; Gitscov, A. M.; Andreev, V. M.; Zhidomorov, G. The phase transition in Cm and (related) lattice defects are studied by low-temperature electron microscopy and electron diffraction. The microstructure of the room-temperature face-centered-cubic (fcc-a,,) phase is very similar to that of a low stacking fault energy fcc alloy; micro twins and stacking faults on the { 11 1) planes are the main defects. The phase transition fcc-a. 4 simple cubic (sc) at 249 K, is confirmed by single-crystal diffraction and the space group of the sc phase was unambiguously determined from the systematic extinctions as Pa3. Moreover a second phase transition, sc 4 fcc-2ao, is discovered. It occurs at a slightly lower temperature. It is suggested that in the sc phase the molecules still have a rotational degree of freedom about their respective (1 11) rotation axis, of which the orientation pattern is already fixed by the Pa3 space group. In the fcc-2a,, phase the rotation angle is found to be frozen in and to alternate between +cp and -cp along the (100) directions. The domain structure of the sc phase consists of eight variants (four translation variants for each of the two orientation variants) present in rather ill-defined regions. As a consequence of the ease of rotation of the molecules, no sharp interfaces are formed between different orientation domains.

The reversible A 2/a to P2~/a phase transition (T~ = 495 K) in titanite is studied by means of electron microscopy and electron diffraction. In the low temperature phase, antiphase boundaries are frequently observed in the image. High resolution lattice imaging further allows the determination of the projected displacement vector of the antiphase boundaries to be [0 1/2 1/2]. The dynamical behaviour of these interfaces in the region close to Tc is investigated, using both diffraction contrast and high resolution techniques. At and just above T~ the interfaces become very broad, very mobile and fluctuate in a dynamical way. When the temperature is further increased these interfaces disappear. No memory effect is present when cycling through the transition, i.e. antiphase boundaries are not pinned in contrast to those observed during the PI-I1 transition in anorthite, CaA12Si2Os.

No abstract

In the diffraction patterns of the f.c.c. phase of C60 the h00 (0k0 and 00l) reflections are consistently absent or very weak, even for the allowed reflections with h = even. It is shown how this phenomenon is sensitively related to the ratio of the molecular size and the lattice parameter. Hereby use is made of two different approaches: the Fourier transform method and an intuitive real space approach. More realistic models, than the uniformly thin shell model, considered till now, are treated. It is shown how the intensities of the h00 reflections vary in terms of the parameters of the model: shell radius, shell profile, as compared to the lattice parameter. The phenomenon is demonstrated on a single crystal fragment, using electron diffraction.