Paraelectric-antiferroelectric phase transition in titanite, CaTiSiO₅

Abstract

The reversible A2/a to P2 ₁/a phase transition (T _c = 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 T _c is investigated, using both diffraction contrast and high resolution techniques. At and just above T _c 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 $P\bar 1 - I\bar 1$ transition in anorthite, CaAl₂Si₂O₈.