A high-temperature study of the low-high leucite phase transition using the transmission electron microscope

That leucite transforms from a tetragonal to a cubic polymorph with increasing temperature has been accepted for nearly a century, but the precise nature of this transition has provoked some disagreement. X-ray diffraction experiments suggest that leucite inverts directly from space group I4₁/a to Ia3d; however, two distinct absorption events appear in most calorimetric experiments, raising the possibility that leucite passes through an intermediate phase with space group I4₁/acd. The transition between low and high leucite was examined in situ during heating experiments with a transmission electron microscope. Dark-field imaging with the 420 diffracted beam revealed pseudomerohedral and merohedral twin types in the low-temperature polymorph, and with increasing temperature the contrast among both sets of twins steadily diminished. We conclude that leucite can indeed occur as a distinct intermediate phase with space group I4₁/acd that is metrically but not symmetrically cubic within the error of our measurements. -from Authors