Phase transitions in leucite: X-ray diffraction studies

Lattice parameters, and intensities of selected X-ray reflexions, have been measured as a function of temperature for natural leucite, to characterise the phase transformation behaviour. At low temperatures leucite has a large ferroelastic distortion, but the temperature evolution of lattice parameters cannot be explained in terms of a purely ferroelastic phase transition; in particular, the considerable change in volume with temperature implies an additional transition mechanism, which we correlate with off-centring of K-ions in the low-temperature phase, and a collapse of the 111 structural channels. The transition behavior can therefore be rationalised in terms of two competing mechanisms: (I) Ferroelastic (consistent with the change m3m 4/mmm); (II) Volume-changing (consistent with m3m 4/m). Coupling of the two order parameters Q_I, and Q_II gives rise to the intermediate 4/mmm tetragonal phase.Our results confirm the existence of an I4₁/aI4₁/acd transition, but the non-disappearance of the 200 reflexion at high temperatures implies that the expected transition from I4₁/acd to Ia3d (cubic) symmetry does not occur. We attribute this to a residual strain field conjugated to the order parameter, due to defects (with possible Al/Si order). Nevertheless, within our experimental resolution, the lattice becomes metrically cubic at 665° C.