Al-Si ordering in the framework of leucite and Cs-exchanged leucite

Ordered distributions of Al atoms in the frameworks of leucite and Cs-exchanged leucite (pollucite) are determined, under the restriction of the Al-O-Al avoidance rule. The observed ²⁹Si MAS NMR spectra of pseudo-tetragonal leucite and Cs-exchanged leucite cannot be explained by an Al-configuration of tetragonal symmetry, but are consistently explained by one of P112₁/a symmetry. On the other hand, an XRD pattern of a single crystal of leucite supports pseudo-tetragonal symmetry. This discrepancy is interpreted by the pseudo-tetragonal crystal consisting of twins of monoclinic microcrystallites. A detailed analysis of the ²⁹Si MAS NMR spectrum of Cs-exchanged leucite of the high temperature phase (pseudo-cubic) shows that the domain size of the monoclinic phase is only several unit cells. The origin of an abnormal twist of the framework of tetragonal leucite and a driving force to transform it to the pseudo-cubic phase at higher temperature are discussed. The degree of the long-range order, S, is newly defined for this system, and estimated from ²⁹Si MAS NMR data as S = ca. 0.8.