Study of Ordering in High-Quartz Solid Solutions by Substitutions Affecting Superlattice Reflections

Ordering of the interstitial and network atoms in stuffed high-quartz solid-solution ( ss ) phases was studied by correlating suitable substitutions in the structure with the resultant changes in the superlattice reflections arising from these phases. In an MgO·Al₂O₃·3SiO₂ high-quartz ss , the special ordering of the interstitial Mg atoms, which produces satellite reflections around superlattice positions, was disturbed by replacement of ∼20 mol% of the MgO by Li₂O or ZnO. The interstitial atoms were further disordered by larger substitutions. The streaked superlattice reflections arising from an Li₂O·Al₂O₃·2SiO₂ high-quartz ss disappeared on replacement of ∼40 mol% of the Li₂O by MgO. This result implies that the interstitial Li atoms are ordered and were disordered by the substitution. The superlattice reflections were much intensified when ∼25 mol% of the SiO₂ was replaced by GeO₂. However, these reflections were intensified to a lesser degree when presumably both SiO₂ and Al₂O₃ were partially replaced by GeO₂ and Ga₂O₃, respectively. These observations strongly suggest an ordering of the network Si and Al atoms, which, together with the ordering of the interstitial Li atoms, contributed to the original streaked superlattice reflections.