Interstitial cristobalite-type compounds (Na 2O) ≤0.33Na[AlSiO 4

Abstract

The series of interstitial compounds (Na 2O) ≤0.33Na[AlSiO 4] exhibits the cubic symmetry of the cristobalite type Na[AlSiO 4]-carnegieite host lattice as concentrations of 0.01–0.33 Na 2O are intercalated at elevated temperatures under closed system conditions (950–1100 K). The tetrahedral framework structure of the host lattice expands correspondingly from a = 7.3001(5) to 7.3029(1)Å. The structure model of the end member, (Na 2O) 0.33Na[AlSiO 4), proposed from structure factor calculations based on X-ray powder diffraction data, shows unique (Na,O)(Na,O) distances in the high-temperature form of 3.17 Å. A total of 6 2 3 sodium atoms and 1 1 3 oxygens statistically occupying the cage like 12-fold oxygen-coordinated position of space group F 43m form a diamond-type sublattice in the high-cristobalite-type “host lattice” [AlSiO 4] 1− 3 d∝ . Superstructure characteristics on X-ray Guinier powder diffraction data indicate complex ordering mechanisms following long-term annealing (200 hr, 370–970 K). At temperatures &2;1350 K in an open system, intercalated Na 2O is released with the subsequent collapse of the Na[AlSiO 4] host lattice from cubic to triclinic, the regular symmetry of Na[AlSiO 4]-carnegieite at temperatures <960 K.