CaAl12Si4O27, a new high-pressure phase containing Al6O19 clusters

CaAl12Si4O27, a new high-pressure phase in the CaO-Al2O3-SiO2 system, was p repared at 1550 degrees C and 14 GPa. It has trigonal symmetry, P-3, with a =7.223(1) and c=8.614(3) Angstrom. Its structure was solved using crystal c hemistry principles and refined using the Rietveld method applied to powder X-ray diffraction data, R-wp = 6.1%, R-B = 2.7%. The structure is a new ty pe based on a close-packed anion lattice with a mixed layer stacking sequen ce ABACA = (hc)(2) and with Ca atoms ordered in one-seventh of the anion si tes in alternate c-stacked layers. The Si atoms are ordered in both octahed ral and tetrahedral sites while the Al atoms are ordered in two independent octahedral sites. Octahedral edge-shared clusters, Al6O19, occur in the st ructure, interconnected within the close-packed layers by corner-linking to SiO4 tetrahedra. The clusters are also connected by corner-sharing to nine -member rings of edge-shared Al- and Si-centered octahedra in adjacent laye rs. The new phase has structural features in common with the barium titaniu m ferrite of similar composition, BaFe11Ti3O23, as well as with the mineral simpsonite, Al4Ta3O13(OH), and alkali metal niobates which contain Nb6O19 clusters. (C) 2000 Academic Press.