CaAl₁₂Si₄O₂₇, a new high-pressure phase containing Al₆O₁₉ clusters

CaAl₁₂Si₄O₂₇, a new high-pressure phase in the CaO-Al₂O₃-SiO₂ system, was prepared at 1550°C and 14 GPa. It has trigonal symmetry, P-3, with a = 7.223(1) and c = 8.614(3) Å. Its structure was solved using crystal chemistry 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 type based on a close-packed anion lattice with a mixed layer stacking sequence ABACA = (hc)₂ and with Ca atoms ordered in one-seventh of the anion sites in alternate c-stacked layers. The Si atoms are ordered in both octahedral and tetrahedral sites while the Al atoms are ordered in two independent octahedral sites. Octahedral edge-shared clusters, Al₆O₁₉, occur in the structure, 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 layers. The new phase has structural features in common with the barium titanium 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.