Cation and anion ordering in the layered oxyfluorides Sr3-xAxAlO4F (A = Ba, Ca)

The synthesis and structural characterization of mixed oxyfluorides of the type Sr(3-x)A(x)AlO(4)F is reported, where A is either calcium or barium. In these compounds the fluoride and oxide ions are ordered onto two distinct crystallographic sites. There is also an ordering of the alkaline earth cations over two crystallographic sites upon substitution of Ba2+ or Ca2+ for Sr2+. The solid solubility limits extend to xsimilar to1 for substitution of both barium and calcium, but the larger Ba2+ cations show a strong site preference for the ten-coordinate strontium sites, while the smaller Ca2+ cations prefer the eight-coordinate strontium sites. (C) 2002 Elsevier Science (USA). All rights reserved.