The crystal structure of SFCA-II, Ca5.1Al9.3Fe3+18.7Fe2+0.9O48 a new homologue of the aenigmatite structure-type, and structure refinement of SFCA-type, Ca2Al5Fe7O20. Implications for the nature of the "ternary-phase solid-solution" previously reported in

Abstract

Determination of the crystal structure of a new calcium ferrite has proved that it has M 3 4 O 4 8 stoichiometry, Ca 5 . 1 Al 9 . 3 Fe 3 + 1 8 . 7 Fe 2 + 0 . 9 O 4 0 . The crystal structure shows that this ferrite is a homologue of SFCA and has a structure which is a composite of the crystal structures of previously described homologues SFCA (M 1 4 O 2 0 ) and SFCA-I (M 2 0 O 2 8 ). Designated as SFCA-II, it has a triclinic cell with a = 10.338, b = 10.482, c = 17.939A, α = 90.384, β = 89.770, γ= 109.398°. SFCA-II contains two types of alternating layers in its structure: the first is composed of octahedral walls that are 5 wide; the second is a continuous layer composed of single ribbons of corner joined tetrahedra (T), alternately separating, single (W) and double (2W) ribbons of so-called winged octahedra (...T-W-T-2W-T...). The crystal structure refinement of a Si-free SFCA-type, Ca 2 Al 5 Fe 7 O 2 0 , formed in the C-A-F system by flux growth at 1290 °C is also presented. At ∼1200-1250°C these three unique but structurally related homologues of SFCA (i.e. SFCA, SFCA-II, and SFCA-I) are seen to span, in that order, the designated compositional range of the so-called ternary-phase solid-solution, Ca 4 Al 4 Fe 2 0 O 4 0 -Ca 4 Al 1 0 Fe 1 4 O 4 0 , which has been regarded as a significant feature of the CaO - Al 2 O 3 - Fe 2 O 3 (C-A-F) phase diagram from -1470 °C to as low as 700°C. Ternary-phase solid-solution in the C-A-F system should be replaced in part by more limited, yet to be determined, ranges of solid solution in at least three unique homologues of the SFCA structure-type.