The crystal structure of SFCA-I, Ca3.18Fe3 14.66Al1.34Fe 2 0.82O28, a homologue of the aenigmatite structure type, and new crystal structure refinements of β-CFF, Ca2.99Fe3 14.30Fe2 0.55O25

Abstract

Determination of the crystal structure of a new calcium ferrite has proved that it has M20O28 stoichiometry, Ca3.18(Fe3+14.66Al1.34)Fe 2+0.82O28. The crystal structure shows that this ferrite is a higher homologue of previously studied SFCA (M14O20), together forming the first two members (n = 0, 1) of the homologous series M14+6nO20+8n. Here designated SFCA-I, it has a triclinic cell with a = 10.43, b = 10.61, c = 11.84 Å, α = 94.14, β = 111.35, γ = 110.27°. SFCA-I (n = 1) contains two types of alternating layers in its structure: the first is composed of octahedral walls that are each 6 octahedra wide; the second is a continuous layer composed of single ribbons of corner joined tetrahedra ..