Structure of Calcium Aluminate, 12CaO.7AL₂O₃

Abstract

Büssem and Eitel¹ determined the unit cell, space group, and approximate structure of calcium aluminate. From the unit cell (body-centred cubic, a = 11.95 Å.) and density (≃ 2.69 gm./c.c.) the cell contents were fixed as Ca₂₄Al₂₈O₆₆. This gives a CaO : Al₂O₃ ratio of 12 : 7 and differs only slightly (0.71 wt. per cent A1₂O₃) from the 5 : 3 ratio proposed on chemical grounds². The most probable space-group appeared to be T⁶_d, or I&4macr;3d. Only 64 of the 66 oxygen atoms in the unit cell could be placed in this space-group; the remaining two were assumed to be distributed statistically. Because of this, the structure has been accepted with reservation, and it seemed worthwhile to check it, especially as more powerful methods of structure analysis are now available.