EPR of Cr3+ in γ-Alum NaAl(SO4)2·12 H2O

Abstract

BY NaAl(S04)2*12 H 2 0 is the only one known r-alum. At room temperature its structure is cubic with space group Pa3 /1,*2/ similarly t o all other alums.The main difference between a-, f? -alum, and sodium-aluminium alum cons i s t s in the orientation of the molecules. In NaAl(S04)2-12 H20 the molecules, which form almost regular tetrahedra, have their c o r n e r s 2directed towards the Na' cations. Besides the (SO4) include also two other groups of atoms, i. e. the octahedra M3+ [%O] and M+[H20l6. The monovalent and trivalent octahedra are coupled with each other by hydrogen bonds: directly and indirectly by (SO4) The M3+ [5016 octahedra in the alums have trigonal symmetry. I1 M k is