Structural studies on the caesium alums, CsMIII[SO4]2·12H2O

Abstract

Structure determinations of the caesium alums CsMIII[SO4]2·12H2O, M = V, Cr, Mn, Fe, Co, Al, Ga, or In, have been carried out at 295(1) K by X-ray diffraction methods. The data obtained include (i) MIII-O distances for the metals in the [M(OH2)6]3+ species [V-O, 1.992(6); Cr-O, 1.959(3); Mn-O, 1.991(6); Fe-O, 1.995(4); Co-O, 1.873(5); Al-O, 1.877(3); Ga-O, 1.944(3); and In-O, 2.112(4) Å], (ii) an estimate of 1.34 Å for the effective co-ordinated radius for the water molecule in the cation, and (iii) the proposal of a new criterion for the classification of the different alum types, based on the disposition of the water molecules about the univalent cation.