Aqua Ions. 2. Structural Manifestations of the Jahn−Teller Effect in the β-Alums

Variable-temperature single-crystal neutron diffraction structures of the alums CsMIII(SO4)2·12D2O, where MIII = Ti, V, Mn, and Ga, are reported. Structural differences are highlighted by the titanium and manganese alums, which undergo cubic (Pa3̄) to orthorhombic (Pbca) phase transitions at ∼13 and ∼156 K, respectively. The structural instability exhibited by these salts is interpreted as arising from cooperative Jahn−Teller interactions, and these measurements characterize the structural changes that result from the coupling between the electronic and vibrational states. Although the symmetry changes associated with the phase transformations are analogous for the Ti and Mn alums, the low-temperature geometries of the tervalent hexaaqua cations are markedly different. Whereas the MnO6 framework is subject to a pronounced tetragonal elongation, changes in the Ti−O bond lengths are very modest; but significant changes in the O−Ti−O bond angles and in the disposition of the coordinated water molecules are identified. The large differences in the transition temperatures and in the low-temperature stereochemistries of the [Ti(OD2)6]3+ and [Mn(OD2)6]3+ cations are related to the sensitivity of the energies of the t2g (O h ) and eg (O h ) orbitals to the various asymmetric vibrations of the hexaaqua complex.