Philip L. W. Tregenna-Piggott, Mary C. M. O'Brien, John R. Pilbrow, Hans U. Güdel, Stephen P. Best and Chris Noble
Paramagnetism of caesium titanium alum and the Jahn-Teller interaction
J. Chem. Phys. 107, 8275-8291 (1997)                 

Abstract: The magnetic properties of [Ti(OH₂)₆]³⁺ within CsTi(SO₄)₂·12H₂O and doped into a range of isostructural diamagnetic lattices, has been reinvestigated using electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Whereas previous attempts to rationalise the variation of the magnetic susceptibility with temperature have employed a ²A_g trigonal ground term, the data are interpreted within a framework used to describe other tervalent hexa-aqua cations whereby the trigonal field is considerable and of a sign such that the e_g orbital set is lower lying, resulting in a ²E_g trigonal ground term. The magnetic susceptibility is then modeled using a novel approach based on the numerical diagonalisation of the vibronic Hamiltonian. Hence, it is shown that the singular magnetic properties of CsTi(SO₄)₂·12H₂O arise from a delicate balance between spin-orbit and Jahn-Teller coupling. Particular use is made of isotopic substitution to obtain information regarding the nature of the vibronic interaction. An excellent fit to both the susceptibility data and ground state g values is provided by a model in which the Jahn-Teller interaction is dominated by coupling to phonon modes involving librational modes of sulphate and water coordinated to titanium(III). This interpretation is further supported by Raman data on the corresponding compounds. This analysis provides the first satisfactory explanation of the magnetic properties of [Ti(OH₂)₆]³⁺ in the b-alum lattice.