COOPERATIVE JAHN-TELLER EFFECT IN TITANIUM ALUM

A low-temperature (ca. 12 K) cubic <(Pa(3)over bar )> to orthorhombic (Pbca) phase transition of the beta-alum CsTi(SO4)(2) . 12H(2)O has be en characterized by High Resolution Powder Neutron Diffraction and EPR measurements. Single crystal Raman spectra of the corresponding rubid ium alum, RbTi(SO4)(2) . 12H(2)O, show that a phase transition from th e beta-alum structure also occurs over the temperature range 5-15 K, w ith the spectroscopic changes remarkably similar for the caesium and r ubidium salts. The structural instability of the titanium(III) alums i s not evident in the corresponding salts of larger or smaller tervalen t cations and hence is interpreted in terms of the electronic structur e of [Ti(OH2)(6)](3+). It is proposed that in the high-temperature cub ic phase the Sg Site symmetry lifts the degeneracy of the t(2g) (O-h) orbitals to leave the e(g) (S-6) orbital set lowest lying. The resulta nt (2)E(g) (S-6) ground term is subject to Jahn-Teller coupling with E g phonon modes. The phase transition is interpreted as arising from a long-range interaction between the Jahn-Teller centers in the lattice giving rise to a cooperative Jahn-Teller effect. The proposed electron ic structure of [Ti(OH2)(6)](3+) in CsTi(SO4)(2) . 12H(2)O is consiste nt with the framework used to describe other tervalent aqua ions but i s at variance with the current interpretation-of 40 years standing-whi ch was based on the premise that the site symmetry of the tervalent ca tion is retained at all temperatures. The long-standing problem of the anomalous ground state g values of [Ti(OH2)(6)](3+) in CsTi(SO4)(2) . 12H(2)O (g(parallel to) = 1.25 and g(perpendicular to) = 1.14) is sho wn to arise as a consequence of the low symmetry distortion which resu lts from a lowering of the site symmetry of [Ti(OH2)(6)](3+)from S-6 < (Pa(3)over bar )> to C-i (Pbca).