Cooperative Jahn-Teller Effect in Titanium Alum

A low-temperature (ca. 12 K) cubic ( Pa 3̄) to orthorhombic ( Pbca ) phase transition of the β-alum CsTi(SO 4 ) 2 ·12H 2 O has been characterized by High Resolution Powder Neutron Diffraction and EPR measurements. Single crystal Raman spectra of the corresponding rubidium alum, RbTi(SO 4 ) 2 ·12H 2 O, show that a phase transition from the β-alum structure also occurs over the temperature range 5−15 K, with the spectroscopic changes remarkably similar for the caesium and rubidium salts. The structural instability of the titanium(III) alums is not evident in the corresponding salts of larger or smaller tervalent cations and hence is interpreted in terms of the electronic structure of [Ti(OH 2 ) 6 ] 3+ . It is proposed that in the high-temperature cubic phase the S 6 site symmetry lifts the degeneracy of the t 2g ( O h ) orbitals to leave the e g ( S 6 ) orbital set lowest lying. The resultant 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 electronic structure of [Ti(OH 2 ) 6 ] 3+ in CsTi(SO 4 ) 2 ·12H 2 O is consistent with the framework used to describe other tervalent aqua ions but is at variance with the current interpretation of 40 years standing which was based on the premise that the site symmetry of the tervalent cation is retained at all temperatures. The long-standing problem of the anomalous ground state g values of [Ti(OH 2 ) 6 ] 3+ in CsTi(SO 4 ) 2 ·12H 2 O ( g ∥ = 1.25 and g ⊥ = 1.14) is shown to arise as a consequence of the low symmetry distortion which results from a lowering of the site symmetry of [Ti(OH 2 ) 6 ] 3+ from S 6 ( Pa 3̄) to C i ( Pbca ).