Cooperative Jahn-Teller Effect in Titanium Alum

Abstract

A low-temperature (ca. 12 K) cubic (Pa3) to orthorhombic (Pbca) phase transition of the β-alum CsTi(SO4)2·12H2O has been characterized by High Resolution Powder Neutron Diffraction and EPR measurements. Single crystal Raman spectra of the corresponding rubidium alum, RbTi(SO4)2·12H2O, 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(OH2)6]3+. It is proposed that in the high-temperature cubic phase the S6 site symmetry lifts the degeneracy of the t2g (Oh) orbitals to leave the eg (S6) orbital set lowest lying. The resultant 2Eg (S6) ground term is subject to Jahn-Teller coupling with Eg phonon modes. The phase transition is interpreted as arising from a long-range inte...