Single-Crystal Raman Spectroscopy of the Rubidium Alums RbMIII(SO4)2·12H2O (MIII = Al, Ga, In, Ti, V, Cr, Fe) between 275 and 1200 cm-1: Correlation between the Electronic Structure of the Tervalent Cation and Structural Abnormalities

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Abstract
Low-temperature single-crystal Raman spectra for RbM(III)(SO(4))(2).12H(2)O (M(III) = Al, Ga, In, Ti, V, Cr, Fe) and RbM(III)(SO(4))(2).12D(2)O (M(III) = Al, V) have been collected and assigned in the range 275-1200 cm(-)(1). These results permit classification of the Ti and V rubidium sulfate alums to the beta modification, whereas the remaining tervalent cations give the expected alpha modification. The dimorphism of the rubidium sulfate alums is explained in terms of the electronic structure of the tervalent cation, where the observation of the beta modification is associated with unequal occupancy of the t(2g) (O(h)()) orbitals. For the rubidium vanadium alums the (3)E(g) <-- (3)A(g) electronic Raman (eR) transition permits quantification of the trigonal field splitting of the t(2g) (O(h)()) orbitals (ca. 1940 cm(-)(1)). The profile of the eR band is sensitive both to changes in temperature and to deuteration. Analysis of the eR band profile suggests a reduced spin-orbit splitting of the (3)E(g) manifold, this being ascribed to excited state Jahn-Teller (J-T) effects. The similarity of the Raman spectra of the cesium and rubidium titanium sulfate alums suggest that they exhibit closely related structural chemistry, with both subject to phase transitions below 80 K. The observation that modes of E(g) symmetry are coupled to the structural change is consistent with the interpretation that the trigonal field leaves an orbital doublet ground term for titanium(III), leading to a cooperative J-T effect.