| Title
| - 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 ElectronicStructure of the Tervalent Cation and Structural Abnormalities
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| Abstract
| - Low-temperature single-crystal Raman spectra forRbMIII(SO4)2·12H2O(MIII = Al, Ga, In, Ti, V, Cr, Fe) andRbMIII(SO4)2·12D2O(MIII = Al, V) have been collected and assigned in therange 275−1200 cm-1.Theseresults permit classification of the Ti and V rubidium sulfate alums tothe β modification, whereas the remainingtervalent cations give the expected α modification. Thedimorphism of the rubidium sulfate alums is explainedin terms of the electronic structure of the tervalent cation, where theobservation of the β modification is associatedwith unequal occupancy of the t2g(Oh) orbitals. For the rubidiumvanadium alums the 3Eg ←3Ag electronicRaman (eR) transition permits quantification of the trigonal fieldsplitting of the t2g (Oh)orbitals (ca. 1940cm-1).The profile of the eR band is sensitive both to changes intemperature and to deuteration. Analysis of the eRband profile suggests a reduced spin−orbit splitting of the3Eg manifold, this being ascribed to excitedstateJahn−Teller (J−T) effects. The similarity of the Raman spectraof the cesium and rubidium titanium sulfatealums suggest that they exhibit closely related structural chemistry,with both subject to phase transitions below80 K. The observation that modes of Eg symmetry arecoupled to the structural change is consistent with theinterpretation that the trigonal field leaves an orbital doublet groundterm for titanium(III), leading to a cooperativeJ−T effect.
- The adoption of the β alummodification by the titanium and vanadium rubidium sulfate alums isunderstood in terms of an anisotropic metal(III)−water πinteraction. This interaction results in degenerate andnondegenerate ground terms for the respective titanium and vanadiumhexaaqua cations. A broadening of the phonon modes ofRbTi(SO4)2·12H2O at lowtemperature suggests the onset of a phase transition which isattributed to a cooperative Jahn−Tellereffect.
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