Vibrational Spectra of Molten Salts. V. Infrared and Raman Spectra of Some Molten Sulfates

Infrared spectra of thin films of pure alkali‐metal sulfates at temperatures just above their melting points have been obtained. These indicate retention of $T_d$ symmetry by the SO₄²⁻ ion, though the ${\nu }_1{\mathrm{(a}}_1)$ and ${\nu }_3{\mathrm{(f}}_2)$ bands move to lower frequencies as the alkali metal is changed from Li to Rb, and the formally forbidden ${\nu }_2\mathrm{(e)}$ mode gives rise to a very weak band, indicating some interionic interactions. Much more striking changes in the spectra are produced by addition of divalent metal or silver sulfate to a K₂SO₄ melt. Spectra from 40 mole % solutions of Ag(I), Mg(II), Ca(II), Sr(II), Ba(II), Zn(II), Cd(II), Pb(II), Cu(II), Co(II), and Ni(II) sulfates have been obtained. These indicate the presence of perturbed SO₄²⁻ ions with $C_{\text{3V}}$ symmetry, and full assignments of the fundamental modes of these $C_{\text{3V}}$ species have been made for the Zn, Cu, Co, and Ni solutions. The Raman spectra of ZnSO₄ at two different concentrations in molten K₂SO₄ further showed small but definite frequency differences from the corresponding infrared spectra. These differences are analyzed in terms of possible metal‐complex formation and the quasilattice structure of these melts.