The Reaction between ZnO and Molten Na₂S₂O₇ or K₂S₂O₇ Forming Na₂Zn(SO₄)₂ or K₂Zn(SO₄)₂, Studied by Raman Spectroscopy and X-ray Diffraction

Reactions between solid zinc oxide and molten sodium or potassium pyrosulfates at 500 °C are shown by Raman spectroscopy to be 1:1 reactions leading to solutions. By lowering the temperature of the solution melts, colorless crystals form. Raman spectra of the crystals are given and tentatively assigned. Crystal structures of the monoclinic salts at room temperature are given. Na2Zn(SO4)2: space group = P2/n (No. 13), Z = 8, a = 8.648(3) Å, b = 10.323(3) Å, c = 15.103(5) Å, β = 90.879(6)°, and wR2 = 0.0945 for 2748 independent reflections. K2Zn(SO4)2: space group = P21/n (No.14), Z = 4, a = 5.3582(11) Å, b = 8.7653(18) Å, c = 16.152(3) Å, β = 91.78(3)°, and wR2 = 0.0758 for 1930 independent reflections. In both compounds, zinc is nearly perfectly trigonally bipyramidal, coordinated to five oxygen atoms, with Zn−O bond lengths ranging from 1.99 to 2.15 Å, equatorial bonds being slightly shorter on the average. The O−Zn−O angles are approximately 90° and 120°. The sulfate groups connect adjacent Zn2+ ions, forming complicated three-dimensional networks. All oxygen atoms belong to nearly perfect tetrahedral SO42- groups, bound to zinc. No oxygen atom is terminally bound to zinc; all zinc oxygens are further connected to sulfur atoms (Zn−O−S bridging). In both structures, some oxygen atoms are uniquely bound to certain S atoms. The sulfate group tetrahedra have quite short (1.42−1.45 Å) terminal S−O bonds in comparison to the longer (1.46−1.50 Å) Zn-bridging S−O bonds. The Na+ or K+ ions adopt positions between the ZnO5 hexahedra and the SO4 tetrahedra, completing the three-dimensional network of the M2Zn(SO4)2 structures. Bond distances and angles compare well with literature values. Empirical correlations between S−O bond distances and average O−S−O bond angles follow a previously found trend.