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 ZnO and molten Na2S2O7 or K2S2O7 at 500 are shown by Raman spectroscopy to be 1:1 reactions leading to solns. By lowering the temp. of the soln. melts, colorless crystals form. Raman spectra of the crystals are given and tentatively assigned. Crystal structures of the monoclinic salts at room temp. are given. Na2Zn(SO4)2: space group P2/n (No. 13), Z = 8, a = 8.648(3), b = 10.323(3), c = 15.103(5) Å, beta = 90.879(6) deg., and wR2 = 0.0945 for 2748 independent reflections. K2Zn(SO4)2: space group P21/n (No.14), Z = 4, a = 5.3582(11), b = 8.7653(18), c = 16.152(3) Å, beta = 91.78(3) deg., and wR2 = 0.0758 for 1930 independent reflections. In both compds., Zn is nearly perfectly trigonally bipyramidal, coordinated to five O atoms, with Zn-O bond lengths ranging from 1.99 to 2.15 Å, equatorial bonds being slightly shorter on the av. The O-Zn-O angles are apprx. 90 deg. and 120 deg.. The sulfate groups connect adjacent Zn2+ ions, forming complicated three-dimensional networks. All O atoms belong to nearly perfect tetrahedral [SO4]2- groups, bound to Zn. No O atom is terminally bound to Zn; all Zn oxygens are further connected to S atoms (Zn-O-S bridging). In both structures, some O 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 av. O-S-O bond angles follow a previously found trend.