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THE CRYSTAL STRUCTURES OF SYNTHETIC POTASSIUM – TRANSITION-METAL ZIPPEITE-GROUP PHASES

O. Maurice Peeters^1,, Renaud Vochten² and Norbert Blaton³

¹ Laboratory for Biocrystallography, Katholieke Universiteit Leuven, O&N2, Herestraat 49-bus 822, B–3000 Leuven, Belgium
² Department of Subatomic and Radiation Physics, Division of Nuclear Material Physics, Universiteit Gent, Proeftuinstraat 86, B–9000 Gent, Belgium
³ Laboratory for Biocrystallography, Katholieke Universiteit Leuven, O&N2, Herestraat 49-bus 822, B–3000 Leuven, Belgium

E-mail address: maurice.peeters{at}pharm.kuleuven.be

Mixed K–Me zippeite-group phases (Me = Mn, Co, Ni, Zn) have been synthesized by adjusting a UO₂SO₄ solution containing MeSO₄ to a pH of 3.8 by means of KOH. The solution was kept for 75 hours at 150°C and an approximate pressure of 3.5 MPa. Single-crystal X-ray studies, chemical analysis and bond-valence calculations revealed the composition K_0.5Me_0.75[(UO₂)₂SO₄O₂]·3H₂O. The crystals are monoclinic, space group C2/c. For Me = Mn, we found a 8.661(6), b 14.375(8), c 17.705(12) Å, β 104.12(5)°; for Me = Co, the cell parameters are a 8.651(5), b 14.188(8), c 17.713(13) Å, β 104.14(6)°, for Me = Ni, they are a 8.662(5), b 14.095(8), c 17.770(9) Å, β 104.18(5)°, and for Me = Zn, they are a 8.650(6), b 14.180(12), c 17.709(13) Å, β 104.14(6)°. The structures were refined to unweighted residuals of 0.0525, 0.0459, 0.0383 and 0.0690, respectively. The structures possess a layer structure parallel to (010) comparable to that of zippeite. We demonstrate that a zippeite-group phase with an interlayer containing monovalent potassium cation and divalent transition metal cations can be synthesized. The interlayer contains two symmetrically distinct Me atoms, one K atom and H₂O molecules. Both Me atoms (Me1, Me2) are coordinated by six oxygen atoms forming distorted octahedra. In the Me1 octahedron, two O atoms are part of (UO₂)²⁺ of adjacent uranyl oxo sulfate layers, and four are part of four H₂O molecules of the interlayer, which is the same configuration as found in the crystal structure of zinc-zippeite, cobalt-zippeite and magnesium-zippeite. In the Me2 octahedron, four O atoms are part of (UO₂)²⁺ of the adjacent uranyl oxo sulfate layers, and two O atoms are part of two equivalent positions of a H₂O group.

Keywords: uranyl sulfate, zippeite-group phases, crystal structure, transition metal.