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

Abstract

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 degrees C and an approximate pressure of 3.5 MPa. Single-crystal X-ray studies, chemical analysis and bond-valence calculations revealed the composition K0.5Me0.75[(UO2)(2)SO4O2]center dot 3H(2)O. 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) angstrom, beta 104.12(5)degrees; for Me = Co, the cell parameters are a 8.651(5), b 14.188(8), c 17.713(13) angstrom, beta 104.14(6)degrees, for Me = Ni, they are a 8.662(5), b 14.095(8), c 17.770(9) angstrom, beta 104.18(5)degrees, and for Me = Zn, they are a 8.650(6), b 14.180(12), c 17.709(13) angstrom, beta 104.14(6)degrees. 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 (0 10) comparable to that of zippeite. We demonstrate that a zippeite-group phase with all interlayer containing monovalent potassium cation and divalent transition metal cations call 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.