Crystal chemistry of the M2+[(UO2)(T6+O4)2(H2O)](H2O)4 (M2+ = Mg, Mn, Fe, Co, Ni and Zn; T6+ = S, Se) compounds: the interplay between chemical composition, pH and structural architecture

Abstract

Crystallization of the M²⁺[(UO₂)(T⁶⁺O₄)₂(H₂O)](H₂O)₄ (M²⁺ = Ni, Mg, Zn, Co, Fe and Mn; T⁶⁺ = S, Se) compounds is characterized by the presence of three different structure types, which are built by similar uranyl-based sulfate/selenate chains linked into pseudolayers through H-bonding networks, but the stacking sequences are different and can be described by the 1M, 2O and 2M polytypic modifications. In order to establish the interplay between chemical composition and structural architecture, eleven new Ni-bearing uranyl sulfate–selenate compounds have been synthesized, as well as a new Zn-bearing uranyl sulfate. To provide the extensive comparison of all the compounds, the crystal structures of previously studied Co and Mn uranyl sulfates of this family of compounds were re-investigated, which allowed reconsideration of their unit-cell parameters and the space-group symmetries. It is shown that crystallization of particular polytypes is governed by the interplay of the ionic radii of M²⁺ and T⁶⁺ cations, while pH of the initial solutions affects the rate of crystallization.