Chemically induced
polytypic phase transitions have been observed
during experimental investigations of crystallization in the mixed
uranyl sulfate-selenate Mg[(UO2)(TO4)2(H2O)](H2O)4 (T = S, Se) system. Three different structure types
form in the system, depending upon the Se:S ratio in the initial aqueous
solution. The phases with the Se/(Se + S) ratios (in mol %) in the
ranges 0–9, 16–47, and 58–100 crystallize in
the space groups P21, Pmn21, and P21/c, respectively. The structures of the phases are based upon the same
type of uranyl-based sulfate/selenate chains that, through hydrogen
bonds, are linked into pseudolayers of the same topological type.
The layers are linked into three-dimensional structures via interlayer
Mg-centered octahedra. The three structure types contain the same
layers but with different stacking sequences that can be conveniently
described as belonging to the 1M, 2O, and 2M polytypic modifications. The Se-for-S substitution
demonstrates a strong selectivity with preferential incorporation
of Se into less tightly bonded T1 site. The larger
ionic radius of Se6+ relative to S6+ induces
rotation of (T1O4) tetrahedra in the adjacent
layers and reconstruction of the structure types. From the information-theoretic
viewpoint, the intermediate Pmn21 structure
type is more complex than the monoclinic end-member structure types.