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B. Stöger, H. Krüger, M. Weil:
"Mg(H₂O)₂[TeO₂(OH)₄]: a polytypic structure with a two-mode disordered stacking arrangement";
Acta Crystallographica Section B, 77 (2021), S. 605 - 623.

Crystals of the hydrous magnesium orthotellurate(VI) Mg(H₂O)2[TeO₂(OH)₄] were grown by slow diffusion of an aqueous MgCl₂ solution into a KOH/Te(OH)₆ solution immobilized in gelatin. The crystal structure is built of sheets of nearly regular corner-sharing [MgO₆] and [TeO₆] octahedra. Half of the bridging O atoms are connected to disordered H atoms, which are located in rhomboidal voids (long and short diameters of ~5.0 and ~2.5 Å, respectively) of these layers. Moreover, the Te^VI atom connects to two OH^- ions and the Mg^II atom to two H₂O molecules. The OH^- ions and H₂O molecules connect adjacent layers forming a disordered hydrogen-bonding network. In a given layer, an adjacent layer may be positioned in four ways, which can be characterized by one of two origin shifts and one of two orientations with respect to [100]. The crystals feature a disordered stacking arrangement, leading to rods of diffuse scattering in the diffraction pattern. The polytypism is explained by application of the order-disorder (OD) theory. Different refinement models are compared and the diffuse scattering is evaluated with structure factor calculations. The correlation coefficient of subsequent origin shifts is ~ -0.33, whereas the orientation of the layers is essentially random. Determining the latter is particularly difficult owing to a small contribution to the diffraction pattern and virtually indistinguishable diffraction patterns for pairs of correlations with the same absolute value. On longer standing in a glass vial, an ordered polytype forms.

polytypism; order-disorder (OD) theory; disorder; diffuse scattering; homometry

http://dx.doi.org/10.1107/S2052520621006223