Mg(H(2)O)(2)[TeO(2)(OH)(4)]: a polytypic structure with a two-mode disordered stacking arrangement

Crystals of the hydrous magnesium orthotellurate(VI) Mg(H(2)O)(2)[TeO(2)(OH)(4)] were grown by slow diffusion of an aqueous MgCl(2) solution into a KOH/Te(OH)(6) solution immobilized in gelatin. The crystal structure is built of sheets of nearly regular corner-sharing [MgO(6)] and [TeO(6)] 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 H(2)O molecules. The OH(−) ions and H(2)O 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.