The Mg member of the isotypic series MTe6O13

MgTe6O13, magnesium hexatellurate(IV), is isotypic with the structures of divalent first-row transition metal analogues MTe6O13 (M = Mn, Fe, Co, Ni and Zn). The asymmetric unit contains one Mg, two Te and five O atoms of which the Mg and one O atom lie on a threefold rotation axis. The structure is made up from slightly distorted [MgO6] octahedra (isolated from each other), distorted [TeO4] bisphenoids and [TeO4 + 1] tetragonal pyramids sharing corners and edges. This arrangement leads to the formation of a dense three-dimensional structure.

MgTe 6 O 13 , magnesium hexatellurate(IV), is isotypic with the structures of divalent first-row transition metal analogues MTe 6 O 13 (M = Mn, Fe, Co, Ni and Zn). The asymmetric unit contains one Mg, two Te and five O atoms of which the Mg and one O atom lie on a threefold rotation axis. The structure is made up from slightly distorted [MgO 6 ] octahedra (isolated from each other), distorted [TeO 4 ] bisphenoids and [TeO 4 + 1 ] tetragonal pyramids sharing corners and edges. This arrangement leads to the formation of a dense three-dimensional structure.
It should be mentioned that the given space group R3m for FeTe 6 O 13 (van der Lee & Astier, 2007) is incorrect. The correct space group in fact is R3 (van der Lee, 2013). Lattice parameters and volumes of the several MTe 6 O 13 phases are in a narrow range (Table 1), as one expects from the similar ionic radii of Mg and the first row transition metals.
The magnesium cation is located on a threefold rotation axis and is surrounded by six oxygen atoms in a slightly distored octahedral environment. Because of the high tellurium content in the structure, the [MgO 6 ] octahedra are isolated from each other. The two distinct tellurium(IV) atoms exhibit different oxygen environments. Te1 is bonded to four oxygen atoms in form of a bisphenoid with distances ranging from 1.8524 (12) to 2.1842 (11) Å. Considering Te···O separations less than 3.1 Å, two remote O atoms (O3, O5) are also present, leading to a pentagonal pyramid [TeO 4 + 2 ] as the resulting coordination polyhedron. Atom Te2 is bonded to four O atoms with distances ranging from 1.8489 (11) to 2.2118 (12) Å, with an additional O atom 2.5908 (12) Å away. The resulting [TeO 4 + 1 ] polyhedron is a distorted tetragonal pyramid. It is augmented to a distorted octahedron if the remote O4 atom at a distance of 3.0387 (12) Å is also considered. For both [TeO x ] polyhedra ( Fig. 1, Table 2) the rules derived by Zemann for the crystal chemistry of oxotellurates(IV) are valid (Zemann, 1971 (Fig. 2). In a simpler view, the structure can be described as being built up from distorted hexagonal layers of the Mg and Te atoms extending parallel to (001) and stacked in an ABCA′B′C′ sequence along [001]. The oxygen atoms are situated in the voids of this arrangement.

Experimental
Magnesium oxide, tellurium dioxide and selenic acid (conc.; 96% wt ) were loaded in the stoichiometric ratio 3:2:1 in a Teflon lined stainless steel autoclave (overall volume 10 ml) that was filled up to two-thirds of its volume with water. The autoclave was then heated at 503 K for one week. Few colourless single crystals of Mg 2 Te 3 O 8 (Lin et al., 2013) and MgTe 6 O 13 , both with a platy habit, were isolated from the colourless solid reaction product. X-ray powder diffraction (XRPD) of the ground bulk material revealed Mg 2 Te 3 O 8 and TeO 2 as main products. Se-containing phases could not be identified by XRPD in the solid reaction product.

Refinement
The atomic coordinates of isotypic ZnTe 6 O 13 (Nawash et al., 2007) were used as starting parameters for the refinement.
Reflection 003 was affected from the beamstop and was omitted from the refinement. The highest positive and negative residual electron densities are located 0.68 and 0.48 Å, respectively, from atom Te1.

Figure 1
The coordination spheres of the two tellurium(IV) atoms. Te-O bonds < 2.6 Å are given as solid lines, Te-O bonds between 2.6 and 3.1 Å as open lines; probability level of the displacement ellipsoids is 74%. [Symmetry codes: i) -y + 1, x-y+1, z; iii) -x + 2/3, -y + 1/3, -z + 1/3; iv) -x + 1, -y + 1, -z; vii) y, -x+y, -z; ix) x-y+2/3, x + 1/3, -z + 1/3.]  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (12)