Garnet-type Na3Te2(FeO4)3

The garnet-type crystal structure of Na3Te2(FeO4)3 shows high similarities with its isotypic analogues Na3Te2[(Fe0.5Al0.5)O4]3 and Na3Te2(GaO4)3.


Structural commentary
The garnet supergroup has the general formula {X 3 }[Y 2 ](Z 3 )' 12 and includes all phases, which crystallize isostructurally with garnet, regardless of the type of elements present at the four atomic sites (Grew et al., 2013). The crystal structure of garnet comprises a three-dimensional framework built of [Y' 6 ] octahedra and (Z' 4 ) tetrahedra in which each octahedron is joined to six others through vertex-sharing tetrahedra. In turn, each tetrahedron shares its vertices with four octahedra, so that the composition of the framework is Y 2 Z 3 ' 12 . Larger X atoms occupy positions in the interstices of the framework and are eightfold coordinated in the form of a distorted dodecahedron (Wells, 1975). In a crystal-chemical sense, the final composition can therefore be expressed as In the title compound, Na takes the X position (multiplicity 24, Wyckoff letter c, site symmetry 2.22), Te the Y position (16 a, .3.), Fe the Z position (24 d, 4..) and O the ' position (96 h, 1). The crystal structure of Na 3 Te 2 (FeO 4 ) 3 is displayed in Fig. 1. Bond-valence sums (Brown, 2002) for all atoms were computed with the parameters of Brese & O'Keeffe (1991). The values (in valence units) of 1.19 for Na, 6.00 for Te, 2.98 for Fe and 2.04 for O are in very good agreement with the expected values of 1, 6, 3 and 2, respectively.
The garnet supergroup includes several chemical classes, which is also reflected by the high number of phases that adopt  (Wedel & Sugiyama, 1999) and Na 3 Te 2 (GaO 4 ) 3 (Frau et al., 2008). The latter two phases comprise Na on the X position and, with respect to the title compound, therefore are the chemically most related compounds. A comparison of relevant bond lengths in the three garnets, together with structural similarity parameters, as revealed by the program compstru (de la Flor et al., 2016) available at the Bilbao Crystallographic Server (Aroyo et al., 2006), is given in Table 1. The cations occupying the Z site apparently influence the two Na-O bond lengths in the crystal structures, although the ionic radii (Shannon, 1976) of Z do not directly correlate with this behaviour. The title compound with Z = Fe (ionic radius 0.49 Å ) has the longest Na-O bonds, followed by the mixedoccupied compound with Z = (Fe,Al) (averaged ionic radius 0.44 Å ) and the compound with Z = Ga (ionic radius 0.47 Å ). On the other hand, the Te-O bond lengths in the three garnet structures are virtually identical.
An X-ray powder diffraction pattern of Na 3 Te 2 (FeO 4 ) 3 has been deposited with the ICDD (PDF 00-048-0300; Gates-Rector & Blanton, 2019) without giving atomic coordinates for the O-atom site or displacement parameters for the atoms. The corresponding unit-cell parameter a = 12.5257 (1) Å determined from room-temperature powder X-ray measurement data is in very good agreement with the one from singlecrystal data (Table 2). In the context of investigating the magnetic ordering of Fe III on the Z sites, neutron powder data recorded at room temperature were also reported for Na 3 Te 2 (FeO 4 ) 3 (Plakhtii et al., 1977).

Synthesis and crystallization
The solid educts Fe(NO 3 ) 3 Á9H 2 O, TeO 2 , H 6 TeO 6 and NaOH were weighed in the molar ratios 2:1:2:15 and placed into a Teflon container (inner volume ca 5 ml). The container was filled to about 2/3 of its volume with water, closed with a Projection of the garnet-type crystal structure of Na 3 Te 2 (FeO 4 ) 3 along [010]. Displacement ellipsoids are drawn at the 90% probability level.
[TeO 6 ] octahedra (red) and (FeO 4 ) tetrahedra (blue) are given in the polyhedral representation, Na atoms as green ellipsoids and O atoms as white ellipsoids. Table 1 Selected bond lengths (Å ) in related garnet-type Na 3 Te 2 (ZO 4 ) 3 oxidotellurates(VI) and their structure similarity parameters relative to Na 3 Te 2 (FeO 4 ) 3 . Teflon lid and embedded into a steel autoclave. The hydrothermal experiment was conducted at 473 K for five days. The solid product was filtered off, washed with water and ethanol and dried in air. It consisted of light-brown microcrystalline material and a few amber-coloured cuboid crystals of Na 3 Te 2 (FeO 4 ) 3 , as well as a very few small yellowish platy crystals of an unknown phase. Preliminary single-crystal measurements of the latter indicated a unit cell with hexagonal metrics (a = 5.252, c = 15.724 Å ) and obvious twinning, which has precluded a structure solution so far. Similar metrics were found for Na 2 GeTeO 6 (Woodward et al., 1998). The powder X-ray diffraction pattern of the bulk revealed Na 3 Te 2 (FeO 4 ) 3 as a side product and the unknown phase (assuming a close relation with Na 2 GeTeO 6 ) as the main phase, in an approximate mass ratio of 0.15:0.85.

Crystal data
Na 3  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.