Garnet-type Na₃Te₂(FeO₄)₃

Na₃Te₂(FeO₄)₃ or Na₃Te₂Fe₃O₁₂, tris­odium ditellurium(VI) triiron(III) dodeca­oxide, was obtained in the form of single-crystals under hydro­thermal conditions. Na₃Te₂(FeO₄)₃ adopts the garnet structure type in space group Iad and comprises one Na (multiplicity 24, Wyckoff letter c, site symmetry 2.22), one Te (16 a, ..), one Fe (24 d, ..) and one O atom (96 h, 1) in the asymmetric unit. The three-dimensional framework structure is built of [TeO₆] octa­hedra and [FeO₄] tetra­hedra by vertex-sharing. The larger Na⁺ cations are situated in the inter­stices of the framework and are eightfold coordinated in the form of a distorted dodeca­hedron. Qu­anti­tative structural comparisons with isotypic Na₃Te₂[(Fe_0.5Al_0.5)O₄]₃ and Na₃Te₂(GaO₄)₃ show a high degree of similarity between the three crystal structures.

1. Chemical context

Layered oxidotellurates(VI) comprising an alkali metal (or ammonium) and a transition metal M, such as (NH₄)₄(VO₂)₂Te₂O₈(OH)₂·2H₂O (Nagarathinam et al., 2022), Li₂Ni₂TeO₆ (Grundish et al., 2019), Na₃Ni_1.5TeO₆ (Grundish et al., 2020) or K₂M₂TeO₆ (M = Ni, Mg, Zn, Co, Cu; Masese et al., 2018) are considered to be promising battery materials. In the quest for new representatives of this group of materials comprising K and Fe^III, we obtained a phase under hydro­thermal conditions with a supposed composition of K₁₂Fe^III₆Te^VI₄O₂₇·3H₂O. However, this phase is not layered but crystallizes in a cubic framework structure with positionally disordered crystal water mol­ecules [Z = 4, space group I3d, a = 14.7307 (12) Å at room temperature; Eder & Weil, 2023], which is closely related to the phase K_12+6xFe₆Te_4–xO₂₇ [x = 0.222 (4), Z = 4, space group I3d, a = 14.7440 (10) Å at 100 K; Albrecht et al., 2021]. With the intention of synthesizing the possible Na-analogue Na₁₂Fe^III₆Te^VI₄O₂₇·3H₂O, we obtained garnet-type Na₃Te₂(FeO₄)₃ instead, and report here its crystal structure and qu­anti­tative comparisons with related crystal structures.

2. Structural commentary

The garnet supergroup has the general formula {X₃}[Y₂](Z₃)φ₁₂ 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 [Yφ₆] octa­hedra and (Zφ₄) 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 Y₂Z₃φ₁₂. 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 {X₃}^[8do][Y₂]^[6o](Z₃^[4t])φ₁₂, or as {X₃}^[8do][Y₂]^[6o](Z^[4t]φ₄)₃. In the title compound, Na takes the X position (multiplicity 24, Wyckoff letter c, site symmetry 2.22), Te the Y position (16 a, ..), Fe the Z position (24 d, ..) and O the φ position (96 h, 1). The crystal structure of Na₃Te₂(FeO₄)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.

Figure 1 Projection of the garnet-type crystal structure of Na3Te2(FeO4)3 along [00]. Displacement ellipsoids are drawn at the 90% probability level. [TeO6] octa­hedra (red) and (FeO4) tetra­hedra (blue) are given in the polyhedral representation, Na atoms as green ellipsoids and O atoms as white ellipsoids.

The garnet supergroup includes several chemical classes, which is also reflected by the high number of phases that adopt the garnet structure type. A search in the ICSD (version 2022-1; Zagorac et al., 2019), using the garnet structure type in space group Iad and with Si on the Z position as search field revealed about 420 entries, and with atoms other than Si on the Z position about 350 entries. With Te on the Y position, only five phases were found, including the mineral yafsoanite [ideally Ca₃Te₂(ZnO₄)₃, Jarosch & Zemann, 1989; Mills et al., 2010], the Li-conducting Nd₃(Te_2–xSb_x)(Li_3+xO₄)₃ (x = 0.05, 0.10) (O'Callaghan et al., 2008), Na₃Te₂[(Fe_0.5Al_0.5)O₄]₃ (Wedel & Sugiyama, 1999) and Na₃Te₂(GaO₄)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 mixed-occupied 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.

Table 1
Selected bond lengths (Å) in related garnet-type Na₃Te₂(ZO₄)₃ oxidotellurates(VI) and their structure similarity parameters relative to Na₃Te₂(FeO₄)₃

	Na3Te2(FeO4)3	Na3Te2[(Al,Fe)O4]3	Na3Te2(GaO4)3
Na1—O1 (4×)	2.4208 (10)	2.396 (3)	2.3907 (17)
Na1—O1 (4×)	2.6226 (10)	2.597 (3)	2.5609 (17)
Te1—O1 (6×)	1.9169 (9)	1.914 (2)	1.9124 (17)
M1—O1 (4×)	1.8680 (9)	1.829 (2)	1.8405 (16)
Degree of lattice distortion, S		0.0064	0.0079
Atomic displacement of O1^a (Å)		0.0205	0.0322
Measure of similarity, Δ		0.001	0.002
Note: (a) The three other atomic sites do not show a displacement due to their site symmetries.

An X-ray powder diffraction pattern of Na₃Te₂(FeO₄)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 single-crystal 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 Na₃Te₂(FeO₄)₃ (Plakhtii et al., 1977).

Table 2
Experimental details

Crystal data
Chemical formula	Na3Te2Fe3O12
Mr	683.72
Crystal system, space group	Cubic, Iad
Temperature (K)	296
a (Å)	12.5276 (9)
V (Å^3)	1966.1 (4)
Z	8
Radiation type	Mo Kα
μ (mm^−1)	10.39
Crystal size (mm)	0.06 × 0.06 × 0.06
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.677, 0.748
No. of measured, independent and observed [I > 2σ(I)] reflections	42303, 569, 446
Rint	0.060
(sin θ/λ)max (Å^−1)	0.934
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.017, 0.041, 1.16
No. of reflections	569
No. of parameters	18
Δρmax, Δρmin (e Å^−3)	1.25, −0.68
Computer programs: APEX3 and SAINT (Bruker, 2016), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), ATOMS for Windows (Dowty, 2006) and publCIF (Westrip, 2010).

3. Synthesis and crystallization

The solid educts Fe(NO₃)₃·9H₂O, TeO₂, H₆TeO₆ 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 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 Na₃Te₂(FeO₄)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 Na₂GeTeO₆ (Woodward et al., 1998). The powder X-ray diffraction pattern of the bulk revealed Na₃Te₂(FeO₄)₃ as a side product and the unknown phase (assuming a close relation with Na₂GeTeO₆) as the main phase, in an approximate mass ratio of 0.15:0.85.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Supporting information

Computing details

Data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: ATOMS for Windows (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Trisodium ditellurium(VI) triiron(III) dodecaoxide

Crystal data

Na3Te2Fe3O12	Mo Kα radiation, λ = 0.71073 Å
Mr = 683.72	Cell parameters from 6128 reflections
Cubic, Ia3d	θ = 4.0–41.1°
a = 12.5276 (9) Å	µ = 10.39 mm^−1
V = 1966.1 (4) Å^3	T = 296 K
Z = 8	Cube, amber
F(000) = 2488	0.06 × 0.06 × 0.06 mm
Dx = 4.620 Mg m^−3

Data collection

Bruker APEXII CCD diffractometer	446 reflections with I > 2σ(I)
ω– and φ–scans	Rint = 0.060
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θmax = 41.6°, θmin = 4.0°
Tmin = 0.677, Tmax = 0.748	h = −23→23
42303 measured reflections	k = −23→23
569 independent reflections	l = −23→23

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0169P)^2 + 1.9053P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.017	(Δ/σ)max < 0.001
wR(F^2) = 0.041	Δρmax = 1.25 e Å^−3
S = 1.16	Δρmin = −0.68 e Å^−3
569 reflections	Extinction correction: SHELXL-2019/2 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
18 parameters	Extinction coefficient: 0.00158 (6)

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Na1	0.250000	0.375000	0.500000	0.01227 (18)
Te1	0.500000	0.500000	0.500000	0.00480 (5)
Fe1	0.250000	0.625000	0.500000	0.00637 (7)
O1	0.35650 (7)	0.53021 (8)	0.45633 (8)	0.00976 (16)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na1	0.0150 (3)	0.0068 (4)	0.0150 (3)	0.000	0.0012 (4)	0.000
Te1	0.00480 (5)	0.00480 (5)	0.00480 (5)	0.00038 (3)	0.00038 (3)	0.00038 (3)
Fe1	0.00652 (9)	0.00605 (14)	0.00652 (9)	0.000	0.000	0.000
O1	0.0068 (4)	0.0106 (4)	0.0119 (4)	0.0023 (3)	−0.0013 (3)	−0.0006 (3)

Geometric parameters (Å, º)

Na1—O1^i	2.4208 (10)	Te1—O1^viii	1.9169 (9)
Na1—O1^ii	2.4208 (10)	Te1—O1^ix	1.9169 (9)
Na1—O1^iii	2.4208 (10)	Te1—O1^x	1.9169 (9)
Na1—O1	2.4208 (10)	Te1—O1^vii	1.9169 (9)
Na1—O1^iv	2.6226 (10)	Te1—O1^xi	1.9169 (9)
Na1—O1^v	2.6226 (10)	Fe1—O1^xii	1.8680 (9)
Na1—O1^vi	2.6226 (10)	Fe1—O1^ii	1.8680 (9)
Na1—O1^vii	2.6226 (10)	Fe1—O1^xiii	1.8680 (9)
Te1—O1	1.9169 (9)	Fe1—O1	1.8680 (9)
O1^i—Na1—O1^ii	153.42 (4)	O1—Te1—O1^viii	91.50 (4)
O1^i—Na1—O1^iii	73.12 (4)	O1—Te1—O1^ix	88.50 (4)
O1^ii—Na1—O1^iii	113.33 (4)	O1^viii—Te1—O1^ix	180.0
O1^i—Na1—O1	113.33 (4)	O1—Te1—O1^x	91.50 (4)
O1^ii—Na1—O1	73.12 (4)	O1^viii—Te1—O1^x	88.50 (4)
O1^iii—Na1—O1	153.42 (4)	O1^ix—Te1—O1^x	91.50 (4)
O1^i—Na1—O1^iv	125.56 (2)	O1—Te1—O1^vii	88.50 (4)
O1^ii—Na1—O1^iv	77.63 (3)	O1^viii—Te1—O1^vii	91.50 (4)
O1^iii—Na1—O1^iv	63.92 (4)	O1^ix—Te1—O1^vii	88.50 (4)
O1—Na1—O1^iv	94.14 (3)	O1^x—Te1—O1^vii	180.0
O1^i—Na1—O1^v	94.14 (3)	O1—Te1—O1^xi	180.0
O1^ii—Na1—O1^v	63.92 (4)	O1^viii—Te1—O1^xi	88.50 (4)
O1^iii—Na1—O1^v	77.63 (3)	O1^ix—Te1—O1^xi	91.50 (4)
O1—Na1—O1^v	125.56 (2)	O1^x—Te1—O1^xi	88.50 (4)
O1^iv—Na1—O1^v	106.99 (4)	O1^vii—Te1—O1^xi	91.50 (4)
O1^i—Na1—O1^vi	63.92 (4)	O1^xii—Fe1—O1^ii	113.83 (3)
O1^ii—Na1—O1^vi	94.14 (3)	O1^xii—Fe1—O1^xiii	101.06 (6)
O1^iii—Na1—O1^vi	125.56 (2)	O1^ii—Fe1—O1^xiii	113.83 (3)
O1—Na1—O1^vi	77.63 (3)	O1^xii—Fe1—O1	113.83 (3)
O1^iv—Na1—O1^vi	169.86 (4)	O1^ii—Fe1—O1	101.06 (6)
O1^v—Na1—O1^vi	73.94 (4)	O1^xiii—Fe1—O1	113.83 (3)
O1^i—Na1—O1^vii	77.63 (3)	Fe1—O1—Te1	135.38 (5)
O1^ii—Na1—O1^vii	125.56 (2)	Fe1—O1—Na1	92.91 (4)
O1^iii—Na1—O1^vii	94.14 (3)	Te1—O1—Na1	107.08 (4)
O1—Na1—O1^vii	63.92 (4)	Fe1—O1—Na1^ix	116.33 (4)
O1^iv—Na1—O1^vii	73.94 (4)	Te1—O1—Na1^ix	99.78 (4)
O1^v—Na1—O1^vii	169.86 (4)	Na1—O1—Na1^ix	98.95 (3)
O1^vi—Na1—O1^vii	106.99 (4)

Symmetry codes: (i) −z+3/4, −y+3/4, −x+3/4; (ii) −x+1/2, y, −z+1; (iii) z−1/4, −y+3/4, x+1/4; (iv) y−1/4, −x+3/4, z+1/4; (v) −z+1/2, x, −y+1; (vi) −y+3/4, −x+3/4, −z+3/4; (vii) z, x, y; (viii) −y+1, −z+1, −x+1; (ix) y, z, x; (x) −z+1, −x+1, −y+1; (xi) −x+1, −y+1, −z+1; (xii) −z+3/4, −y+5/4, x+1/4; (xiii) z−1/4, −y+5/4, −x+3/4.