Ilmenite-type Na₂(Fe_2/3Te_4/3)O₆

Na₂(Fe_2/3Te_4/3)O₆ (Z = 3) or Na₃(FeTe₂)O₉ (Z = 2), tris­odium iron(III) ditellurium(VI) nona­oxide, adopts the ilmenite (FeTiO₃, Z = 6) structure type with the Ti site (site symmetry 3.) replaced by Na and the Fe site (site symmetry 3.) replaced by a mixed-occupied (Fe^III,Te^VI) site in a Fe:Te ratio of 1:2. Whereas the [(Fe,Te)O₆] octa­hedron is only slightly distorted, the [NaO₆] octa­hedron shows much stronger distortions, as revealed by a larger spread of the bond lengths and some distortion parameters.

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Structure description

Crystals of Na₂(Fe_2/3Te_4/3)O₆ were inadvertently obtained during hydro­thermal synthesis attempts originally aiming at a phase with composition Na₁₂Fe^III₆Te^VI₄O₂₇·3H₂O and for which possible relationships with the potassium phase K₁₂Fe^III₆Te^VI₄O₂₇·3H₂O (Eder, 2023) were to be investigated.

Na₂(Fe_2/3Te_4/3)O₆ (Z = 3) or Na₃(FeTe₂)O₉ (Z = 2) crystallizes in the ilmenite structure type (FeTiO₃, Z = 6). The Na⁺ cations take the Ti sites and the occupationally disordered (Fe^III/Te^VI) atoms (ratio Fe^III:Te^VI = 1:2) take the Fe sites of the ilmenite structure. The latter is a twofold superstructure of the corundum structure where two-thirds of the octa­hedral voids of the hexa­gonal close packed (hcp) structure defined by O atoms are occupied (Wells, 1975). The two types of metal sites in the ilmenite structure, both with site symmetry 3. (multiplicity 6, Wyckoff letter c), have a distorted octa­hedral oxygen environment. The [(Fe,Te)O₆] octa­hedron is only slightly distorted, the [NaO₆] octa­hedron more clearly as evidenced by their bond lengths distribution [(Fe,Te)—O = 1.951 (3) Å (3×), 1.993 (3) Å (3×); Na—O1 = 2.297 (3) Å (3×), 2.545 (4) Å (3×)], and by qu­anti­tative distortion parameters (Robinson et al., 1971) [quadratic elongation: ([(Fe,Te)O₆] = 1.018; [NaO₆] = 1.062; angle variance: [(Fe,Te)O₆] = 62.99°²; [NaO₆] = 204.27°²]. The polyhedral volume of [(Fe,Te)O₆] amounts to 9.950 ų, and that of [NaO₆] to 17.378 ų as calculated with the VOLCAL option in PLATON (Spek, 2020).

The only other Te-containing compounds adopting the ilmenite structure type deposited with the Inorganic Crystal Structure Database (ICSD, release 2023–1; Zagorac et al., 2019) are Na₂(Ti^IVTe^VI)O₆ and α-Na₂(Ge^IVTe^VI)O₆ (Woodward et al., 1999).

The ilmenite-type crystal structure of Na₂(Fe_2/3Te_4/3)O₆ is shown in Figs. 1 and 2.

Figure 1 Crystal structure of ilmenite-type Na2(Fe2/3Te4/3)O6 in a projection along [00] showing the layer stacking along [001]. [(Fe,Te)O6] units are shown in the polyhedral representation (red octa­hedra), Na atoms (blue) with bonds to the O atoms. Displacement ellipsoids are drawn at the 74% probability level.

Figure 2 Projection of the crystal structure of Na2(Fe2/3Te4/3)O6 onto (001), showing only one layer of [(Fe,Te)O6] units (red polyhedra) and of Na atoms (blue). For clarity, Na—O bonds are omitted. Displacement ellipsoids are drawn at the 74% probability level.

Synthesis and crystallization

Hydro­thermal synthesis conditions were the same as detailed for garnet-type Na₃Te₂(FeO₄)₃ (Eder & Weil, 2023). Small amounts of yellowish (nearly colourless) crystals of Na₂(Fe_2/3Te_4/3)O₆ with a plate-like form were harvested from the reaction mixture that also contained very few colourless crystals of NaFe^III(Te^IVO₃)₂ (Weil & Stöger, 2008).

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1.

Table 1
Experimental details

Crystal data
Chemical formula	Na2(Fe2/3Te4/3)O6
Mr	349.33
Crystal system, space group	Trigonal, R:H
Temperature (K)	300
a, c (Å)	5.2598 (8), 15.778 (3)
V (Å^3)	378.02 (14)
Z	3
Radiation type	Mo Kα
μ (mm^−1)	9.76
Crystal size (mm)	0.05 × 0.04 × 0.01
Data collection
Diffractometer	Stoe Stadivari
Absorption correction	Multi-scan (LANA; Koziskova et al., 2016)
Tmin, Tmax	0.519, 0.596
No. of measured, independent and observed [I > 2σ(I)] reflections	3266, 454, 383
Rint	0.043
(sin θ/λ)max (Å^−1)	0.731
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.027, 0.066, 1.03
No. of reflections	454
No. of parameters	17
Δρmax, Δρmin (e Å^−3)	1.64, −1.30
Computer programs: X-AREA (Stoe, 2021), SHELXT (Sheldrick, 2015a), SHELXL Sheldrick, 2015b), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010).

All crystals under investigation were systematically twinned with at least two twin domains present. During the integrating and scaling process of the finally chosen crystal, a pseudo-Laue class of .m was suggested by X-AREA (Stoe, 2021) with similar R_int values compared to the Laue class corresponding to the actual structure (). From this pseudo-symmetry imposed by the twinning, one possible twin law (m₍₂₁₀₎) with a transformation of a, −a−b, c was derived. The intensity data were integrated on basis of a hexa­gonal primitive unit-cell of same dimensions to include the reflections of both twin domains (Fig. 3). By applying the twin law given above, the ratios of the respective domains refined to values of 0.540:0.460 (2). For the sake of charge-neutrality, the mixed-occupied (Fe^III/Te^VI) site was constrained to a ratio of 1:2 for Fe:Te. The two atom types located at this site were refined with common displacement parameters.

Figure 3 (a) Reconstructed reciprocal h0l plane of Na2(Fe2/3Te4/3)O6; (b) reflections belonging to the two twin domains are marked in green squares (domain 1) and orange circles (domain 2); reflections of the two domains overlap in every third row h.

Structure data of Na₂(Fe_2/3Te_4/3)O₆ were standardized with STRUCTURE-TIDY (Gelato & Parthé, 1987).

Structural data

Computing details

Trisodium iron(III) ditellurium(VI) nonaoxide

Crystal data

Na2Fe0.6666Te1.3333O6	Dx = 4.604 Mg m^−3
Mr = 349.33	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3:H	Cell parameters from 3735 reflections
a = 5.2598 (8) Å	θ = 3.9–31.3°
c = 15.778 (3) Å	µ = 9.76 mm^−1
V = 378.02 (14) Å^3	T = 300 K
Z = 3	Plate, yellowish
F(000) = 470	0.05 × 0.04 × 0.01 mm

Data collection

Stoe Stadivari diffractometer	383 reflections with I > 2σ(I)
Radiation source: Axo_Mo	Rint = 0.043
rotation method, ω scans	θmax = 31.3°, θmin = 4.7°
Absorption correction: multi-scan (LANA; Koziskova et al., 2016)	h = −4→7
Tmin = 0.519, Tmax = 0.596	k = −7→5
3266 measured reflections	l = −22→23
454 independent reflections

Refinement

Refinement on F^2	17 parameters
Least-squares matrix: full	0 restraints
R[F^2 > 2σ(F^2)] = 0.027	w = 1/[σ^2(Fo^2) + (0.0446P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.066	(Δ/σ)max < 0.001
S = 1.03	Δρmax = 1.64 e Å^−3
454 reflections	Δρmin = −1.30 e Å^−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.

Refinement. Refined as a 2-component inversion twin.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Te	0.000000	0.000000	0.16114 (4)	0.0138 (2)	0.6667
Fe	0.000000	0.000000	0.16114 (4)	0.0138 (2)	0.3333
Na	0.000000	0.000000	0.3619 (2)	0.0215 (6)
O	0.3411 (6)	0.0563 (7)	0.0969 (2)	0.0181 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Te	0.0101 (2)	0.0101 (2)	0.0212 (3)	0.00503 (11)	0.000	0.000
Fe	0.0101 (2)	0.0101 (2)	0.0212 (3)	0.00503 (11)	0.000	0.000
Na	0.0183 (9)	0.0183 (9)	0.0278 (15)	0.0091 (4)	0.000	0.000
O	0.0126 (14)	0.0156 (13)	0.0260 (15)	0.0068 (11)	0.0020 (11)	−0.0026 (12)

Geometric parameters (Å, º)

Te—O	1.951 (3)	Na—O^vi	2.297 (3)
Te—O^i	1.951 (3)	Na—O^vii	2.297 (3)
Te—O^ii	1.951 (3)	Na—O^viii	2.297 (3)
Te—O^iii	1.993 (3)	Na—O^iii	2.545 (4)
Te—O^iv	1.993 (3)	Na—O^iv	2.545 (4)
Te—O^v	1.993 (3)	Na—O^v	2.545 (4)
O—Te—O^i	95.42 (13)	O^vi—Na—O^iii	98.45 (11)
O—Te—O^ii	95.42 (13)	O^vii—Na—O^iii	91.78 (15)
O^i—Te—O^ii	95.42 (13)	O^viii—Na—O^iii	156.33 (16)
O—Te—O^iii	98.82 (19)	O^vi—Na—O^iv	91.78 (15)
O^i—Te—O^iii	79.08 (15)	O^vii—Na—O^iv	156.34 (16)
O^ii—Te—O^iii	165.14 (16)	O^viii—Na—O^iv	98.45 (11)
O—Te—O^iv	79.08 (15)	O^iii—Na—O^iv	65.98 (14)
O^i—Te—O^iv	165.15 (16)	O^vi—Na—O^v	156.34 (16)
O^ii—Te—O^iv	98.82 (19)	O^vii—Na—O^v	98.45 (11)
O^iii—Te—O^iv	88.08 (14)	O^viii—Na—O^v	91.77 (15)
O—Te—O^v	165.14 (16)	O^iii—Na—O^v	65.98 (14)
O^i—Te—O^v	98.82 (19)	O^iv—Na—O^v	65.98 (14)
O^ii—Te—O^v	79.08 (15)	Te—O—Te^iv	100.92 (15)
O^iii—Te—O^v	88.08 (14)	Te—O—Na^ix	120.25 (14)
O^iv—Te—O^v	88.08 (14)	Te^iv—O—Na^ix	123.88 (14)
O^vi—Na—O^vii	99.80 (15)	Te—O—Na^iv	142.72 (16)
O^vi—Na—O^viii	99.80 (15)	Te^iv—O—Na^iv	87.65 (11)
O^vii—Na—O^viii	99.80 (15)	Na^ix—O—Na^iv	81.55 (11)

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

 

Footnotes

‡Present address: Department of Quantum Matter Physics, Ecole de Physique, University of Geneva, 24, Quai Ernest-Ansermet, CH – 1211 Geneva 4, Switzerland.