Hydro­flux synthesis and crystal structure of Tl₃IO

Single-crystals of thallium(I) iodide oxide Tl₃IO were obtained as by-product in a hydro­flux synthesis at 473 K for 10 h. A potassium hydroxide hydro­flux with a water-base molar ratio of 1.6 and the starting materials TlNO₃, RhI₃ and Ba(NO₃)₂ was used, resulting in a few black needle-shaped crystals. X-ray diffraction on a single-crystal revealed the hexa­gonal space group P6₃/mmc (No. 194) with lattice parameters a = 7.1512 (3) Å and c = 6.3639 (3) Å. Tl₃IO crystallizes as hexa­gonal anti-perovskite (anti-BaNiO₃ type) and is thus structurally related to the alkali-metal halide/auride oxides M₃XO (M = K, Rb, Cs; X = Cl, Br, I, Au). The oxygen atoms center thallium octa­hedra. The [OTl₆] octa­hedra share trans faces, forming a linear chain along [001]. Twelve thallium atoms surround each iodine atom in an [ITl₁₂] anti-cubocta­hedron. Thallium and iodine atoms together form a hexa­gonal close-sphere packing, in which every fourth octa­hedral void is occupied by oxygen.

1. Chemical context

The class of alkali-metal halide/auride oxides comprises several compounds with the general formula M₃XO (M = K, Rb, Cs; X = Cl, Br, I, Au) (Sitta et al., 1991a,b; Feldmann & Jansen, 1995a,b,c; Sabrowsky et al., 1996), Li₃BrO (Wortmann et al., 1989), Na₃X′O (X′ = Cl, Br) (Sabrowsky et al., 1988; Hippler et al., 1990). These ternary oxides crystallize typically as anti-perovskites, i.e. in the cubic anti-CaTiO₃ type. The cesium derivatives Cs₃BrO, Cs₃IO and Cs₃AuO adopt hexa­gonal anti-perovskite structures (anti-BaNiO₃ type), whereas the Cs₃ClO crystallizes as anti-NH₄CdCl₃ type and thus does not form a perovskite structure. The crystal structure of Rb₃IO has both face and corner-sharing [ORb₆] octa­hedra (anti-BaFeO_3–x type). The adopted structure type depends on the size of the alkali-metal and halide/auride ions and their ratio. The stability range of the different perovskite phases can be estimated by using Goldschmidt's tolerance factor, where larger M and X ions tend to destabilize the cubic anti-perovskite structure resulting in the hexa­gonal polymorph (Babel, 1969; Feldmann & Jansen, 1995b).

For the synthesis of the title compound, the hydro­flux method was used, which can be classified as inter­mediate between hydro­thermal and flux synthesis (Chance et al., 2013). An approximately equimolar mixture of alkali-metal hydroxide (typically NaOH or KOH) and water is used as reaction medium (Albrecht et al., 2020a). Good solubility of oxides and hydroxides, highly crystalline reaction products suitable for single-crystal X-ray diffraction analysis, comparably low reaction temperatures and a pressureless setup are essential advantages of the hydro­flux method. In this communication, we report on the synthesis and crystal structure analysis of the thallium(I) iodide oxide Tl₃IO.

2. Structural commentary

Single-crystal X-ray diffraction on a black needle revealed the composition Tl₃IO and a hexa­gonal structure in the space group P6₃/mmc (no. 194) with lattice parameters a = 7.1512 (3) Å and c = 6.3639 (3) Å at 100 (1) K. Tl₃IO crystallizes as hexa­gonal anti-perovskite (anti-BaNiO₃ type; Fig. 1, Tables 1 and 2). The asymmetric unit consists of three atoms, thallium (site symmetry mm2, Wyckoff position 6h), iodine (m2, 2d) and oxygen (m., 2a). The oxygen atoms center thallium octa­hedra. The [OTl₆] octa­hedra share trans faces, forming a linear chain along [001]. Twelve thallium atoms surround each iodine atom in a [ITl₁₂] anti-cubocta­hedron (triangular orthobicupola). Thallium and iodine atoms together form a hexa­gonal close-sphere packing, in which every fourth octa­hedral void is occupied by oxygen. Thus, also the thallium atom centers an anti-cubocta­hedron, which has the composition [Tl(I₄Tl₈)].

Table 1
Atomic coordinates and equivalent isotropic displacement parameters (in 10 ⁴ Ų) in Tl₃IO at 100 (1) K

Atom	Wyckoff symbol	x	y	z	Uiso/Ueq
Tl	6h	0.1608 (1)	0.3216 (1)	1/4	47 (1)
I	2d	2/3	1/3	1/4	53 (1)
O	2a	0	0	0	68 (7)

Table 2
Anisotropic displacement parameters (in 10 ⁴ Ų) in Tl₃IO at 100 (1) K

Atom	U11	U22	U33	U23	U12	U13
Tl	38 (1)	24 (1)	73 (1)	0	0	12 (1)
I	42 (1)	42 (1)	77 (2)	0	0	21 (1)
O	68 (10)	68 (10)	68 (19)	0	0	34 (5)

Figure 1 Crystal structure of Tl3IO in P63/mmc, highlighting the one-dimensional chains consisting of [OTl6] octa­hedra. Ellipsoids enclose 99% of the probability density of the atoms.

The [OTl₆] octa­hedron is slightly elongated along the chain direction. The O—Tl bond length of 2.549 (1) Å is about 1% longer than in Tl₂O, at 2.517 (1) Å (Sabrowsky, 1971). The Tl—O—Tl angles along the chain parallel to c are 94.8 (1)°. The shortest Tl⋯Tl distances in Tl₃IO are with 3.449 (1) Å, very similar to those in thallium metal, which has Tl⋯Tl distances of 3.405 (1) and 3.455 (1) Å in its hexa­gonal sphere packing (Barrett, 1958). Accordingly, the [ITl₁₂] anti­cubocta­hedra are also stretched along [001], with Tl—I distances of 3.576 (1) Å and 3.833 (1) Å. Although thallium(I) has a larger ionic radius (1.70 Å for c.n. = 12; Shannon, 1976) than potassium (1.64 Å for c.n. = 12), the M—O, M⋯M and the average M—I distances in Tl₃IO are smaller than in K₃IO by 3.5%, 7.5% and 1%, respectively (Feldmann & Jansen, 1995b).

3. Synthesis and crystallization

Thallium(I) iodide oxide, Tl₃IO, was synthesized in a potassium hydroxide hydro­flux. The reaction was carried out in a PTFE-lined 50 mL Berghof type DAB-2 stainless steel autoclave starting from TlNO₃ (0.38 mmol; abcr, 99.5%), RhI₃ (0.06 mmol; abcr, 99%), and Ba(NO₃)₂ (0.19 mmol; VEB Laborchemie Apolda, 99%). Water and potassium hydroxide (86%, Fisher Scientific) in the molar ratio of 1.6:1.0 were added to these compounds. The sealed autoclave was heated at a heating rate of 2 K min⁻¹ to 473 K and after 10 h cooled to room temperature at a rate of 0.1 K min⁻¹. The reaction product after washing with water mainly consisted of thallium(I) iodide, thallium(III) oxide, barium carbonate and a brown powder of an unidentified rhodium-containing compound. A few black single crystals of Tl₃IO with a needle-like morphology were found, which are sensitive to water and other protic solvents. In contact with water, the Tl₃IO crystals immediately turn yellow, probably due to the formation of thallium(I) hydroxide and thallium(I) iodide, which are both yellow. Energy-dispersive X-ray spectroscopy on Tl₃IO single-crystals revealed a disproportionately high oxygen content, indicating surface decomposition.

Several experiments failed to exchange rhodium(III) iodide with other iodine sources like potassium iodide, copper(I) iodide or silver(I) iodide. Likewise, experiments without barium nitrate were not successful. However, when both starting materials were used, Tl₃IO was obtained reproducibly, also at reaction temperatures of 423 K or 523 K. Similarly, the hydro­thermal synthesis of Na₃[Tl(OH)₆] starting from thallium(I) sulfate required heavy metal salts like bis­muth nitrate (Giesselbach, 2002). The oxidation of thallium(I) to thallium(III) in this reaction was achieved by oxygen in alkaline solutions (Rich, 2007).

The alkali-metal oxide halides M₃XO are reported to be very sensitive to traces of moisture or carbon dioxide due to their highly basic nature (Feldmann & Jansen, 1995b). Remarkably, Tl₃IO crystallizes in the presence of water from the hydro­flux. In other experiments, we synthesized a water sensitive oxo­hydroxoferrate (Albrecht et al., 2019) or an oxidation sensitive manganate(V) from hydro­flux (Albrecht et al., 2020b). Obviously, the activity of water is dramatically reduced in these aqueous salt melts.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3.

Table 3
Experimental details

Crystal data
Chemical formula	Tl3IO
Mr	756.01
Crystal system, space group	Hexagonal, P63/mmc
Temperature (K)	100
a, c (Å)	7.1512 (3), 6.3639 (3)
V (Å^3)	281.85 (3)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	90.87
Crystal size (mm)	0.09 × 0.05 × 0.03
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2016)
Tmin, Tmax	0.113, 0.749
No. of measured, independent and observed [I > 2σ(I)] reflections	15853, 480, 442
Rint	0.049
(sin θ/λ)max (Å^−1)	0.995
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.018, 0.041, 1.29
No. of reflections	480
No. of parameters	10
Δρmax, Δρmin (e Å^−3)	2.80, −2.01
Computer programs: APEX3 and SAINT (Bruker, 2016), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).

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: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Thallium(I) iodide oxide

Crystal data

Tl3IO	Dx = 8.908 Mg m^−3
Mr = 756.01	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63/mmc	Cell parameters from 8223 reflections
a = 7.1512 (3) Å	θ = 3.2–46.5°
c = 6.3639 (3) Å	µ = 90.87 mm^−1
V = 281.85 (3) Å^3	T = 100 K
Z = 2	Needle, black
F(000) = 608	0.09 × 0.05 × 0.03 mm

Data collection

Bruker APEXII CCD diffractometer	442 reflections with I > 2σ(I)
φ and ω scans	Rint = 0.049
Absorption correction: multi-scan (SADABS; Bruker, 2016)	θmax = 45.0°, θmin = 3.3°
Tmin = 0.113, Tmax = 0.749	h = −14→14
15853 measured reflections	k = −11→14
480 independent reflections	l = −11→12

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0193P)^2] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.018	(Δ/σ)max < 0.001
wR(F^2) = 0.041	Δρmax = 2.80 e Å^−3
S = 1.29	Δρmin = −2.01 e Å^−3
480 reflections	Extinction correction: SHELXL-2016/6 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
10 parameters	Extinction coefficient: 0.0035 (2)

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
Tl	0.16078 (2)	0.32156 (2)	0.250000	0.00465 (5)
I	0.666667	0.333333	0.250000	0.00534 (7)
O	0.000000	0.000000	0.000000	0.0068 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tl	0.00378 (6)	0.00238 (6)	0.00733 (9)	0.00119 (3)	0.000	0.000
I	0.00418 (9)	0.00418 (9)	0.00765 (18)	0.00209 (5)	0.000	0.000
O	0.0068 (10)	0.0068 (10)	0.0068 (19)	0.0034 (5)	0.000	0.000

Geometric parameters (Å, º)

Tl—Tl^i	3.4494 (3)	Tl—Tl^vi	3.7538 (1)
Tl—Tl^ii	3.7538 (1)	Tl—Tl^vii	3.7018 (3)
Tl—Tl^iii	3.7538 (1)	Tl—Tl^viii	3.7018 (3)
Tl—Tl^iv	3.4494 (3)	Tl—O	2.5490 (1)
Tl—Tl^v	3.7538 (1)	Tl—O^ix	2.5490 (1)
Tl^i—Tl—Tl^iv	60.0	O—Tl—Tl^viii	132.580 (2)
Tl^iv—Tl—Tl^iii	62.648 (2)	O^ix—Tl—Tl^viii	132.580 (2)
Tl^vii—Tl—Tl^vi	90.0	O^ix—Tl—Tl^iv	47.420 (2)
Tl^i—Tl—Tl^viii	180.0	O^ix—Tl—Tl^i	47.420 (2)
Tl^v—Tl—Tl^iii	115.917 (5)	O—Tl—Tl^iii	42.580 (2)
Tl^iv—Tl—Tl^viii	120.0	O—Tl—Tl^iv	47.420 (2)
Tl^ii—Tl—Tl^v	149.235 (3)	O^ix—Tl—Tl^vii	132.580 (2)
Tl^i—Tl—Tl^vii	120.0	O—Tl—Tl^vii	132.580 (2)
Tl^iii—Tl—Tl^vi	149.235 (3)	O—Tl—Tl^i	47.420 (2)
Tl^iv—Tl—Tl^vii	180.0	O—Tl—Tl^v	108.774 (4)
Tl^iv—Tl—Tl^vi	90.0	O—Tl—Tl^ii	42.580 (2)
Tl^viii—Tl—Tl^vii	60.0	O—Tl—Tl^vi	108.774 (4)
Tl^v—Tl—Tl^vi	54.704 (4)	O^ix—Tl—Tl^vi	42.580 (2)
Tl^i—Tl—Tl^ii	62.648 (2)	O^ix—Tl—Tl^iii	108.774 (4)
Tl^i—Tl—Tl^iii	90.0	O—Tl—O^ix	77.242 (5)
Tl^iv—Tl—Tl^ii	90.0	Tl^x—O—Tl^i	94.839 (4)
Tl^viii—Tl—Tl^iii	90.0	Tl^x—O—Tl^iii	85.161 (4)
Tl^viii—Tl—Tl^ii	117.352 (2)	Tl—O—Tl^x	180.0
Tl^ii—Tl—Tl^iii	54.704 (4)	Tl^iv—O—Tl^i	85.161 (4)
Tl^vii—Tl—Tl^ii	90.0	Tl—O—Tl^ii	94.840 (4)
Tl^i—Tl—Tl^vi	62.648 (2)	Tl^iv—O—Tl^iii	94.839 (4)
Tl^i—Tl—Tl^v	90.0	Tl^x—O—Tl^ii	85.160 (4)
Tl^viii—Tl—Tl^vi	117.352 (2)	Tl^ii—O—Tl^i	94.839 (4)
Tl^iv—Tl—Tl^v	62.648 (2)	Tl—O—Tl^iv	85.160 (4)
Tl^ii—Tl—Tl^vi	115.917 (5)	Tl—O—Tl^iii	94.840 (4)
Tl^viii—Tl—Tl^v	90.0	Tl^x—O—Tl^iv	94.840 (4)
Tl^vii—Tl—Tl^v	117.352 (2)	Tl^ii—O—Tl^iii	85.161 (4)
Tl^vii—Tl—Tl^iii	117.352 (2)	Tl^ii—O—Tl^iv	180.0
O^ix—Tl—Tl^ii	108.774 (4)	Tl^i—O—Tl^iii	180.0
O^ix—Tl—Tl^v	42.580 (2)	Tl—O—Tl^i	85.160 (4)

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