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. 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.


Chemical context
The class of alkali-metal halide/auride oxides comprises several compounds with the general formula M 3 XO (M = K, Rb, Cs; X = Cl, Br, I, Au) (Sitta et al., 1991a,b;Feldmann & Jansen, 1995a,b,c;Sabrowsky et al., 1996), Li 3 BrO (Wortmann et al., 1989), Na 3 X 0 O (X 0 = Cl, Br) (Sabrowsky et al., 1988;Hippler et al., 1990). These ternary oxides crystallize typically as anti-perovskites, i.e. in the cubic anti-CaTiO 3 type. The cesium derivatives Cs 3 BrO, Cs 3 IO and Cs 3 AuO adopt hexagonal anti-perovskite structures (anti-BaNiO 3 type), whereas the Cs 3 ClO crystallizes as anti-NH 4 CdCl 3 type and thus does not form a perovskite structure. The crystal structure of Rb 3 IO has both face and corner-sharing [ORb 6 ] 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 antiperovskite 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, ISSN 2056-9890 we report on the synthesis and crystal structure analysis of the thallium(I) iodide oxide Tl 3 IO.

Synthesis and crystallization
Thallium(I) iodide oxide, Tl 3 IO, 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 TlNO 3 (0.38 mmol; abcr, 99.5%), RhI 3 (0.06 mmol; abcr, 99%), and Ba(NO 3 ) 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 À1 to 473 K and after 10 h cooled to room temperature at a rate of 0.1 K min À1 . 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 Tl 3 IO with a needlelike morphology were found, which are sensitive to water and other protic solvents. In contact with water, the Tl 3 IO 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 Tl 3 IO singlecrystals 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, Tl 3 IO was obtained reproducibly, also at reaction temperatures of 423 K or 523 K. Similarly, the hydrothermal synthesis of Na 3 [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 M 3 XO are reported to be very sensitive to traces of moisture or carbon dioxide due to their highly basic nature (Feldmann & Jansen, 1995b). Remarkably, Tl 3 IO 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.  Table 1 Atomic coordinates and equivalent isotropic displacement parameters (in 10 4 Å 2 ) in Tl 3 IO at 100 (1) K.

Atom
Wyckoff symbol x y z U iso /U eq   Figure 1 Crystal structure of Tl 3 IO in P6 3 /mmc, highlighting the one-dimensional chains consisting of [OTl 6 ] octahedra. Ellipsoids enclose 99% of the probability density of the atoms.

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

Funding information
This work was financially supported by the Deutsche Forschungsgemeinschaft (project-id: 438795198).

Crystal data
Tl 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )