Crystal structure of potassium orthoselenate(IV), K2SeO3

Potassium orthoselenate(IV), K2SeO3, crystallizes isostructural with Na2SO3 and K2TeO3 in the trigonal space group P with lattice parameters a = 6.1063 (4) Å and c = 6.9242 (4) Å at 100 K.


Chemical context
Ternary alkali metal selenates(IV) are a long-known but poorly studied class of compounds. After the discovery of the first salts of selenic acid by Berzelius, comprehensive studies on these salts were not carried out until the beginning of the 1930s, when Janitzki reported the syntheses of sodium and potassium salts of selenic acid (Janitzki, 1932). Moreover, the composition and solubility of hydrates and anhydrates of these selenates(IV) were determined. However, only two crystal structures of ternary alkali metal selenates(IV) are known to date, viz. K 2 Se 2 O 5 (Rider et al., 1985) and Na 2 SeO 3 (Helmholdt et al., 1999;Wickleder, 2002). The latter compound was synthesized by annealing a mixture of Na 2 O and SeO 2 at 773 K.
In this communication, we report on the synthesis and crystal structure of potassium orthoselenate(IV), K 2 SeO 3 . The title compound was synthesized using the hydroflux approach, an ultra-alkaline reaction medium consisting of an approximately equimolar mixture of water and alkali metal hydroxide Chance et al., 2013). Advantages of the hydroflux method are the good solubility of oxides and hydroxides, the fast and simple reaction at moderate temperatures, and the formation of single-crystals suitable for X-ray diffraction. Moreover, the high hydroxide concentration within the hydroflux reduces the activity of water, leading to the unexpected fact that water-sensitive products can be isolated, e.g. K 2 [Fe 2 O 3 (OH) 2 ] (Albrecht et al., 2019), Tl 3 IO (Albrecht et al., 2020), or K 2 Te 3 (Albrecht & Ruck, 2021).
It is noted that the X-ray powder diffraction pattern of ground K 2 SeO 3 crystals ( Fig. 3) differs significantly from previously published data (Hanawalt et al., 1938;Klushina et al., 1968).

Database survey
K 2 SeO 3 crystallizes isostructural with Na 2 SO 3 (Zachariasen & Buckley, 1931;Larsson & Kierkegaard, 1969) and K 2 TeO 3 (Andersen et al., 1989). On a more general level, the structure of K 2 SeO 3 can be related to the Ni 2 In type in space group P6 3 / mmc (Laves & Wallbaum, 1942), with the K + ions on the Ni positions and [SeO 3 ] 2anions occupying the positions of the In atoms. The orientation of the selenate(IV) groups is responsible for the symmetry reduction to P3; the higher pseudosymmetry is mirrored in the respective twin laws.

Synthesis and crystallization
Potassium orthoselenate(IV), K 2 SeO 3 , was synthesized in a potassium hydroxide hydroflux with a molar water-base ratio of 1.7. The reaction was carried out in a PTFE-lined 50 mL Berghof-type DAB-2 stainless steel autoclave to prevent evaporation of water. The starting material SeO 2 (4 mmol, abcr, 99.8%) was dissolved in 3 ml of water before adding 6.3 g of KOH (Fischer Scientific, 86%). After closing the autoclave, the reaction mixture was heated to 473 K at a rate of 2 K min À1 and, after 8 h, cooled to room temperature at a rate of À1 K min À1 . The solid reaction product was washed twice with 2 ml of methanol on a Schlenk frit under inert conditions to remove adherent hydroflux. The colorless, block-shaped crystals of K 2 SeO 3 (Fig. 4) dissolve readily in water, but dissolve in methanol a little slower than the hydroflux. Scanning electron microscopy showed that the surface of the crystals was etched by the washing process (Fig. 5). Due to its hygroscopicity, the product was dried in dynamic vacuum and stored under argon. Pure K 2 SeO 3 was obtained with a yield of about 50%. Energy-dispersive X-ray spectroscopy on selected crystals confirmed the chemical composition within the limits of the method.
For the Rietveld refinement, the program JANA2006 was used ( Coordination polyhedra of the potassium atoms, with displacement ellipsoids drawn at the 99% probability level.

Figure 1
Crystal structure of K 2 SeO 3 at 100 K, with displacement ellipsoids drawn at the 99% probability level; the unit cell is outlined. performed using a SU8020 (Hitachi) with a triple detector system for secondary and low-energy backscattered electrons (U a = 5 kV). The composition of selected single crystals was determined by semi-quantitative energy dispersive X-ray analysis (U a = 15 kV) using a Silicon Drift Detector X-MaxN (Oxford Instruments). The data were processed applying the AZtec software package (Oxford Instruments, 2013).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The investigated crystal was found to be a fourfold twin: twinning by merohedry plus twofold rotation along [001]. The crystal, thus, partially conserves the hexagonal (pseudo-)symmetry of the Ni 2 In type.

Funding information
Funding for this research was provided by: Deutsche Forschungsgemeinschaft (grant No. 438795198).

Figure 4
Photograph of K 2 SeO 3 crystals.

Figure 5
Scanning electron microscopy image after the washing process.