Dipotassium hydrogencarbonate fluoride monohydrate

Single crystals of the title compound, K2(HCO3)F·H2O, were obtained as a secondary product after performing flux synthesis experiments aimed at the preparation of potassium rare earth silicates. The basic building unit of the structure is an [(HCO3)(H2O)F]2− zigzag chain running parallel to [001]. Both types of anions as well as the water molecules reside on mirror planes perpendicular to [010] at y = 0.25 and y = 0.75, respectively. Linkage between the different constituents of the chains is provided by O—H⋯O and O—H⋯F hydrogen bonding. The K+ cations are located between the chains and are coordinated by two F and five O atoms in form of a distorted monocapped trigonal prism.

Single crystals of the title compound, K 2 (HCO 3 )FÁH 2 O, were obtained as a secondary product after performing flux synthesis experiments aimed at the preparation of potassium rare earth silicates. The basic building unit of the structure is an [(HCO 3 )(H 2 O)F] 2À zigzag chain running parallel to [001]. Both types of anions as well as the water molecules reside on mirror planes perpendicular to [010] at y = 0.25 and y = 0.75, respectively. Linkage between the different constituents of the chains is provided by O-HÁ Á ÁO and O-HÁ Á ÁF hydrogen bonding. The K + cations are located between the chains and are coordinated by two F and five O atoms in form of a distorted monocapped trigonal prism.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2728). OH-group, which is also involved in hydrogen bonding. Consequently, the O1-C-O3 and O2-C-O3 angles are significantly smaller than 120°, whereas the O1-C-O2 angle is larger compared with the ideal undistorted case.
The potassium cations are coordinated by two fluorine and five oxygen atoms. The resulting coordination polyhedron can be described as a distorted monocapped trigonal prism (Fig. 2). Bond valence sum calculations were performed for the K + cations using the bond valence parameters for K-O and K-F of Brown & Altermatt (1985). The resulting value in valence units (v.u.) is 1.10 and thus is reasonably close to the expected value of 1.0 v.u..
A slightly different description of the structure results when anion-centred polyhedra are considered as well. Actually, each Fanion is surrounded by two hydrogen atoms (belonging to a water molecule and a hydrogencarbonate group, respectively) and four additional equatorial potassium atoms in form of a distorted octahedron (Fig. 3). Edge-sharing between adjacent octahedra results in columns running along [010]. The linkage between the octahedral columns with the laterally attached H 2 O and (HCO 3 )-groups is realised by O4-H41··· O1 hydrogen bonds (Fig. 4).
As can be seen from Fig. 6, in both compounds the chains are located on mirror planes.

Experimental
Single crystals of K 2 (HCO 3 )F.H 2 O were obtained by chance as a by-product during the preparation of K-REE-silicates (REE is an rare earth element) from a KF flux following the approach of Vidican et al. (2003). After the removal of the platinum crucible the solidified melt cake was immediately crashed in an agate mortar and transferred to a glass slide under a polarizing binocular. Due to the hygroscopic character of potassium fluoride under the given experimental conditions (temperature: 296 K, relative humidity: 43%) deliquescence of the halide became obvious after 30 min. After 2 h, 50 µm sized platy birefringent single crystals were observed in the solution droplets which continued to grow until the solution was completely evaporated. The sample was checked regularly over a period of two weeks. However, no indication for weathering or alteration could be detected. In order to study the compound in more detail a single-crystal of good optical quality showing sharp extinction when imaged between crossed polarizers was selected and mounted on the tip of a glass fibre using nail hardener as glue. A preliminary unit cell determination at ambient temperature using on Oxford Diffraction Gemini Ultra single-crystal diffractometer resulted in a set of lattice parameters that could not be found in the recent WEB-based version of the Inorganic Crystal Structure Database (ICSD). Therefore, we decided to perform a full data collection for structure solution. For the low temperature measurement the crystal was flash-cooled in a 173 (2) K dried air stream generated by an Oxford Cryosystems Desktop Cooler.

Computing details
Data collection: CrysAlis PRO (Agilent, 2011); cell refinement: CrysAlis PRO (Agilent, 2011); data reduction: CrysAlis PRO (Agilent, 2011); program(s) used to solve structure: SIR2002 (Burla et al., 2003); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2011); software used to prepare material for publication: publCIF (Westrip, 2010);.   Side view of the coordination polyhedron around a single K + cation. [Symmetry codes: (i) -x + 1, -y + 1, -z; (ii) -x + 1, -y + 1, -z + 1; (iii) -x + 2, -y + 1, -z +1].   Projection of edge-sharing anion-centered chains (linked by H 2 O and HCO 3 -moieties) perpendicular to (100). Small blue spheres correspond to the K + cations.   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.31 e Å −3 Δρ min = −0.31 e Å −3 Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.