Potassium gallium hydrogenphosphate fluoride, K₂Ga[H(HPO₄)₂]F₂

Microporous gallium phosphates have been widely studied because they find many potential applications in, for example, catalysis, ion-exchangers and adsorbents (Cheetham et al., 1999). The fluoride ion has been discovered to play a major role in the gallium phosphate system, and various series of complex fluoride gallium phosphates with organic molecules substituting for the alkali cations have been synthesized by Férey and other researchers. On the other hand, the known alkali (ammonium) fluoride gallium phosphates show considerable structural variety (LiGa₃F₃(OH)(H₂O)₂(PO₄)₂ (Beitone et al., 2001), LiGa(PO₄)F_0.74(OH)_0.26 (Beitone et al., 2002), KGaF_1-δ(OH)_δPO₄ (Harrison et al., 1995), K[(GaPO₄){F_0.25(GaPO₄)}₄] (Sun et al., 2003), (NH₄)_0.93(H₃O)_0.07Ga(PO₄)(OH)_0.5F_0.5 (Férey et al., 1993), (NH₄)Ga(PO₄)F and (NH₄)₂Ga₂(PO₄)(HPO₄)F₃ (Loiseau et al., 2000)).

Recently, we have been systematically studying the reactivity of alkali and ammonium cations in hydrothermal synthesis and the crystal chemistry of complex gallium and indium phosphates without fluoride agents (Filaretov, Zhizhin, Komissarova et al., 2002; Zhizhin et al., 2000; Filaretov et al., 2006; Rusakov et al., 2006) and with fluoride ions as templates (Filaretov, Zhizhin, Olenev et al., 2002; Komissarova et al., 2002). In this context, we report here the hydrothermal synthesis and characterization of the first potassium gallium hydrogenphosphate fluoride, K₂Ga[H(HPO₄)₂]F₂, (I).

The crystal structure of (I) belongs to the K₂Fe[H(HPO₄)₂]F₂ structure type (Mi et al., 2005). The asymmetric unit contains four O, two H and one each of K, Ga, P and F atoms. The structure can be described as negatively charged infinite {[Ga[H(HPO₄)₂]F₂]^2-}_n columns extending along [100]. The columns are built of corner-sharing Ga[F₂O₄] octahedra and tetrahedral HPO₄ units fused together via Ga—O—P bonds (Figs. 1 and 2). Each Ga[F₂O₄] octahedron shares two of its four O vertices with two adjacent phosphate tetrahedra, while each HPO₄ tetrahedron links two of its non-hydroxy O vertices to two Ga[F₂O₄] octahedra. The charge-balancing K⁺ cations are located in the space between the columns, thus completing a three-dimensional structure. Additional sturdiness of the structure, i.e. reinforcement of the connections between adjacent columns, is provided by two hydrogen bonds.

According to the narrow range of K—O and K—F distances, the K⁺ cation (site symmetry 1) adopts a sevenfold coordination comprising two F and five O nearest neighbours, with an average K—O,F bond length of 2.76 (2) Å. The bond valence sum (BVS) of 1.16 for K calculated by the Brese & O'Keeffe (1991) formalism shows that its valence requirement is satisfied by this coordination. However, this coordination number is not unique because the outer O4 atom {as in K₂Fe[H(HPO₄)2]F₂; Fig. 2 of Mi et al. (2005)} might or might not be part of the coordination environment to 3.2562 (9) Å. If it is included, the BVS value of the [7 + 1]-coordinate K⁺ cation rises to 1.21, but this coordination still agrees with the maximum gap in the K—O distance, assuming a cut-off of 3.35 Å (Donnay & Allmann, 1970). The ninth O neighbour at 3.4222 (9) Å is clearly beyond this cut-off (Table 1). The coordination environment of the K⁺ cation is so asymmetric that no easy polyhedral description can be proposed.

As shown by the Ga—O and Ga—F distances (Table 1), the Ga³⁺ cation (site symmetry 1) is characterized by a typical and nearly regular octahedral geometry (Harrison et al., 1995; Loiseau et al., 2000; Beitone et al., 2002; Filaretov et al., 2006; Rusakov et al., 2006) with four long equatorial Ga—O bonds and two slightly shorter Ga—F bonds. The BVS value of 3.03 for Ga agrees with that expected for trivalent gallium. The P atom reveals its normal tetrahedral coordination [average P—O = 1.54 (2) Å] and participates in P—O1—Ga, P—O2—Ga and two terminal P—O bonds. The BVS of P is unexceptional (4.79, expected 5.00). On the basis of its length, the P—O4 bond is assumed to be protonated. The BVS values for O1 (1.96), O2 (2.03), O3 (1.54) and O4 (1.28) are all reasonably close to expected ideal valences and confirm that atoms O3 and O4 represent hydroxy groups. Atoms O4 and H1, both in general positions, participate in a moderately strong O4—H1 ··· F hydrogen bond (Table 2). In contrast, the P—O3 distance (Table 1) is typical for P—O distances in PO₄ tetrahedra. This is so because O3 is involved in a very short symmetry-restricted hydrogen bond (Table 2).

A difference electron-density map calculated after full-matrix least-squares refinement including all non-H atoms with anisotropic displacement parameters clearly shows a maximum near O4 between O4 and F (interpreted as atom H1) and two maxima close to the inversion centre at (0,1/2,0) between two O3 atoms related by inversion (Fig. 3). These two maxima were suggestive of disordered hydrogen positions and were therefore refined with a fixed isotropic displacement parameter and a half occupancy. This kind of disordered hydrogen is characteristic of the K₂Fe[H(HPO₄)₂]F₂ and Li₂Fe[(PO₄)(HPO₄)] structures (Mi et al., 2004). One might suspect that the structural formula Li₂Fe[(PO₄)(HPO₄)] does not take into account the fact that the H atom is shared between two structurally identical P sites. Probably, a more correct formula should read Li₂Fe[H(PO₄)₂], or as a new Sc analogue, Li₂Sc[H(PO₄)₂] according to recent results of ours. The crystal chemical function of the H2 atoms is to interconnect the {[Ga[H(HPO₄)₂]F₂]^2-}ⁿ columns in the ab plane (Fig. 3). Similarly, very short symmetry-restricted hydrogen bonds are well known and have recently been reported for structures of hydrogenphosphates, arsenates and borates (Mi et al., 2004; Schwendtner & Kolitsch, 2005; Massa et al., 2006). They were also found in the novel hydrogenphosphates Li₂Sc[H(PO₄)₂] and Ba₄Sc₂[H(PO₄)₂](HPO₄)₄(H₂PO₄)·2H₂O (Filaretov et al., unpublished results).

In conclusion, the K₂Ga[H(HPO₄)₂]F₂ structure reveals an original arrangement which had not yet been found amongst well known complex gallium phosphates.