Ammonium iron(III) phosphate(V) fluoride, (NH4)0.5[(NH4)0.375K0.125]FePO4F, with ammonium partially substituted by potassium

The title compound, ammonium potassium iron(III) phosphate fluoride, (NH4)0.875K0.125FePO4F, is built from zigzag chains ∞ 1{[FeO4F2]7−}, with Fe3+ in a distorted octahedral coordination, extending along both the [011] and [01] directions. These chains are made up of alternating trans-[FeO4F2] and cis-[FeO4F2] octahedra via shared F-atom corners, and are linked by PO4 tetrahedra, resulting in an open-framework structure with channels along the [010] and [100] directions. There are two crystallographically independent ammonium sites: one in the [010] channels and the other, partially substituted by K+ ions, in the [100] channels. The ammonium in the [010] channels is fixed to the framework via eight hydrogen bonds (six N—H⋯O and two N—H⋯F).

Data collection: CrysAlis CCD (Oxford Diffraction, 2005); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2005) and ATOMS (Dowty, 2004); software used to prepare material for publication: SHELXL97.  (Loiseau et al., 1994(Loiseau et al., , 2000, and (NH 4 )V(PO 4 )F (Alda et al., 2003) in the ammonium series, and KAl(PO 4 )F (Slovokhotova et al., 1991), KGa(PO 4 )F (Harrison et al., 1995), KFe(PO 4 )F (Matvienko et al., 1979), and KCr(PO 4 )F (Slobodyanik et al., 1991) in the potassium series. However, to the best of our knowledge, sodium fluorophosphate compounds with the KTP-type structure have not been reported to date. Our attempt to synthesize KTP-type sodium fluorophophates failed as well, instead we obtained a solid-solution ammonium fluorophosphate, The negative charge of the framework is compensated by ammonium ions and potassium cations. Two crystallographically independent ammonium sites are found. One resides in the channels along [010], linking to the framework via hydrogen bonds (N-H···O(F)). If this ammonium ion is considered as a normal cation, its coordination number is 8 with an average bond distance of 2.969Å (i.e., to neighboring oxygen and fluorine atoms). The other ammonium site, partially occupied by K + ions, resides in the channels along [100]. The potassium (ammonium) has a coordination number of 8 with a mean bond distance of 2.882Å. Therefore, the ammonium site in the [010] channels is slightly larger in size than its counterpart in the [100] channels. Consequently, potassium ions preferentially substitute for the ammonium ions in the [100] channels. We suggest that the potassium-to-ammonium ratio in the title compound is controlled by the size effect. It is possible that the size effect in hydrothermally synthesized crystals at lower temperatures is more pronounced than those from a solid state reaction route.

Experimental
Transparent, colorless single crystals of the title compound were synthesized hydrothermally. A mixture of 0.055 g NaBF 4 , 1.035 g NH 4 H 2 PO 4 , 0.342 g NH 4 HF 2 and 0.048 g Fe 2 O 3 in an approximate molar ratio of Na: NH 4 : P: Fe = 0.5: 15: 9: supplementary materials sup-2 6, was dissolved in 9 ml distilled water while stirring. The prepared solution was transferred to a Teflon-lined stainless steel autoclave (internal volume 30 ml, degree of filling 33%) and held at 453 K for three days under autogenous pressure. Then the autoclave was cooled to room temperature by turning off the power. Products were filtered off, washed with distilled water and dried at room temperature. The presence of potassium in the crystal that was used for single crystal X-ray data collection, was determined by semi-quantitative chemical analysis on an Oxford Instruments Energy Dispersive Spectrometer(EDS)(calcd K: 2.56 %, Obsd K: ~3%). Sodium was not detectable in the crystal by EDS. It is supposed that potassium should be introduced into the sample as impurities in the reagents.

Refinement
There are two possible space groups Pna2 1 (abc) and Pnna(acb) for the KTP-type compounds. Initially, the centrosymmetric Pnna( No.52) space group was selected according to the observed systematic absences. All the framework atoms fit to the centrosymmetric model, except that the ammonium ions resided in the channel are in disordered manner and have abnormally short (1.46 Å) N-N distances. Furthermore, the structural refinement converged to only R1(gt)=0.0783 and wR(all)=0.1813, which are much higher than those from the noncentrosymmetric model (i.e., all anisotropic atoms resulting in R1(gt)=0.031,and wR(all)=0.054 using 155 parameters). Therefore, the noncentrosymmetric space group Pna2 1 (No.33) was chosen to solve and refine the crystal structure as an inversion twin with twin components 0.48 (3)/0.52 (3). During the structure refinement, five constrained parameters were set, one for the constraint of N1 and K1 atoms (i.e., EXYZ and EADP) sharing the same site, the other four for geometrical constraints (i.e., HFIX: fixed bond distance 0.89 Å) of N2-H1, N2-H2, N2-H3, and N2-H4, respectively. Hydrogen coordinate parameters linked to N2 were obtained from difference electron density synthesis and refined by constrained N-H bond distances, their displacement parameters refined via setting to a common variable. Hydrogen atoms linked to N1 were not determined in the present paper.  . Coordination environment of Fe and P atoms, with displacement ellipsoids drawn at the 50% probability level (symmetry codes: (i) x, 1+y, z; (ii) 1/2-x, 1/2+y, 1/2+z; (iii) -1/2+x, 1/2-y, z; (iv) 1/2+x, 1/2-y, z).

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.
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 > 2sigma(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. The crystal structure of the title compound was refined by a inversion twin matrix so the Flack parameter is equal to zero.