KMg0.09Fe1.91(PO4)2

KMg0.09Fe1.91(PO4)2, potassium [iron(II)/magnesium] iron(III) bis(orthophosphate), is a solid solution derived from compounds with general formula KM IIFe(PO4)2 (M II = Fe, Cu), in which the Mg atoms substitute Fe atoms only in the octahedrally surrounded sites. The framework of the structure is built up from [FeO5] trigonal bipyramids and [MO6] (M = (Fe, Mg) octahedra sharing corners and edges and connected by two types of bridging PO4 tetrahedra. The K+ cations are nine-coordinated and are situated in channels running along [101].


Experimental
Crystal data

Refinement
The atomic positions and labelling of atoms are the same as in Badri et al. (2011) to simplify any comparison.
In the first steps of structure refinement, the Mg atoms were placed into the same positions as Fe. The coordinates and the ADPs of both Mg1 and Fe1, Mg2 and Fe2 sites were constrained to be equal. The corresponding occupancies were freely refined but constrained to unity. It was found that the Mg occupancy in the M1 position is negative. Thus the occupancy of Fe in this site was set to 1. The Mg quantity was refined only in the position M2. At the same time, we refined the occupancy of K1 site freely; it was found to be 1.
The remaining highest positive electron density was found at a distance of 0.69 Å from Fe1 and the highest negative density at 0.44 Å from P1.

Computing details
Data  Elementary fragments in the titled compound.

Potassium [iron(II)/magnesium] iron(III) bis(orthophosphate)
Crystal data  (14) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'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 > σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (