Reinvestigation of KMg1/3Nb2/3OPO4 1

The crystal structure of potassium magnesium niobium oxide phosphate, KMg1/3Nb2/3OPO4, which was described in the space group P4322 [McCarron & Calabrese, (1993 ▶). J. Solid State Chem. 102, 354–361], has been redetermined in the revised space group P41. Accordingly, the assignment of the space group P4322 and, therefore, localization of K at a single half-occupied position, as noted in the previous study, proved to be an artifact. As a consequence, two major and two minor positions of K are observed due to the splitting along [001], as first noted for KTiOPO4 structure analogues. It has been shown that the geometry of the {M II 1/3Nb2/3O6/2}∞ framework is almost unaffected by the lowering of symmetry.

The crystal structure of potassium magnesium niobium oxide phosphate, KMg 1/3 Nb 2/3 OPO 4 , which was described in the space group P4 3 22 [McCarron & Calabrese, (1993). J. Solid State Chem. 102, 354-361], has been redetermined in the revised space group P4 1 . Accordingly, the assignment of the space group P4 3 22 and, therefore, localization of K at a single half-occupied position, as noted in the previous study, proved to be an artifact. As a consequence, two major and two minor positions of K are observed due to the splitting along [001], as first noted for KTiOPO 4 structure analogues. It has been shown that the geometry of the {M II 1/3 Nb 2/3 O 6/2 } 1 framework is almost unaffected by the lowering of symmetry.

Experimental
Crystal data This work aims to unify the description of all isostructural series of KM II 1/3 Nb 2/3 OPO 4 (M = Mg, Mn, Co) based on a K-split structural model. Improvements brought by this model were tested on Mg-containing isostructure of the series.
Unlike the previous work, all refiments were carried out on F 2 obs , the absolute structure of the title compound was estimated on the basis of the Flack parameter with the TWIN/BASF instruction (Sheldrick, 2008) by sparse-matrix least squares (Flack & Bernardinelli, 2000). The initial coordinates were taken from the Babaryk et al., (2007a). The initial ratio of Mg and Nb occupancies was fixed at the ideal value 1/2 while occupancies of K(1) and K(2) were free and refined to the following convergence parameters: R = 0.038, wR = 0.082, S = 1.113 and residual electron densities Δρ max /Δρ min = 1.41/-0.86 e Å 3 [σ(Δρ) = 0.14 e/Å 3 ]. The highest observed peak (1.41 e Å 3 ) is distant from 0.92Å from K1 and the lowest one at a distance of 0.27 Å from K(2) (Fig. 1a,c) and corresponds to splitting along the c-axis that was earlier shown (Norberg et al., 2005) for KTiOPO 4 only in a case of synchrotron radiation experiments. In the modified model occupancies of K atoms were set of 0.25 equally and further refinement was performed with linear restraints on the occupancies to fit the charge balance. The refinement converged to R = 0.026, wR = 0.055, S = 1.195 and residual electron densities Δρ max /Δρ min = 0.62/-0.54 e Å 3 [σ(Δρ) = 0.11 e Å 3 ] ( Fig. 1 (Table 1) produce rather comparable values in this study and previous ones, thus, the geometry of {M II 1/3 Nb 2/3 O 6/2 } ∞ is almost not affected by the lowering of symmetry. The major differences between the previously reported and this investigation are concerned with K-splittig phenomena. To the the best of our knowledge this is the first example among tetragonal KTP-analogues (Peuchert et al., 1995;Babaryk et al., 2007b) where it is observed. K-splitting phenomena for KTiOPO 4 family of compounds is intesively studied over the last decade. Examples of splitting at room temperature for KTP-isostructures become more evident, verbi causa, RbSbOGeO 4 (Belokoneva et al., 1997), RbTiOAsO 4 (Streltsov et al., 2000), CsTiOAsO 4 (Nordborg, 2000), KTiOPO 4 (Norberg & Ishizawa, 2005) using synchrotron irradiation experiments. In case X-ray study of KTP such splitting was found only upon heating at 673 and 973 K (Delarue et al., 1999). Insert of 7-11% at. of Nb into KTP allow to introduce the cation-split model for refinement and, noteworthy, occupation of minor K-position growth with increasing Nb-content in KTiOPO 4 (Alekseeva et al., 2003). Recent precision investigation (30 K) of 7% at. Nb-doped KTP crystals (Dudka et al., 2005) showed K distribution over the split position is almost indentical to results obtained at room temperature. Thus, it is likely to introduce the K-split model for refinement of Nb-containing KTP analogues as well as it was applied above.
The final distribution of K over the positions K1A/K1B is equiproportional to K1B/K2B ones that are of ca. 7/3. The arrangement of K atoms is almost symmetrical, the distance between major K1A and minor K1B positions is 0.70 (2) Å that is almost equal to 0.69 (3)  K2B is also observed in this study similar to that reported earlier (Norberg & Ishizawa, 2005). Both K1A and K2A atoms formed eight K-O contacts (Fig. 4a) according to a scheme [2+6]: two of them are in a range of 2.592 (7)-2.713 (7) Å corresponding to ionic-covalent type and six are longer ones within the limits 2.862 (1)-3.138 (1) Å demonstrating ionic bonding type. In counterpart to this each K1B and K2B are closed with seven O atoms (Fig. 4b) One can conclude that the cation subblattice arrangement is silmilar either for KTP isostructures or its aliovalent analogues, however a measure of K-splitting "strength" depends on the charge of the polyvalent metal constituent.

Experimental
For preparation of the title compound 1.072 (1) g of KPO 3 , 0.122 (1) g of MgO, 0.806 (1) g of Nb 2 O 5 and 10.000 (1) g of K 2 Mo 2 O 7 were mixed together and well grounded in an agate mortar. All the reagents were of analytical grade purity. The mixture was melted in a 50 ml platinum crucible at 1273 K. Melted solution was cooled at a rate 20 K×h -1 up to 1133 (5) K. Crystalline precipitate was freed from the liquid flux by decantation. The crystals were washed from the rest of the solidified component with hot 5%-aqueous (NaPO 3 ) x . Crystalline phase consists of well-shaped colourless biaxial crystals (with size distribution from 0.2 to 0.4 mm of length) in major unless the rear pale-red prismatic crystals of K 5+x Nb 8-x Mg x P 5 O 34 (unpublished results). It is likely to obtaine pure compound at more slow cooling of the initial melt.
Evaluation of the elements quantities was performed on an X-ray fluorescence energy dispersive spectrometer Elvax Light and established Mo presence at a level of 0.01% at., which has not been taken into account at the structure refinements.

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 > σ(F 2 ) is used only for calculating Rfactors(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.