The aluminoarsenate Na1.67K1.33Al3(AsO4)4

The title compound, sodium potassium trialuminium tetrakis(orthoarsenate), was prepared by solid-state reactions. The anionic framework consists of corrugated layers parallel to (010) and is made up of corner-sharing AlO6 octahedra (site symmetries .2. and 2/m..) that are connected to isolated AsO4 tetrahedra (site symmetries .2. and m..) through edge- and corner-sharing. The alkali cations are occupationally disordered. The two K+ cations [site symmetries .2. and m..; occupancies 0.314 (7) and 0.035 (12)] are situated in the interlayer space, whereas the smaller Na+ cations [both with site symmetry m..; occupancies = 0.725 (14) and 0.112 (14)] are located in the cavities of the anionic framework. The K+ cations are surrounded by six and seven O atoms, the Na+ cations by seven and nine O atoms. The resulting coordination polyhedra of the two types of cations are highly distorted.


Comment
Na 1.67 K 1.33 Al 3 (AsO 4 ) 4 is isostructural with K 3 Cr 3 (AsO 4 ) 4 (Friaa et al. (2003)) and belongs to the A I M III (XO 4 ) 4 family of compounds (A I = alkali metal, M III = Al, Cr, Fe,.. X = As, P) . Its structure consists of AlO 6 octahedra and AsO 4 tetrahedra sharing corners and edges to form a two-dimensional framework ( Fig.1) that consists of corrugated layers extending parallel to (010). Each layer lies on a c-glide plane perpendicular to the b axis at y = 0 and y = 1/2. In the asymmetric unit two AlO 6 octahedra are linked to an As1O 4 tetrahedron by sharing corners and to an As2O 4 tetrahedron by sharing edges.
The alkali cations are occupationally disordered with occupancies of 0.725 (14), 0.112 (14), 0.314 (7) and 0.035 (12) for Na1, Na2, K1 and K2, leading to the chemical formula Na 1.67 K 1.33 Al 3 (AsO 4 ) 4 . Smaller than K + , the seven-and ninecoordinated Na1 + and Na2 + (r Na+ = 1.02 Å), are located in cavities surrounded by the anionic framework (Fig. 2 A similar distribution of cations was observed in K 1.8 Sr 0.6 Al 3 (AsO 4 ) 4 (Haj Abdallah et al., 2012). In fact, smaller than K + , the Sr 2+ cations (r Sr2+ = 1.18 Å) are located in the anionic cavities, on the other hand, K + occupy the interlayer space. The cationic distribution in these two structures make it reasonable to conclude that the interlayer space is reserved for bigger cations and the anionic cavities for smaller cations. To our knowledge, this is the first published structure with occupationally disordered alkali sites in the A I M III (XO 4 ) 4 family of compounds. Reports of some new members of this family will be published soon.

Experimental
Crystals of Na 1.67 K 1.33 Al 3 (AsO 4 ) 4 were obtained from a mixture of K 2 CO 3 , Al 2 O 3 and NH 4 H 2 AsO 4 in the stoichiometric molar ratio K/Al/As=2/3/4. The mixture was finely ground and heated in a porcelain crucible at 723 K for 4 h to eliminate volatile products. The temperature was held at 1173 K during 10 days until fusion was reached. The sample was slowly cooled in a speed of 5 K/ h to 923 K and finally quenched to room temperature. A long wash with boiling water liberated colorless crystals. The qualitative analysis by electron microscope probe of a selected crystal revealed the presence of the different elements in the compound composition. It is most likely that the incorporated sodium stems from the crucible.

Refinement
Except for K2 and Na2, both with a very low occupation, all atoms were refined with anisotropic displacement parameters. Constraints were applied to the Na + and K + cation occupation rates to achieve electro-neutrality. The highest and lowest values of the electron densities occurare located 0.97 Å and 0.80 Å, respectively, from As2.

Figure 1
Projection of the Na  Cavities limited by the anionic framework hosting Na + cations in the Na 1.67 K 1.33 Al 3 (AsO 4 ) 4 structure.

Sodium potassium trialuminium tetrakis(orthoarsenate)
Crystal data Na 1 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.  (2)