The aluminoarsenate K1.8Sr0.6Al3(AsO4)4

The title compound, potassium strontium trialuminium tetraarsenate, was prepared by solid-state reaction. The structure consists of AlO6 octahedra (site symmetries 2.. and 2/m) and two AsO4 tetrahedra (.2. and m..) sharing corners and edges to form a two-dimensional structure parallel to (010). The cations are occupationally disordered and are located in the interlayer space. For both types of cations, distorted coordination polyhedra are observed.

The title compound, potassium strontium trialuminium tetraarsenate, was prepared by solid-state reaction. The structure consists of AlO 6 octahedra (site symmetries 2.. and 2/m) and two AsO 4 tetrahedra (.2. and m..) sharing corners and edges to form a two-dimensional structure parallel to (010). The cations are occupationally disordered and are located in the interlayer space. For both types of cations, distorted coordination polyhedra are observed.

Anissa Haj Abdallah and Amor Haddad Comment
The crystal structure of K 1.8 Sr 0.6 Al 3 (AsO 4 ) 4 is a two-dimensional network formed by curved layers perpendicular to the b axis. Each layer consists of AlO 6 octahedra and AsO 4 tetrahedra sharing corners and edges (Fig. 1). This structural arrangement leads to six-membered windows within the layer (Fig. 2). It is isostructural with the compounds K 3 Cr 3 (AsO 4 ) 4 (Friaa et al., 2003) and K 3 Fe 3 (AsO 4 ) 4 (Ouerfelli et al., 2005). The asymmetric unit is the link-up of two AlO 6 octahedra and As2O 4 tetrahedron by corners and As1O 4 tetrahedron by edges (Fig. 3). The common edge O4···O4, is the shortest oxygen distance. The As1O 4 tetrahedron lies on two fold axis and is distorted. It has two type of As-O bonds, two short distances and two long distances involving the bridging oxygen atoms O4. The As2O 4 tetrahedron is located on mirror plane. It shares two oxygen atoms O5 with two equivalent Al1O 6 octahedra and one oxygen atom O1, which is situated on mirror plane, with Al2O 6 octahedron. The fourth oxygen atom O2 which lies on mirror plane is not coordinated and points to the interlayer space. The Al1O 6 octahedron is strongly distorted which the O4-Al1-O4 angle 75.6 ° is more acute than those according to an ideal octahedron. The mean distance of Al1-O 1.930 (4) Å. The Al2O 6 octahedron which lies on a crystallographic center of symmetry is less distorted than the Al1O 6 octahedron which lies on a two fold axis. The structure is characterized by two cationic sites. The site (0, 1/2, 1/2) located in the interlayer space and the site (1/4, 1/2, 1/4) confined in windows. The difference to be indicated is the cationic distribution in the similar structure compounds. Both strontium cations Sr1 and Sr2 occupy the windows and the potassium cations K1 and K2 are located in interlayers space of the structure. The bond valence sum of the K1, K2 and Sr1 are in a good agreement with their oxidation states (Brown & Altermatt, 1985). But for the Sr2, the bond strength is higher than the expected value +2 suggesting that the SrO 9 appear too small for this ion which is acceptable given the small number of Sr atoms that occupy this site. The valence sums around the Al atoms are both low (2.6 vu) but attempts to introduce other elements onto these sites were not successfull. The resulting difference electron density contains several large features reflecting the difficulty in determining the precise locations of Sr and and K.

Experimental
The purpose is to obtain a phase from the molars proportions of (2:0.5:4) of a mixture of KNO 3 , Sr(NO 3 ) 2 and NH 4 H 2 AsO 4 . The mixture was finely ground and calcined at 723 K in a porcelain crucible. Then the temperature was held at 943 K during 26 h. A slow cooling in a speed of 2 K/h until 893 K and of 5 K/h until 823 K was proceeded. The fusion was reached. A long wash in the boiling water allowed us to isolate some parallelepipedic colourless crystals with acceptable size for an analysis by X-ray diffraction on single-crystal. The qualitative analysis by electron microscope

Refinement
The localization of the strontium and potassium atoms is delicate because of the existence of disorder. The Fourier difference synthesis reveals an intense peak and three nearby peaks relatively less intense. It was then necessary to take considering the peaks which could have a structural meaning because of their environments in atoms of oxygen. A constraint, respecting the electroneutrality, was applied to the occupation rates of the cations. Furthermore, the refinement anisotropy leading to very deformed ellipsoids, the condition EADP allowed by the program SHELX was applied. The high value of the electron densities occurs at 1.68 e/A 3 from heavy atom As1 is due to the effects of Fourier series termination.    The asymmetric unit of the structure of K 1.8 Sr 0.6 Al 3 (AsO 4 ) 4 .  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.68 e Å −3 Δρ min = −1.45 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0006 (3)

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.