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

Mohamad Alem Bouhassine; Habib Boughzala

doi:10.1107/S1600536813033850

. 2013 Dec 24;70(Pt 1):i6. doi: 10.1107/S1600536813033850

The aluminoarsenate Na_1.67K_1.33Al₃(AsO₄)₄

Mohamad Alem Bouhassine ^a, Habib Boughzala ^a,^*

PMCID: PMC3914036 PMID: 24526941

Abstract

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.

Related literature

For further information on this structure type, see: Friaa et al. (2003 ▶); Haj Abdallah & Haddad (2012 ▶). For background to the bond-valence method, see: Brown (2002 ▶).

Experimental

Crystal data

Na_1.67K_1.33Al₃(AsO₄)₄
M _r = 726.95
Orthorhombic,
a = 10.493 (1) Å
b = 20.395 (4) Å
c = 6.335 (4) Å
V = 1355.7 (9) Å³
Z = 4
Mo Kα radiation
μ = 10.53 mm⁻¹
T = 293 K
0.40 × 0.10 × 0.07 mm

Data collection

Enraf–Nonius CAD-4 diffractometer
Absorption correction: ψ scan (North et al., 1968 ▶) T _min = 0.130, T _max = 0.332
1723 measured reflections
780 independent reflections
717 reflections with I > 2σ(I)
R _int = 0.062
2 standard reflections every 120 min intensity decay: 1.1%

Refinement

R[F ² > 2σ(F ²)] = 0.038
wR(F ²) = 0.112
S = 1.15
780 reflections
84 parameters
1 restraint
Δρ_max = 2.38 e Å⁻³
Δρ_min = −1.33 e Å⁻³

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994 ▶); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995 ▶); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: DIAMOND (Brandenburg, 2006 ▶); software used to prepare material for publication: publCIF (Westrip, 2010 ▶).

Supplementary Material

Crystal structure: contains datablock(s) I, New_Global_Publ_Block. DOI: 10.1107/S1600536813033850/wm2786sup1.cif

e-70-000i6-sup1.cif^{(25.4KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536813033850/wm2786Isup2.hkl

e-70-000i6-Isup2.hkl^{(39.1KB, hkl)}

Additional supporting information: crystallographic information; 3D view; checkCIF report

Table 1. Selected bond lengths (Å).

As1—O1	1.642 (5)
As1—O2ⁱ	1.678 (3)
As1—O2	1.678 (3)
As1—O3	1.694 (4)
As2—O4ⁱⁱ	1.648 (3)
As2—O4	1.648 (3)
As2—O5	1.729 (3)
As2—O5ⁱⁱ	1.729 (3)
Al1—O3	1.827 (5)
Al1—O3ⁱⁱⁱ	1.827 (5)
Al1—O5ⁱⁱⁱ	1.947 (3)
Al1—O5^iv	1.947 (3)
Al1—O5ⁱ	1.947 (3)
Al1—O5	1.947 (3)
Al2—O2ⁱⁱ	1.817 (3)
Al2—O2	1.817 (3)
Al2—O4^v	1.924 (3)
Al2—O4^vi	1.924 (3)
Al2—O5ⁱⁱ	1.991 (3)
Al2—O5	1.991 (3)

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Symmetry codes: (i) Inline graphic ; (ii) ; (iii) ; (iv) ; (v) ; (vi) .

supplementary crystallographic information

1. Comment

Na_1.67K_1.33Al₃(AsO₄)₄ is isostructural with K₃Cr₃(AsO₄)₄ (Friaa et al. (2003)) and belongs to the A^IM^III(XO₄)₄ family of compounds (A^I = alkali metal, M^III = Al, Cr, Fe,.. X = As, P) . Its structure consists of AlO₆ octahedra and AsO₄ 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 AlO₆ octahedra are linked to an As1O₄ tetrahedron by sharing corners and to an As2O₄ 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_1.67K_1.33Al₃(AsO₄)₄. Smaller than K⁺, the seven- and nine-coordinated Na1⁺ and Na2⁺ (r_Na+ = 1.02 Å), are located in cavities surrounded by the anionic framework (Fig. 2). On the other hand, the K1⁺ and K2⁺ cations (r_K+ = 1.38 Å) are surrounded, respectively, by seven and six O atoms and are located in larger sites in the corrugated interlayer space. Using the bond valence method (Brown, 2002), the calculated bond-valence sum values (in valence units) of 5.17, 4.98, 3.04, 3.03, 0.86, 1.11, 0.92, 0.86, respectively, for As1, As2, Al1, Al2, Na1, Na2, K1 and K2 are in good agreement with the expected oxidation states.

A similar distribution of cations was observed in K_1.8Sr_0.6Al₃(AsO₄)₄ (Haj Abdallah et al., 2012). In fact, smaller than K⁺, the Sr²⁺ 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^IM^III(XO₄)₄ family of compounds. Reports of some new members of this family will be published soon.

2. Experimental

Crystals of Na_1.67K_1.33Al₃(AsO₄)₄ were obtained from a mixture of K₂CO₃, Al₂O₃ and NH₄H₂AsO₄ 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.