K3Al2As3O12

Berthold Stöger; Matthias Weil

doi:10.1107/S1600536812000438

. 2012 Jan 11;68(Pt 2):i15. doi: 10.1107/S1600536812000438

K₃Al₂As₃O₁₂

Berthold Stöger ^a, Matthias Weil ^a,^*

PMCID: PMC3274838 PMID: 22346791

Abstract

Single crystals of K₃Al₂As₃O₁₂, tripotassium dialuminotriarsenate(V), were obtained unintentionally by the reaction of KAsO₃ with a corundum crucible at 973 K. The asymmetric unit contains three K, two Al, three As and 12 O atoms. The structure of the title compound is isotypic with those of other K₃ M′₂ X ₃O₁₂ (M′ = Al, Ga; X = P, As) structures and is made up of a three-dimensional network of corner-sharing [AlO₄] and [AsO₄] tetrahedra. The three K⁺ cations are located in channels running along the [100], [001], [101] and [10 Inline graphic ] directions, exhibiting different coordination numbers of 9, 8 and 6, respectively. All corresponding [KO_x] polyhedra are considerably distorted.

Related literature

For a recent review on NASICON-type materials, see: Anantharamulu et al. (2011 ▶). For isotypic K₃ M′₂ X ₃O₁₂ structures, see: Beaurain et al. (2008 ▶) and Yakubovich et al. (2008 ▶) for K₃Ga₂P₃O₁₂; Boughzala et al. (1997 ▶) for the solid solution K₃Al₂(As_1.92P_1.08)O₁₂; Devi & Vidyasagar (2000 ▶) for K₃Al₂P₃O₁₂. For the isopointal structure of Tl₃Al₂P₃O₁₂, see: Devi & Vidyasagar (2000 ▶). For background to the bond-valence method, see: Brown & Altermatt (1985 ▶).

Experimental

Crystal data

K₃Al₂As₃O₁₂
M _r = 588
Orthorhombic,
a = 8.7943 (2) Å
b = 17.4400 (2) Å
c = 8.6610 (3) Å
V = 1328.36 (6) Å³
Z = 4
Mo Kα radiation
μ = 8.63 mm⁻¹
T = 100 K
0.10 × 0.06 × 0.01 mm

Data collection

Bruker APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2008 ▶) T _min = 0.49, T _max = 0.92
52293 measured reflections
9623 independent reflections
8606 reflections with I > 3σ(I)
R _int = 0.038

Refinement

R[F ² > 3σ(F ²)] = 0.018
wR(F ²) = 0.039
S = 0.80
9623 reflections
181 parameters
Δρ_max = 0.33 e Å⁻³
Δρ_min = −0.31 e Å⁻³
Absolute structure: Flack (1983 ▶), 3882 Friedel pairs
Flack parameter: 0.008 (3)

Data collection: APEX2 (Bruker, 2008 ▶); cell refinement: SAINT-Plus (Bruker, 2008 ▶); data reduction: SAINT-Plus; method used to solve structure: coordinates taken from an isotypic structure; program(s) used to refine structure: JANA2006 (Petříček et al., 2006 ▶); molecular graphics: ATOMS (Dowty, 2006 ▶); software used to prepare material for publication: publCIF (Westrip, 2010 ▶).

Supplementary Material

Crystal structure: contains datablock(s) global, I. DOI: 10.1107/S1600536812000438/gw2112sup1.cif

e-68-00i15-sup1.cif^{(15.5KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536812000438/gw2112Isup2.hkl

e-68-00i15-Isup2.hkl^{(461.4KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Comment

During crystal growth studies of KAsO₃, we inadvertently obtained single crystals with composition K₃Al₂As₃O₁₂ from an attacked corundum crucible. Many oxides with general formula M_xM'₂X₃O₁₂ crystallize with three-dimensional framework structures (Devi & Vidyasagar, 2000) and are of technological interest. Most notably, compounds crystallizing in the NASICON (Na₃Zr₂Si₂PO₁₂) structure type are excellent ion conductors and have been intensely studied. A recent review on compounds with the NASICON structure has been given by Anantharamulu et al. (2011).

The structure of K₃Al₂As₃O₁₂ is isotypic with the phosphate analogue K₃Al₂P₃O₁₂ (Devi & Vidyasagar, 2000), the mixed arsenate/phosphate solid solution K₃Al₂As_1.92P_1.08O₁₂ (Boughzala et al., 1997) and K₃Ga₂P₃O₁₂ (Beaurain et al., 2008; Yakubovich et al., 2008). Trithallium dialuminotriphosphate, Tl₃Al₂P₃O₁₂ (Devi & Vidyasagar, 2000), can be considered as isopointal to the title compound, because it features distinctly different coordinations of the Al sites and the cationic network due to the electron lone pairs of the Tl⁺ ions.

Whereas in the NASICON structure type the M' site is octahedrally and the X site tetrahedrally coordinated, in the title compound both sites exhibit a tetrahedral coordination. Two crystallographically different [AlO₄] and three [AlO₄] tetrahedra, all on general positions, are linked via their corners to a complex three-dimensional network, whereby [AlO₄] units connect only to [AsO₄] units and vice-versa. This network can be decomposed into undulating sheets normal to [010] (Al1, Al2, As1, As2) which are connected by [AsO₄] units (As3) (Fig. 1).

The three different K⁺ cations are located in channels running along the [100] and [001] (K1, K2) (Fig. 1) as well as the [101] and [101] (K3) (Fig. 2) directions. Considering K–O distances up to 3.5 Å as relevant for first coordination spheres, the K⁺ cations are coordinated by 9 (K1), 8 (K1) and 6 (K3) O atoms, respectively, all in the form of irregular [KO_x] polyhedra. The total bond valence sums (parameters: R₀ = 2.132 Å, b = 0.37 (Brown & Altermatt, 1985)), 1.08 (K1), 1.04 (K2) and 0.90 (K3) valence units (v.u.) are close to the expected value of 1 v.u. and point to a slight undersaturation of K3. The coordination of the K⁺ cations is very similar in all isotypic structures. The main difference in these structures pertains to the bond lengths of the XO₄ tetrahedra. Corresponding mean bond lengths are 1.746 Å for AlO₄ and 1.680 Å for AsO₄ tetrahedra in the title compound; 1.737 Å for AlO₄ and 1.527 Å for PO₄ tetrahedra in K₃Al₂P₃O₁₂; 1.730 Å for AlO₄ and 1.615 Å for (As/P)O₄ tetrahedra in K₃Al₂As_1.92P_1.08O₁₂; 1.816 Å for GaO₄ and 1.535 Å for PO₄ tetrahedra in K₃Ga₂P₃O₁₂.

Experimental

K₂CO₃ and H₃AsO₄ were obtained commercially and used without purification. 10 g 80%_wt H₃AsO₄ were titrated against an aqueous K₂CO₃ solution using methyl red as indicator. The water was evaporated and the residue recrystallized from water to obtain KH₂AsO₄. This solid was then heated in a corundum crucible at 973 K, cooled to 633 K over 24 h and quenched. Few colourless crystals of the title compound were isolated from the reaction mixture.