The aluminoarsenate K_1.8Sr_0.6Al₃(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.

Related literature

For further information on this structure type, see: K₃Cr₃(AsO₄)₄ (Friaa et al., 2003); K₃Fe₃(AsO₄)₄ (Ouerfelli et al., 2005). For similar structures, see: K₃Fe₃(PO₄)₄·H₂O (Lii, 1995); Na₃Fe₃(PO₄)₄ (Lajmi et al., 2002). For background to the bond-valence method, see: Brown & Altermatt (1985).

Experimental

Crystal data

• K_1.8Sr_0.6Al₃(AsO₄)₄

• M_r = 759.57

• Orthorhombic, C m c e

• a = 10.567 (3) Å

• b = 20.531 (4) Å

• c = 6.388 (1) Å

• V = 1385.9 (5) ų

• Z = 4

• Mo Kα radiation

• μ = 12.67 mm⁻¹

• T = 293 K

• 0.36 × 0.22 × 0.14 mm

Data collection

• Enraf–Nonius CAD-4 diffractometer

• Absorption correction: ψ scan (North et al., 1968) T_min = 0.088, T_max = 0.292

• 1510 measured reflections

• 796 independent reflections

• 704 reflections with I > 2σ(I)

• R_int = 0.024

• 2 standard reflections every 120 min intensity decay: 0.4%

Refinement

• R[F² > 2σ(F²)] = 0.039

• wR(F²) = 0.112

• S = 1.09

• 796 reflections

• 88 parameters

• Δρ_max = 1.68 e Å⁻³

• Δρ_min = −1.45 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, 1998); software used to prepare material for publication: WinGX (Farrugia, 1999).

Supporting information

S1. Comment

The crystal structure of K_1.8Sr_0.6Al₃(AsO₄)₄ is a two-dimensional network formed by curved layers perpendicular to the b axis. Each layer consists of AlO₆ octahedra and AsO₄ 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 K₃Cr₃(AsO₄)₄ (Friaa et al., 2003) and K₃Fe₃(AsO₄)₄ (Ouerfelli et al., 2005). The asymmetric unit is the link-up of two AlO₆ octahedra and As2O₄ tetrahedron by corners and As1O₄ tetrahedron by edges (Fig. 3). The common edge O4···O4, is the shortest oxygen distance. The As1O₄ 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 As2O₄ tetrahedron is located on mirror plane. It shares two oxygen atoms O5 with two equivalent Al1O₆ octahedra and one oxygen atom O1, which is situated on mirror plane, with Al2O₆ octahedron. The fourth oxygen atom O2 which lies on mirror plane is not coordinated and points to the interlayer space. The Al1O₆ 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 Al2O₆ octahedron which lies on a crystallographic center of symmetry is less distorted than the Al1O₆ 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 SrO₉ 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.

Related literature

For further information on this structure type, see: K₃Cr₃(AsO₄)₄ (Friaa et al., 2003); K₃Fe₃(AsO₄)₄ (Ouerfelli et al., 2005). For similar bidimensional networks, see: K₃Fe₃(PO₄)₄.H₂O (Lii, 1995); Na₃Fe₃(PO₄)₄ (Lajmi et al., 2002). For background to the bond-valence method, see: Brown & Altermatt (1985).

S2. Experimental

The purpose is to obtain a phase from the molars proportions of (2:0.5:4) of a mixture of KNO₃, Sr(NO₃)₂ and NH₄H₂AsO₄. 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 probe of a selected crystal revealed the presence of aluminium atom which comes from the crucible, as well as the different elements of the compound composition.

S3. 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/A³ from heavy atom As1 is due to the effects of Fourier series termination.

Computing details

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994); cell refinement: CAD-4 EXPRESS (Enraf–Nonius, 1994); 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, 1998); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. : Projection of the structure of K1.8Sr0.6Al3(AsO4)4 along the c axis.
	Fig. 2. : A view of a layer showing the windows.
	Fig. 3. : The asymmetric unit of the structure of K1.8Sr0.6Al3(AsO4)4.

potassium strontium trialuminium tetraarsenate

Crystal data

K1.8Sr0.6Al3(AsO4)4	F(000) = 1424
Mr = 759.57	Dx = 3.641 Mg m^−3
Orthorhombic, Cmce	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2bc 2	Cell parameters from 25 reflections
a = 10.567 (3) Å	θ = 2.2–27°
b = 20.531 (4) Å	µ = 12.67 mm^−1
c = 6.388 (1) Å	T = 293 K
V = 1385.9 (5) Å^3	Parallelipedic, colourless
Z = 4	0.36 × 0.22 × 0.14 mm

Data collection

Enraf–Nonius CAD-4 diffractometer	704 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.024
Graphite monochromator	θmax = 27.0°, θmin = 3.8°
ω/2θ scans	h = −13→0
Absorption correction: ψ scan (North et al., 1968)	k = −26→0
Tmin = 0.088, Tmax = 0.292	l = −8→8
1510 measured reflections	2 standard reflections every 120 min
796 independent reflections	intensity decay: 0.4%

Refinement

Refinement on F^2	Primary atom site location: real-space vector search
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.039	w = 1/[σ^2(Fo^2) + (0.0826P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.112	(Δ/σ)max < 0.001
S = 1.09	Δρmax = 1.68 e Å^−3
796 reflections	Δρmin = −1.45 e Å^−3
88 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
As1	0.7500	0.54444 (4)	0.2500	0.0058 (3)
As2	0.0000	0.65488 (4)	−0.05958 (11)	0.0070 (3)
Al1	0.7500	0.40809 (11)	0.2500	0.0054 (5)
Al2	0.0000	0.5000	0.0000	0.0069 (6)
Sr1	0.010 (3)	0.6619 (4)	0.4463 (8)	0.056 (4)	0.12 (2)
Sr2	0.006 (8)	0.5644 (17)	0.477 (4)	0.056 (4)	0.03 (2)
K1	0.2111 (8)	0.7265 (4)	0.3993 (12)	0.062 (2)	0.33 (1)
K2	0.258 (2)	0.7305 (10)	0.190 (3)	0.062 (2)	0.12 (1)
O1	0.0000	0.5866 (3)	0.0858 (7)	0.0090 (10)
O2	0.0000	0.7176 (3)	0.1009 (9)	0.0180 (12)
O3	0.7104 (3)	0.59095 (18)	0.0477 (5)	0.0092 (7)
O4	0.6367 (3)	0.48557 (16)	0.2962 (5)	0.0078 (7)
O5	−0.1259 (3)	0.65351 (18)	−0.2222 (5)	0.0120 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
As1	0.0061 (4)	0.0055 (4)	0.0057 (4)	0.000	0.0008 (2)	0.000
As2	0.0047 (4)	0.0081 (4)	0.0083 (4)	0.000	0.000	−0.0014 (2)
Al1	0.0042 (10)	0.0075 (11)	0.0046 (9)	0.000	0.0008 (6)	0.000
Al2	0.0026 (13)	0.0090 (15)	0.0088 (13)	0.000	0.000	0.0000 (11)
Sr1	0.058 (7)	0.074 (5)	0.034 (2)	0.038 (9)	−0.004 (5)	0.003 (2)
Sr2	0.058 (7)	0.074 (5)	0.034 (2)	0.038 (9)	−0.004 (5)	0.003 (2)
K1	0.074 (5)	0.051 (3)	0.060 (5)	0.037 (4)	−0.002 (4)	0.011 (3)
K2	0.074 (5)	0.051 (3)	0.060 (5)	0.037 (4)	−0.002 (4)	0.011 (3)
O1	0.008 (2)	0.008 (2)	0.010 (2)	0.000	0.000	0.0038 (19)
O2	0.022 (3)	0.014 (3)	0.018 (3)	0.000	0.000	−0.004 (2)
O3	0.0094 (17)	0.0098 (18)	0.0084 (15)	0.0029 (15)	−0.0011 (13)	0.0003 (13)
O4	0.0086 (16)	0.0060 (16)	0.0088 (15)	−0.0033 (15)	0.0017 (14)	0.0011 (11)
O5	0.0089 (18)	0.0115 (17)	0.0155 (18)	−0.0023 (16)	−0.0024 (14)	0.0051 (13)

Geometric parameters (Å, º)

As1—O3^i	1.661 (3)	Sr1—O2	2.489 (8)
As1—O3	1.661 (3)	Sr1—O5^xiii	2.565 (17)
As1—O4^i	1.727 (3)	Sr1—O3^ix	2.57 (2)
As1—O4	1.727 (3)	Sr1—O2^xiv	2.666 (10)
As2—O2	1.646 (6)	Sr1—O3^xi	2.75 (3)
As2—O1	1.682 (5)	Sr1—O1	2.775 (9)
As2—O5^ii	1.688 (3)	Sr2—O3^ix	2.23 (9)
As2—O5	1.688 (3)	Sr2—O4^xv	2.67 (4)
Al1—O5^iii	1.831 (4)	Sr2—O4^ix	2.75 (4)
Al1—O5^iv	1.831 (4)	Sr2—O1	2.54 (2)
Al1—O3^v	1.947 (3)	Sr2—O5^xii	2.94 (5)
Al1—O3^vi	1.947 (3)	Sr2—O3^xi	2.36 (8)
Al1—O4^i	2.013 (4)	K1—O5^xiv	2.735 (7)
Al1—O4	2.013 (4)	K1—O3^ix	2.804 (8)
Al2—O1^vii	1.861 (5)	K1—O2^xiv	2.819 (8)
Al2—O1	1.861 (5)	K1—O2	2.940 (9)
Al2—O4^viii	1.967 (3)	K2—O5^xvi	2.69 (2)
Al2—O4^ix	1.967 (3)	K2—O2	2.80 (3)
Al2—O4^x	1.967 (3)	K2—O5^xiv	2.817 (19)
Al2—O4^xi	1.967 (3)	K2—O2^xvii	2.90 (2)
Sr1—O5^xii	2.450 (15)	K2—O3^xviii	3.02 (2)
O3^i—As1—O3	109.8 (2)	O5^xiii—Sr1—O3^ix	114.3 (3)
O3^i—As1—O4^i	111.16 (17)	O5^xii—Sr1—O2^xiv	76.4 (3)
O3—As1—O4^i	116.37 (16)	O2—Sr1—O2^xiv	84.2 (2)
O3^i—As1—O4	116.37 (16)	O5^xiii—Sr1—O2^xiv	74.6 (3)
O3—As1—O4	111.16 (17)	O3^ix—Sr1—O2^xiv	123.7 (9)
O4^i—As1—O4	91.2 (2)	O5^xii—Sr1—O3^xi	112.0 (5)
O2—As2—O1	108.0 (3)	O2—Sr1—O3^xi	102.6 (7)
O2—As2—O5^ii	113.34 (17)	O5^xiii—Sr1—O3^xi	58.5 (5)
O1—As2—O5^ii	109.03 (16)	O3^ix—Sr1—O3^xi	113.5 (3)
O2—As2—O5	113.34 (17)	O2^xiv—Sr1—O3^xi	116.9 (8)
O1—As2—O5	109.03 (16)	O5^xii—Sr1—O1	134.2 (6)
O5^ii—As2—O5	104.0 (2)	O2—Sr1—O1	61.2 (2)
O5^iii—Al1—O5^iv	92.6 (2)	O5^xiii—Sr1—O1	128.7 (9)
O5^iii—Al1—O3^v	93.80 (16)	O3^ix—Sr1—O1	74.3 (3)
O5^iv—Al1—O3^v	87.00 (16)	O2^xiv—Sr1—O1	145.3 (3)
O5^iii—Al1—O3^vi	87.00 (16)	O3^xi—Sr1—O1	71.6 (4)
O5^iv—Al1—O3^vi	93.80 (16)	O3^ix—Sr2—O4^xv	69 (2)
O3^v—Al1—O3^vi	178.8 (3)	O3^ix—Sr2—O4^ix	67.4 (18)
O5^iii—Al1—O4^i	96.06 (14)	O4^xv—Sr2—O4^ix	90.0 (17)
O5^iv—Al1—O4^i	170.55 (17)	O3^ix—Sr2—O1	85.0 (19)
O3^v—Al1—O4^i	88.66 (15)	O4^xv—Sr2—O1	147 (3)
O3^vi—Al1—O4^i	90.43 (15)	O4^ix—Sr2—O1	59.7 (7)
O5^iii—Al1—O4	170.55 (17)	O4^xv—Sr2—O5^xii	61.2 (11)
O5^iv—Al1—O4	96.06 (14)	O4^ix—Sr2—O5^xii	125 (3)
O3^v—Al1—O4	90.43 (15)	O1—Sr2—O5^xii	123 (2)
O3^vi—Al1—O4	88.66 (15)	O3^ix—Sr2—O3^xi	151.3 (17)
O4^i—Al1—O4	75.6 (2)	O4^xv—Sr2—O3^xi	130 (2)
O1^vii—Al2—O1	180.0 (3)	O4^ix—Sr2—O3^xi	125 (2)
O1^vii—Al2—O4^viii	87.08 (14)	O1—Sr2—O3^xi	82.4 (17)
O1—Al2—O4^viii	92.92 (14)	O5^xii—Sr2—O3^xi	108.5 (13)
O1^vii—Al2—O4^ix	92.92 (14)	O5^xiv—K1—O3^ix	158.0 (4)
O1—Al2—O4^ix	87.08 (14)	O5^xiv—K1—O2^xiv	60.2 (2)
O4^viii—Al2—O4^ix	85.5 (2)	O3^ix—K1—O2^xiv	110.3 (3)
O1^vii—Al2—O4^x	87.08 (14)	O5^xiv—K1—O2	67.8 (2)
O1—Al2—O4^x	92.92 (14)	O3^ix—K1—O2	90.8 (3)
O4^viii—Al2—O4^x	94.5 (2)	O2^xiv—K1—O2	73.8 (2)
O4^ix—Al2—O4^x	180.0	O5^xvi—K2—O2	123.0 (7)
O1^vii—Al2—O4^xi	92.92 (14)	O5^xvi—K2—O5^xiv	57.4 (4)
O1—Al2—O4^xi	87.08 (14)	O2—K2—O5^xiv	68.8 (5)
O4^viii—Al2—O4^xi	180.00 (19)	O5^xvi—K2—O2^xvii	69.0 (6)
O4^ix—Al2—O4^xi	94.5 (2)	O2—K2—O2^xvii	160.9 (8)
O4^x—Al2—O4^xi	85.5 (2)	O5^xiv—K2—O2^xvii	114.9 (8)
O5^xii—Sr1—O2	145.1 (10)	O5^xvi—K2—O3^xviii	144.3 (9)
O5^xii—Sr1—O5^xiii	64.0 (2)	O2—K2—O3^xviii	87.6 (7)
O2—Sr1—O5^xiii	137.6 (9)	O5^xiv—K2—O3^xviii	156.4 (10)
O5^xii—Sr1—O3^ix	62.4 (5)	O2^xvii—K2—O3^xviii	87.4 (5)
O2—Sr1—O3^ix	108.1 (7)

Symmetry codes: (i) −x+3/2, y, −z+1/2; (ii) −x, y, z; (iii) x+1, −y+1, −z; (iv) −x+1/2, −y+1, z+1/2; (v) x, −y+1, −z; (vi) −x+3/2, −y+1, z+1/2; (vii) −x, −y+1, −z; (viii) x−1/2, −y+1, z−1/2; (ix) x−1/2, y, −z+1/2; (x) −x+1/2, −y+1, z−1/2; (xi) −x+1/2, y, −z+1/2; (xii) −x, y, z+1; (xiii) x, y, z+1; (xiv) −x, −y+3/2, z+1/2; (xv) x−1/2, −y+1, z+1/2; (xvi) x+1/2, −y+3/2, −z; (xvii) x+1/2, y, −z+1/2; (xviii) −x+1, y, z.