New mixed aluminium–chromium diarsenate

Potassium chromium aluminium diarsenate, KCr_1/4Al_3/4As₂O₇, was prepared by solid-state reaction. The structure consists of (Cr_1/4/Al_3/4)O₆ octa­hedra and As₂O₇ diarsenate groups sharing corners to build up a three-dimensional anionic framework. The potassium cations are located in wide channels running along the c-axis direction. The crystal structure is isostructural with the triclinic A^IM^IIIX₂O₇ (A^I = alkali metal; M^III = Al, Cr, Fe; X = As, P) compounds. However, the M^III octa­hedrally coordinated site is 25% partially occupied by chromium and 75% by aluminium.

1. Chemical context

In recent years, inorganic metal phosphates/arsenates with formula A^IM^IIIX₂O₇ (A^I = alkali metal; M^III = Al, Cr, Fe; X = As, P) have been part of intensive research activities, with crystals grown either from high-temperature solid-state reactions or under aqueous solution conditions. The crystal chemistry of these compounds with X₂O₇ groups reveals a large structural variety accompanied in some cases by inter­esting magnetic, electric, optical, or thermal expansion properties. Focusing on compounds with M^III = Cr, it is noticeable that the corresponding diphosphates have been studied extensively, in contrast to the scarcely studied chromium diarsenates. The title structure is isostructural with the A^ICr^IIIAs₂O₇ family; nevertheless, in this crystal structure some of the chromium ions are partly substituted by aluminium in an octahedrally coordinated site. Herein, the preparation and crystal structure of KCr_1/4Al_3/4As₂O₇ is reported. It is one of a series of new potassium chromium–aluminum diarsenate compounds recently isolated by our group.

2. Structural commentary

The structure of KCr_1/4Al_3/4As₂O₇ can be described as a three-dimensional framework of [(Cr/Al)As₂O₇]⁻ anions built up from corner-sharing (Cr/Al)O₆ octa­hedra and As₂O₇ groups. The (Cr/Al)O₆ octa­hedron shares its six corners with five diarsenate groups while the As₂O₇ anion shares all of its six corners with five octa­hedra; the inter­connection between the polyhedra results in centrosymmetric (Cr/Al)As₂O₁₁ units (Fig. 1). The framework can also be described as been formed by polyhedral parallel layers, as in many isoformular compounds, leaving empty channels running along the c axis in which the K⁺ cations are located (Fig. 2).

Figure 1 A view of the asymmetric unit of the title compound completed by equivalent atomic positions, showing the principal structural units. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) −x + 1, −y + 1, −z + 1; (ii) x, y + 1, z; (iii) −x + 1, −y + 1, −z + 2; (iv) x + 1, y, z.]

Figure 2 Projection of the KCr1/4Al3/4As2O7 structure showing the channels parallel to [001] in which the K^+ cations are located.

In this structure, the aluminium Al^III and the chromium Cr^III cations share the same (2i) crystallographic site. These cations are surrounded by oxygen atoms in an octa­hedral coordination with an average bond length (Cr/Al)—O of 1.920 (14) Å. The presence of the Cr^III cations is proved by the shortening of the Cr—O bond length compared to Al^III—O. In fact, according to similar studies (Bouhassine & Boughzala, 2014, 2015) the average Al^III—O and Cr^III—O bond lengths in octa­hedral coordination are 1.907 and 1.979 Å, respectively.

The two arsenic atoms in the unit cell are tetra­hedrally coordinated. The AsO₄ polyhedra connected via the bridging O4 atom into a diarsenate As₂O₇ anion. Like in the related triclinic structures of KAlAs₂O₇ (Boughzala & Jouini, 1995) and RbAlAs₂O₇ (Boughzala et al., 1993), the As—O distances involving the bridging O4 atom are the longest (Table 1). The As1—O4—As2 bridging angle of 118.50 (14)° in the title structure is similar to that in the reported isotypic structures of CsCrAs₂O₇ [118.7 (2)°; Bouhassine & Boughzala, 2015] and KAlAs₂O₇ [118.3 (2)°; Boughzala & Jouini, 1995]. The O—As—O bond angles for As1 and As2 span the ranges 103.99 (12) to 117.41 (13) and 106.34 (13) to 113.63 (12), respectively, reflecting a slight distortion of each AsO₄ tetra­hedron.

Table 1
Selected geometric parameters (Å, °)

As1—O2	1.659 (2)	As2—O4	1.761 (3)
As1—O3^i	1.669 (3)	(Cr/Al)—O6^ii	1.898 (3)
As1—O1	1.669 (3)	(Cr/Al)—O3	1.915 (3)
As1—O4	1.776 (2)	(Cr/Al)—O1	1.919 (3)
As2—O6	1.654 (2)	(Cr/Al)—O5	1.925 (3)
As2—O7	1.663 (2)	(Cr/Al)—O2^iii	1.925 (3)
As2—O5	1.675 (2)	(Cr/Al)—O7^iv	1.940 (3)
As2—O4—As1	118.50 (14)
Symmetry codes: (i) x+1, y, z; (ii) x, y+1, z; (iii) -x+1, -y+1, -z+1; (iv) -x+1, -y+1, -z+2.

The (Cr/Al) cations are in a slightly distorted octa­hedral oxygen coordination with (Cr/Al)—O distances ranging from 1.898 (3) to 1.940 (3) Å, and with O—(Cr/Al)—O angles ranging from 85.28 (11) to 92.23 (12)° and from 177.25 (11) to 176.41 (11)°. Each (Cr/Al)O₆ octa­hedron is linked by its six vertices to five As₂O₇ anions. Two corners are joined to the same diarsenate group (Fig. 3). On the other hand, each As₂O₇ anion is surrounded by five (Cr/Al)O₆ units (Fig. 4).

Figure 3 The environment of the (Cr/Al)O6 octa­hedron in the structure of KCr1/4Al3/4As2O7.

Figure 4 The environment of the diarsenate group As2O7 in the title structure.

It is worth mentioning that members of the related alumin­ium diarsenate family A^IAlAs₂O₇ (A^I = K, Rb, Tl, Cs; Boughzala & Jouini, 1992) crystallize in the triclinic space group P and are classified as type II (Durif & Averbuch-Pouchot, 1996); the diarsenate groups have a different conformational orientation compared to that of the title structure, which belongs to the type I family of A^IM^IIIX₂O₇ diarsenates. In fact, the diarsenate tetra­hedra are in a nearly eclipsed conformation with an O1—As1—As2—O5 torsion angle of 25.4 (2)°, as shown in Fig. 5. The corresponding angle is 158.8 (2)° for KAlAs₂O₇ (Boughzala & Jouini, 1995).

Figure 5 View parallel to the As1—As2 direction, emphasizing the nearly eclipsed conformation of the diarsenate anion.

The potassium cations lodge in two independent special positions in the unit cell, located in wide channels that are delimited by the anionic framework and run along the c-axis direction. The K1 and K2 cations are surrounded by eight and ten oxygen atoms, respectively (Fig. 6), with K—O distances ranging from 2.769 (3) to 3.246 (3) Å and from 2.806 (3) to 3.205 (3) Å, respectively, forming irregular coordination polyhedra, as often occurs with this cation in homologous structures.

Figure 6 The eight- and ten-coordinated K1 and K2 atoms (polyhedral plot) in the structure of KCr1/4Al3/4As2O7.

3. Database survey

The structure of KAlP₂O₇ (Ng & Calvo, 1973) was the first to be published for the A^IM^IIIX₂O₇ family (X = As, P). Afterwards, based on different substitutions and combinations, a large number of different phases were isolated and characterized crystallographically. Replacement of the cations can improve the structural and physical properties, but also affects the coordination numbers, the distortion of the coordination polyhedra and the conformation of the X₂O₇ groups. In addition, the crystal symmetry can be affected. The structures are triclinic, in space group P with two formulas units, for the diarsenate compounds A^IAlAs₂O₇ (A^I = K, Rb, Tl, Cs) (Boughzala & Jouini, 1992; Boughzala et al., 1993; Boughzala & Jouini; 1995), whereas the diphosphates are generally monoclinic. The isotypic A^ICrP₂O₇ phases crystallize in space group P2₁/c for A^I = Na (Bohatý et al., 1982), K (Gentil et al., 1997), Rb (Zhao & Li, 2011) and Cs (Linde & Gorbunova, 1982). The same applies for the A^IFeP₂O₇ phases for A^I = Na (Gabelica-Robert et al., 1982) and K (Riou et al., 1988). However, the two Li-containing phases LiMP₂O₇ show a symmetry reduction to space group P2₁ (M = Cr, Ivashkevich et al., 2007; M = Fe, Riou et al., 1990). CsCrAs₂O₇ (Bouhassine & Boughzala, 2015) is the first phase of the A^ICrAs₂O₇ family to crystallize in the P2₁/c space group.

4. Synthesis and crystallization

The crystals of the title compound were obtained from heating a mixture of KNO₃, Cr₂O₃ and NH₄H₂AsO₄, with a K:Cr:As molar ratio of 2:1:2. In order to eliminate volatile products, the sample was placed in a porcelain crucible and slowly heated under atmospheric conditions to 673 K and kept for 12 h. In a second step, the crucible was progressively heated at 1123 K for 10 days and then slowly cooled down at a rate of 5 K/24h to 923 K and finally allowed to cool radiatively to room temperature. A long wash with boiling water liberated green crystals. Manifestly, the aluminium present in the studied composition is coming from the porcelain crucible.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The 2i site was initially refined as being entirely occupied by chromium ions with reliability factor R(F²) = 0.053. Trying to improve the convergence factor, the occupation rate of the 2i site was refined, leading to R(F²) = 0.023 and a partial occupancy of 67%. Occupied by just Cr^III, this occupancy is insufficient to achieve electric neutrality in the empirical formula. To ensure the electroneutrality, many propositions were considered such as the existence of some vacancies in the positions of the oxygen atoms, or the contribution of more than one oxidation state of chromium in the 2i site. The most reasonable idea was to consider a competitive presence of Cr^III and Al^III in the same crystallographic site endowed with the same U_ij parameters. The aluminium has obviously diffused from the porcelain crucible. The last refinement steps lead to the final formula KCr_1/4Al_3/4As₂O_7. The presence of both aluminium and chromium in the structure was confirmed by TEM as shown in Fig. 7.

Table 2
Experimental details

Crystal data
Chemical formula	KCr0.25Al0.75As2O7
Mr	334.17
Crystal system, space group	Triclinic, P
Temperature (K)	293
a, b, c (Å)	6.243 (3), 6.349 (3), 8.153 (4)
α, β, γ (°)	96.57 (2), 104.45 (3), 103.08 (4)
V (Å^3)	299.8 (8)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	12.37
Crystal size (mm)	0.40 × 0.30 × 0.20
Data collection
Diffractometer	Enraf–Nonius CAD-4
Absorption correction	ψ scan (North et al., 1968)
Tmin, Tmax	0.079, 0.182
No. of measured, independent and observed [I > 2σ(I)] reflections	1628, 1479, 1306
Rint	0.014
(sin θ/λ)max (Å^−1)	0.702
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.023, 0.059, 1.08
No. of reflections	1479
No. of parameters	104
Δρmax, Δρmin (e Å^−3)	0.59, −1.09
Computer programs: CAD-4 EXPRESS (Enraf–Nonius, 1994), XCAD4 (Harms & Wocadlo, 1995), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 2008) and publCIF (Westrip, 2010).

Figure 7 Qualitative elemental composition determined by TEM (Fei Tecnai G20 STEM microscope). (Low-energy unlabelled peaks are related to oxygen and the one around 8 keV is attributed to the copper sample holder).

Supporting information

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, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Potassium chromium aluminium diarsenate, KCr_1/4Al_3/4As₂O₇

Crystal data

KCr0.25Al0.75As2O7	Z = 2
Mr = 334.17	F(000) = 314
Triclinic, P1	Dx = 3.702 Mg m^−3
Hall symbol: -p 1	Mo Kα radiation, λ = 0.71073 Å
a = 6.243 (3) Å	Cell parameters from 25 reflections
b = 6.349 (3) Å	θ = 3.8–27°
c = 8.153 (4) Å	µ = 12.37 mm^−1
α = 96.57 (2)°	T = 293 K
β = 104.45 (3)°	Triclinic, green
γ = 103.08 (4)°	0.40 × 0.30 × 0.20 mm
V = 299.8 (8) Å^3

Data collection

Enraf–Nonius CAD-4 diffractometer	1306 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.014
Graphite monochromator	θmax = 30.0°, θmin = 2.6°
ω/2θ scans	h = 0→7
Absorption correction: ψ scan (North et al., 1968)	k = −8→8
Tmin = 0.079, Tmax = 0.182	l = −11→11
1628 measured reflections	2 standard reflections every 120 min
1479 independent reflections	intensity decay: 1.1%

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.023	Secondary atom site location: difference Fourier map
wR(F^2) = 0.059	w = 1/[σ^2(Fo^2) + (0.0303P)^2 + 0.5924P] where P = (Fo^2 + 2Fc^2)/3
S = 1.08	(Δ/σ)max < 0.001
1479 reflections	Δρmax = 0.59 e Å^−3
104 parameters	Δρmin = −1.09 e Å^−3

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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	0.82649 (6)	0.53279 (5)	0.68995 (4)	0.00590 (10)
As2	0.48557 (6)	0.24607 (5)	0.83512 (4)	0.00587 (10)
Cr	0.37535 (14)	0.69197 (13)	0.72579 (10)	0.0054 (3)	0.255 (6)
Al	0.37535 (14)	0.69197 (13)	0.72579 (10)	0.0054 (3)	0.745 (6)
O1	0.6626 (4)	0.7081 (4)	0.6757 (3)	0.0109 (5)
O2	0.7856 (4)	0.3500 (4)	0.5149 (3)	0.0105 (5)
O3	0.0934 (4)	0.6909 (4)	0.7793 (3)	0.0097 (5)
O4	0.7722 (4)	0.3747 (4)	0.8485 (3)	0.0108 (5)
O5	0.3164 (4)	0.3800 (4)	0.7206 (3)	0.0100 (5)
O6	0.4234 (5)	−0.0032 (4)	0.7201 (3)	0.0113 (5)
O7	0.4552 (4)	0.2466 (4)	1.0321 (3)	0.0106 (5)
K1	1.0000	1.0000	1.0000	0.0484 (5)
K2	0.0000	0.0000	0.5000	0.0364 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
As1	0.00519 (19)	0.00627 (16)	0.00641 (15)	0.00121 (12)	0.00247 (12)	0.00063 (11)
As2	0.00655 (19)	0.00535 (15)	0.00580 (15)	0.00124 (12)	0.00216 (12)	0.00115 (10)
Cr	0.0047 (5)	0.0053 (4)	0.0062 (4)	0.0015 (3)	0.0019 (3)	0.0005 (3)
Al	0.0047 (5)	0.0053 (4)	0.0062 (4)	0.0015 (3)	0.0019 (3)	0.0005 (3)
O1	0.0127 (13)	0.0117 (11)	0.0122 (11)	0.0066 (10)	0.0060 (10)	0.0049 (9)
O2	0.0122 (13)	0.0107 (11)	0.0079 (10)	0.0041 (10)	0.0022 (9)	−0.0015 (8)
O3	0.0070 (12)	0.0095 (11)	0.0097 (10)	−0.0010 (9)	0.0023 (9)	−0.0026 (8)
O4	0.0056 (12)	0.0159 (12)	0.0119 (11)	0.0017 (10)	0.0026 (9)	0.0079 (9)
O5	0.0089 (13)	0.0076 (10)	0.0113 (11)	0.0025 (9)	−0.0010 (9)	0.0011 (8)
O6	0.0163 (14)	0.0064 (11)	0.0109 (11)	0.0033 (10)	0.0045 (10)	−0.0016 (8)
O7	0.0106 (13)	0.0139 (11)	0.0069 (10)	0.0011 (10)	0.0038 (9)	0.0012 (9)
K1	0.0302 (9)	0.0408 (9)	0.0615 (11)	−0.0093 (7)	0.0253 (8)	−0.0310 (8)
K2	0.0354 (9)	0.0148 (6)	0.0393 (8)	0.0055 (6)	−0.0209 (6)	−0.0002 (5)

Geometric parameters (Å, º)

As1—O2	1.659 (2)	O3—K2^v	3.202 (3)
As1—O3^i	1.669 (3)	O4—K1^iv	3.246 (3)
As1—O1	1.669 (3)	O5—K2	2.834 (3)
As1—O4	1.776 (2)	O6—Al^iv	1.898 (3)
As1—K1	3.4388 (16)	O6—Cr^iv	1.898 (3)
As1—K2^ii	3.5783 (15)	O6—K2	2.806 (3)
As2—O6	1.654 (2)	O7—Al^vii	1.940 (3)
As2—O7	1.663 (2)	O7—Cr^vii	1.940 (3)
As2—O5	1.675 (2)	O7—K1^iii	2.849 (3)
As2—O4	1.761 (3)	K1—O3^i	2.769 (3)
As2—K2	3.441 (2)	K1—O3^ix	2.769 (3)
As2—K1^iii	3.7159 (17)	K1—O7^vii	2.849 (3)
As2—K1^iv	3.8935 (18)	K1—O7^ii	2.849 (3)
Cr—O6^v	1.898 (3)	K1—O1^x	3.032 (3)
Cr—O3	1.915 (3)	K1—O4^xi	3.246 (3)
Cr—O1	1.919 (3)	K1—O4^v	3.246 (3)
Cr—O5	1.925 (3)	K1—As1^x	3.4388 (16)
Cr—O2^vi	1.925 (3)	K1—As2^ii	3.7160 (17)
Cr—O7^vii	1.940 (3)	K1—As2^vii	3.7160 (17)
Cr—K2^v	3.6674 (17)	K2—O6^xii	2.806 (3)
Cr—K1	3.902 (2)	K2—O5^xii	2.834 (3)
O1—K1	3.032 (3)	K2—O2^xiii	2.848 (3)
O1—K2^ii	3.205 (3)	K2—O2^viii	2.848 (3)
O2—Al^vi	1.925 (3)	K2—O3^iv	3.202 (3)
O2—Cr^vi	1.925 (3)	K2—O3^xiv	3.202 (3)
O2—K2^i	2.848 (3)	K2—O1^vi	3.205 (3)
O3—As1^viii	1.669 (3)	K2—O1^iii	3.205 (3)
O3—K1^viii	2.769 (3)	K2—As2^xii	3.441 (2)
O2—As1—O3^i	116.44 (13)	O3^ix—K1—O4^xi	119.81 (7)
O2—As1—O1	117.41 (13)	O7^vii—K1—O4^xi	93.40 (8)
O3^i—As1—O1	104.67 (13)	O7^ii—K1—O4^xi	86.60 (8)
O2—As1—O4	105.16 (13)	O1^x—K1—O4^xi	82.64 (7)
O3^i—As1—O4	103.99 (12)	O1—K1—O4^xi	97.36 (7)
O1—As1—O4	108.17 (12)	O3^i—K1—O4^v	119.81 (7)
O2—As1—K1	164.68 (9)	O3^ix—K1—O4^v	60.19 (7)
O3^i—As1—K1	52.76 (9)	O7^vii—K1—O4^v	86.60 (8)
O1—As1—K1	61.84 (9)	O7^ii—K1—O4^v	93.40 (8)
O4—As1—K1	88.94 (9)	O1^x—K1—O4^v	97.36 (7)
O2—As1—K2^ii	94.71 (9)	O1—K1—O4^v	82.64 (7)
O3^i—As1—K2^ii	63.46 (9)	O4^xi—K1—O4^v	180.000 (1)
O1—As1—K2^ii	63.56 (9)	O3^i—K1—As1^x	151.32 (5)
O4—As1—K2^ii	159.92 (8)	O3^ix—K1—As1^x	28.68 (6)
K1—As1—K2^ii	71.00 (4)	O7^vii—K1—As1^x	112.10 (6)
O6—As2—O7	113.63 (12)	O7^ii—K1—As1^x	67.90 (6)
O6—As2—O5	106.34 (13)	O1^x—K1—As1^x	29.03 (5)
O7—As2—O5	111.83 (13)	O1—K1—As1^x	150.97 (5)
O6—As2—O4	106.77 (13)	O4^xi—K1—As1^x	109.51 (6)
O7—As2—O4	109.74 (12)	O4^v—K1—As1^x	70.49 (6)
O5—As2—O4	108.26 (12)	O3^i—K1—As1	28.68 (5)
O6—As2—K2	53.92 (10)	O3^ix—K1—As1	151.32 (6)
O7—As2—K2	116.01 (10)	O7^vii—K1—As1	67.90 (6)
O5—As2—K2	54.99 (9)	O7^ii—K1—As1	112.10 (6)
O4—As2—K2	134.24 (8)	O1^x—K1—As1	150.97 (5)
O6—As2—K1^iii	83.81 (10)	O1—K1—As1	29.03 (5)
O7—As2—K1^iii	46.83 (9)	O4^xi—K1—As1	70.49 (6)
O5—As2—K1^iii	88.35 (10)	O4^v—K1—As1	109.51 (6)
O4—As2—K1^iii	156.13 (8)	As1^x—K1—As1	180.0
K2—As2—K1^iii	69.32 (4)	O3^i—K1—As2^ii	68.09 (6)
O6—As2—K1^iv	72.50 (10)	O3^ix—K1—As2^ii	111.91 (6)
O7—As2—K1^iv	85.22 (9)	O7^vii—K1—As2^ii	154.81 (5)
O5—As2—K1^iv	160.88 (9)	O7^ii—K1—As2^ii	25.19 (5)
O4—As2—K1^iv	55.72 (8)	O1^x—K1—As2^ii	78.58 (6)
K2—As2—K1^iv	126.37 (3)	O1—K1—As2^ii	101.42 (6)
K1^iii—As2—K1^iv	110.23 (3)	O4^xi—K1—As2^ii	94.66 (6)
O6^v—Cr—O3	89.05 (12)	O4^v—K1—As2^ii	85.34 (6)
O6^v—Cr—O1	88.35 (12)	As1^x—K1—As2^ii	87.50 (4)
O3—Cr—O1	177.25 (11)	As1—K1—As2^ii	92.50 (4)
O6^v—Cr—O5	177.24 (11)	O3^i—K1—As2^vii	111.91 (6)
O3—Cr—O5	90.41 (12)	O3^ix—K1—As2^vii	68.09 (6)
O1—Cr—O5	92.23 (12)	O7^vii—K1—As2^vii	25.19 (5)
O6^v—Cr—O2^vi	85.28 (11)	O7^ii—K1—As2^vii	154.81 (5)
O3—Cr—O2^vi	88.97 (12)	O1^x—K1—As2^vii	101.42 (6)
O1—Cr—O2^vi	91.72 (12)	O1—K1—As2^vii	78.58 (6)
O5—Cr—O2^vi	92.00 (11)	O4^xi—K1—As2^vii	85.34 (6)
O6^v—Cr—O7^vii	91.24 (11)	O4^v—K1—As2^vii	94.66 (6)
O3—Cr—O7^vii	91.73 (11)	As1^x—K1—As2^vii	92.50 (4)
O1—Cr—O7^vii	87.42 (12)	As1—K1—As2^vii	87.50 (4)
O5—Cr—O7^vii	91.49 (11)	As2^ii—K1—As2^vii	180.0
O2^vi—Cr—O7^vii	176.44 (11)	O6—K2—O6^xii	180.0
O6^v—Cr—K2^v	48.75 (9)	O6—K2—O5	56.38 (8)
O3—Cr—K2^v	60.75 (8)	O6^xii—K2—O5	123.62 (8)
O1—Cr—K2^v	117.88 (8)	O6—K2—O5^xii	123.62 (8)
O5—Cr—K2^v	128.80 (9)	O6^xii—K2—O5^xii	56.38 (8)
O2^vi—Cr—K2^v	50.16 (8)	O5—K2—O5^xii	180.0
O7^vii—Cr—K2^v	127.49 (8)	O6—K2—O2^xiii	54.52 (8)
O6^v—Cr—K1	71.06 (9)	O6^xii—K2—O2^xiii	125.48 (8)
O3—Cr—K1	128.54 (8)	O5—K2—O2^xiii	109.72 (8)
O1—Cr—K1	49.56 (8)	O5^xii—K2—O2^xiii	70.28 (8)
O5—Cr—K1	111.31 (9)	O6—K2—O2^viii	125.48 (8)
O2^vi—Cr—K1	133.48 (9)	O6^xii—K2—O2^viii	54.52 (8)
O7^vii—Cr—K1	44.10 (8)	O5—K2—O2^viii	70.28 (8)
K2^v—Cr—K1	119.78 (4)	O5^xii—K2—O2^viii	109.72 (8)
As1—O1—Cr	130.98 (15)	O2^xiii—K2—O2^viii	180.0
As1—O1—K1	89.13 (11)	O6—K2—O3^iv	52.34 (7)
Cr—O1—K1	101.64 (10)	O6^xii—K2—O3^iv	127.66 (7)
As1—O1—K2^ii	88.65 (10)	O5—K2—O3^iv	93.39 (7)
Cr—O1—K2^ii	139.96 (11)	O5^xii—K2—O3^iv	86.61 (7)
K1—O1—K2^ii	81.57 (8)	O2^xiii—K2—O3^iv	52.42 (7)
As1—O2—Al^vi	136.04 (15)	O2^viii—K2—O3^iv	127.58 (7)
As1—O2—Cr^vi	136.04 (15)	O6—K2—O3^xiv	127.66 (7)
Al^vi—O2—Cr^vi	0.00 (8)	O6^xii—K2—O3^xiv	52.34 (7)
As1—O2—K2^i	125.20 (12)	O5—K2—O3^xiv	86.61 (8)
Al^vi—O2—K2^i	98.57 (10)	O5^xii—K2—O3^xiv	93.39 (7)
Cr^vi—O2—K2^i	98.57 (10)	O2^xiii—K2—O3^xiv	127.58 (7)
As1^viii—O3—Cr	131.84 (14)	O2^viii—K2—O3^xiv	52.42 (7)
As1^viii—O3—K1^viii	98.56 (11)	O3^iv—K2—O3^xiv	180.0
Cr—O3—K1^viii	129.00 (12)	O6—K2—O1^vi	80.54 (8)
As1^viii—O3—K2^v	88.74 (10)	O6^xii—K2—O1^vi	99.46 (8)
Cr—O3—K2^v	87.81 (9)	O5—K2—O1^vi	64.92 (8)
K1^viii—O3—K2^v	85.79 (7)	O5^xii—K2—O1^vi	115.08 (8)
As2—O4—As1	118.50 (14)	O2^xiii—K2—O1^vi	92.66 (7)
As2—O4—K1^iv	97.65 (10)	O2^viii—K2—O1^vi	87.34 (7)
As1—O4—K1^iv	131.00 (12)	O3^iv—K2—O1^vi	131.29 (7)
As2—O5—Cr	127.51 (15)	O3^xiv—K2—O1^vi	48.71 (7)
As2—O5—K2	96.06 (11)	O6—K2—O1^iii	99.46 (8)
Cr—O5—K2	135.48 (11)	O6^xii—K2—O1^iii	80.54 (8)
As2—O6—Al^iv	145.71 (15)	O5—K2—O1^iii	115.08 (8)
As2—O6—Cr^iv	145.71 (15)	O5^xii—K2—O1^iii	64.92 (8)
Al^iv—O6—Cr^iv	0.00 (5)	O2^xiii—K2—O1^iii	87.34 (7)
As2—O6—K2	97.63 (11)	O2^viii—K2—O1^iii	92.66 (7)
Al^iv—O6—K2	100.69 (11)	O3^iv—K2—O1^iii	48.71 (7)
Cr^iv—O6—K2	100.69 (11)	O3^xiv—K2—O1^iii	131.29 (7)
As2—O7—Al^vii	143.23 (16)	O1^vi—K2—O1^iii	180.00 (7)
As2—O7—Cr^vii	143.23 (16)	O6—K2—As2^xii	151.55 (5)
Al^vii—O7—Cr^vii	0.00 (8)	O6^xii—K2—As2^xii	28.45 (5)
As2—O7—K1^iii	107.98 (11)	O5—K2—As2^xii	151.05 (5)
Al^vii—O7—K1^iii	107.62 (10)	O5^xii—K2—As2^xii	28.95 (5)
Cr^vii—O7—K1^iii	107.62 (10)	O2^xiii—K2—As2^xii	97.04 (6)
O3^i—K1—O3^ix	180.000 (1)	O2^viii—K2—As2^xii	82.96 (6)
O3^i—K1—O7^vii	95.75 (8)	O3^iv—K2—As2^xii	112.12 (6)
O3^ix—K1—O7^vii	84.25 (8)	O3^xiv—K2—As2^xii	67.88 (6)
O3^i—K1—O7^ii	84.25 (8)	O1^vi—K2—As2^xii	104.03 (6)
O3^ix—K1—O7^ii	95.75 (8)	O1^iii—K2—As2^xii	75.97 (6)
O7^vii—K1—O7^ii	180.0	O6—K2—As2	28.45 (5)
O3^i—K1—O1^x	126.04 (8)	O6^xii—K2—As2	151.55 (5)
O3^ix—K1—O1^x	53.96 (8)	O5—K2—As2	28.95 (5)
O7^vii—K1—O1^x	126.18 (8)	O5^xii—K2—As2	151.05 (5)
O7^ii—K1—O1^x	53.82 (8)	O2^xiii—K2—As2	82.96 (6)
O3^i—K1—O1	53.96 (8)	O2^viii—K2—As2	97.04 (6)
O3^ix—K1—O1	126.04 (8)	O3^iv—K2—As2	67.88 (6)
O7^vii—K1—O1	53.82 (8)	O3^xiv—K2—As2	112.12 (6)
O7^ii—K1—O1	126.18 (8)	O1^vi—K2—As2	75.97 (6)
O1^x—K1—O1	180.0	O1^iii—K2—As2	104.03 (6)
O3^i—K1—O4^xi	60.19 (7)	As2^xii—K2—As2	180.0

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