research communications\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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ISSN: 2056-9890

New mixed aluminium–chromium diarsenate

aLaboratoire de Materiaux et Cristallochimie, Faculté des Sciences de Tunis, Université de Tunis El Manar, 2092 Manar II Tunis, Tunisia
*Correspondence e-mail: habib.boughzala@ipein.rnu.tn

Edited by T. J. Prior, University of Hull, England (Received 7 January 2017; accepted 1 February 2017; online 10 February 2017)

Potassium chromium aluminium diarsenate, KCr1/4Al3/4As2O7, was prepared by solid-state reaction. The structure consists of (Cr1/4/Al3/4)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 AIMIIIX2O7 (AI = alkali metal; MIII = Al, Cr, Fe; X = As, P) compounds. However, the MIII 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 AIMIIIX2O7 (AI = alkali metal; MIII = 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 MIII = 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 AICrIIIAs2O7 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 KCr1/4Al3/4As2O7 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 KCr1/4Al3/4As2O7 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[link]). 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[link]).

[Figure 1]
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]
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 AlIII and the chromium CrIII 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 CrIII cations is proved by the shortening of the Cr—O bond length compared to AlIII—O. In fact, according to similar studies (Bouhassine & Boughzala, 2014[Bouhassine, M. A. & Boughzala, H. (2014). Acta Cryst. E70, i6.], 2015[Bouhassine, M. A. & Boughzala, H. (2015). Acta Cryst. E71, 636-639.]) the average AlIII—O and CrIII—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[Boughzala, H. & Jouini, T. (1995). Acta Cryst. C51, 179-181.]) and RbAlAs2O7 (Boughzala et al., 1993[Boughzala, H., Driss, A. & Jouini, T. (1993). Acta Cryst. C49, 425-427.]), the As—O distances involving the bridging O4 atom are the longest (Table 1[link]). 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[Bouhassine, M. A. & Boughzala, H. (2015). Acta Cryst. E71, 636-639.]] and KAlAs2O7 [118.3 (2)°; Boughzala & Jouini, 1995[Boughzala, H. & Jouini, T. (1995). Acta Cryst. C51, 179-181.]]. 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—O3i 1.669 (3) (Cr/Al)—O6ii 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)—O2iii 1.925 (3)
As2—O5 1.675 (2) (Cr/Al)—O7iv 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[link]). On the other hand, each As2O7 anion is surrounded by five (Cr/Al)O6 units (Fig. 4[link]).

[Figure 3]
Figure 3
The environment of the (Cr/Al)O6 octa­hedron in the structure of KCr1/4Al3/4As2O7.
[Figure 4]
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 AIAlAs2O7 (AI = K, Rb, Tl, Cs; Boughzala & Jouini, 1992[Boughzala, H. & Jouini, T. (1992). C. R. Acad. Sci. Paris Ser. II, pp. 1419-1422.]) crystallize in the triclinic space group P[\overline{1}] and are classified as type II (Durif & Averbuch-Pouchot, 1996[Durif, A. & Averbuch-Pouchot, M.-T. (1996). In Topics in Phosphate Chemistry. Singapore: World Scientific Publishing Co.]); the diarsenate groups have a different conformational orientation compared to that of the title structure, which belongs to the type I family of AIMIIIX2O7 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[link]. The corresponding angle is 158.8 (2)° for KAlAs2O7 (Boughzala & Jouini, 1995[Boughzala, H. & Jouini, T. (1995). Acta Cryst. C51, 179-181.]).

[Figure 5]
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[link]), 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]
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[Ng, H. N. & Calvo, C. (1973). Can. J. Chem. 51, 2613-2620.]) was the first to be published for the AIMIIIX2O7 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[\overline{1}] with two formulas units, for the diarsenate compounds AIAlAs2O7 (AI = K, Rb, Tl, Cs) (Boughzala & Jouini, 1992[Boughzala, H. & Jouini, T. (1992). C. R. Acad. Sci. Paris Ser. II, pp. 1419-1422.]; Boughzala et al., 1993[Boughzala, H., Driss, A. & Jouini, T. (1993). Acta Cryst. C49, 425-427.]; Boughzala & Jouini; 1995[Boughzala, H. & Jouini, T. (1995). Acta Cryst. C51, 179-181.]), whereas the diphosphates are generally monoclinic. The isotypic AICrP2O7 phases crystallize in space group P21/c for AI = Na (Bohatý et al., 1982[Bohatý, L., Liebertz, J. & Fröhlich, R. (1982). Z. Kristallogr. 161, 53-59.]), K (Gentil et al., 1997[Gentil, S., Andreica, D., Lujan, M., Rivera, J. P., Kubel, F. & Schmid, H. (1997). Ferroelectrics, 204, 35-44.]), Rb (Zhao & Li, 2011[Zhao, D. & Li, F. (2011). Z. Kristallogr. 226, 443-444.]) and Cs (Linde & Gorbunova, 1982[Linde, S. A. & Gorbunova, Yu. E. (1982). Izv. Akad. Nauk SSSR Neorg. Mater. 18, 464-467.]). The same applies for the AIFeP2O7 phases for AI = Na (Gabelica-Robert et al., 1982[Gabelica-Robert, M., Goreaud, M., Labbe, Ph. & Raveau, B. (1982). J. Solid State Chem. 45, 389-395.]) and K (Riou et al., 1988[Riou, D., Labbe, P. & Goreaud, M. (1988). Eur. J. Solid. State Inorg. Chem. 25, 215-229.]). However, the two Li-containing phases LiMP2O7 show a symmetry reduction to space group P21 (M = Cr, Ivashkevich et al., 2007[Ivashkevich, L. S., Selevich, K. A., Lesnikovich, A. I. & Selevich, A. F. (2007). Acta Cryst. E63, i70-i72.]; M = Fe, Riou et al., 1990[Riou, D., Nguyen, N., Benloucif, R. & Raveau, B. (1990). Mater. Res. Bull. 25, 1363-1369.]). CsCrAs2O7 (Bouhassine & Boughzala, 2015[Bouhassine, M. A. & Boughzala, H. (2015). Acta Cryst. E71, 636-639.]) is the first phase of the AICrAs2O7 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[link]. 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 CrIII, 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 CrIII and AlIII in the same crystallographic site endowed with the same Uij parameters. The aluminium has obviously diffused from the porcelain crucible. The last refinement steps lead to the final formula KCr1/4Al3/4As2O7. The presence of both aluminium and chromium in the structure was confirmed by TEM as shown in Fig. 7[link].

Table 2
Experimental details

Crystal data
Chemical formula KCr0.25Al0.75As2O7
Mr 334.17
Crystal system, space group Triclinic, P[\overline{1}]
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)
V3) 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[North, A. C. T., Phillips, D. C. & Mathews, F. S. (1968). Acta Cryst. A24, 351-359.])
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−1) 0.702
 
Refinement
R[F2 > 2σ(F2)], wR(F2), 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[Enraf-Nonius (1994). CAD-4 EXPRESS. Enraf-Nonius, Delft, The Netherlands.]), XCAD4 (Harms & Wocadlo, 1995[Harms, K. & Wocadlo, S. (1995). XCAD4. University of Marburg, Germany.]), SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]), SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]), DIAMOND (Brandenburg, 2008[Brandenburg, K. (2008). DIAMOND. Crystal Impact GbR, Bonn, Germany.]) and publCIF (Westrip, 2010[Westrip, S. P. (2010). J. Appl. Cryst. 43, 920-925.]).
[Figure 7]
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 top

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, KCr1/4Al3/4As2O7 top
Crystal data top
KCr0.25Al0.75As2O7Z = 2
Mr = 334.17F(000) = 314
Triclinic, P1Dx = 3.702 Mg m3
Hall symbol: -p 1Mo Kα radiation, λ = 0.71073 Å
a = 6.243 (3) ÅCell parameters from 25 reflections
b = 6.349 (3) Åθ = 3.8–27°
c = 8.153 (4) ŵ = 12.37 mm1
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) Å3
Data collection top
Enraf–Nonius CAD-4
diffractometer
1306 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.014
Graphite monochromatorθmax = 30.0°, θmin = 2.6°
ω/2θ scansh = 07
Absorption correction: ψ scan
(North et al., 1968)
k = 88
Tmin = 0.079, Tmax = 0.182l = 1111
1628 measured reflections2 standard reflections every 120 min
1479 independent reflections intensity decay: 1.1%
Refinement top
Refinement on F20 restraints
Least-squares matrix: fullPrimary atom site location: structure-invariant direct methods
R[F2 > 2σ(F2)] = 0.023Secondary atom site location: difference Fourier map
wR(F2) = 0.059 w = 1/[σ2(Fo2) + (0.0303P)2 + 0.5924P]
where P = (Fo2 + 2Fc2)/3
S = 1.08(Δ/σ)max < 0.001
1479 reflectionsΔρmax = 0.59 e Å3
104 parametersΔρmin = 1.09 e Å3
Special details top

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 (Å2) top
xyzUiso*/UeqOcc. (<1)
As10.82649 (6)0.53279 (5)0.68995 (4)0.00590 (10)
As20.48557 (6)0.24607 (5)0.83512 (4)0.00587 (10)
Cr0.37535 (14)0.69197 (13)0.72579 (10)0.0054 (3)0.255 (6)
Al0.37535 (14)0.69197 (13)0.72579 (10)0.0054 (3)0.745 (6)
O10.6626 (4)0.7081 (4)0.6757 (3)0.0109 (5)
O20.7856 (4)0.3500 (4)0.5149 (3)0.0105 (5)
O30.0934 (4)0.6909 (4)0.7793 (3)0.0097 (5)
O40.7722 (4)0.3747 (4)0.8485 (3)0.0108 (5)
O50.3164 (4)0.3800 (4)0.7206 (3)0.0100 (5)
O60.4234 (5)0.0032 (4)0.7201 (3)0.0113 (5)
O70.4552 (4)0.2466 (4)1.0321 (3)0.0106 (5)
K11.00001.00001.00000.0484 (5)
K20.00000.00000.50000.0364 (4)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
As10.00519 (19)0.00627 (16)0.00641 (15)0.00121 (12)0.00247 (12)0.00063 (11)
As20.00655 (19)0.00535 (15)0.00580 (15)0.00124 (12)0.00216 (12)0.00115 (10)
Cr0.0047 (5)0.0053 (4)0.0062 (4)0.0015 (3)0.0019 (3)0.0005 (3)
Al0.0047 (5)0.0053 (4)0.0062 (4)0.0015 (3)0.0019 (3)0.0005 (3)
O10.0127 (13)0.0117 (11)0.0122 (11)0.0066 (10)0.0060 (10)0.0049 (9)
O20.0122 (13)0.0107 (11)0.0079 (10)0.0041 (10)0.0022 (9)0.0015 (8)
O30.0070 (12)0.0095 (11)0.0097 (10)0.0010 (9)0.0023 (9)0.0026 (8)
O40.0056 (12)0.0159 (12)0.0119 (11)0.0017 (10)0.0026 (9)0.0079 (9)
O50.0089 (13)0.0076 (10)0.0113 (11)0.0025 (9)0.0010 (9)0.0011 (8)
O60.0163 (14)0.0064 (11)0.0109 (11)0.0033 (10)0.0045 (10)0.0016 (8)
O70.0106 (13)0.0139 (11)0.0069 (10)0.0011 (10)0.0038 (9)0.0012 (9)
K10.0302 (9)0.0408 (9)0.0615 (11)0.0093 (7)0.0253 (8)0.0310 (8)
K20.0354 (9)0.0148 (6)0.0393 (8)0.0055 (6)0.0209 (6)0.0002 (5)
Geometric parameters (Å, º) top
As1—O21.659 (2)O3—K2v3.202 (3)
As1—O3i1.669 (3)O4—K1iv3.246 (3)
As1—O11.669 (3)O5—K22.834 (3)
As1—O41.776 (2)O6—Aliv1.898 (3)
As1—K13.4388 (16)O6—Criv1.898 (3)
As1—K2ii3.5783 (15)O6—K22.806 (3)
As2—O61.654 (2)O7—Alvii1.940 (3)
As2—O71.663 (2)O7—Crvii1.940 (3)
As2—O51.675 (2)O7—K1iii2.849 (3)
As2—O41.761 (3)K1—O3i2.769 (3)
As2—K23.441 (2)K1—O3ix2.769 (3)
As2—K1iii3.7159 (17)K1—O7vii2.849 (3)
As2—K1iv3.8935 (18)K1—O7ii2.849 (3)
Cr—O6v1.898 (3)K1—O1x3.032 (3)
Cr—O31.915 (3)K1—O4xi3.246 (3)
Cr—O11.919 (3)K1—O4v3.246 (3)
Cr—O51.925 (3)K1—As1x3.4388 (16)
Cr—O2vi1.925 (3)K1—As2ii3.7160 (17)
Cr—O7vii1.940 (3)K1—As2vii3.7160 (17)
Cr—K2v3.6674 (17)K2—O6xii2.806 (3)
Cr—K13.902 (2)K2—O5xii2.834 (3)
O1—K13.032 (3)K2—O2xiii2.848 (3)
O1—K2ii3.205 (3)K2—O2viii2.848 (3)
O2—Alvi1.925 (3)K2—O3iv3.202 (3)
O2—Crvi1.925 (3)K2—O3xiv3.202 (3)
O2—K2i2.848 (3)K2—O1vi3.205 (3)
O3—As1viii1.669 (3)K2—O1iii3.205 (3)
O3—K1viii2.769 (3)K2—As2xii3.441 (2)
O2—As1—O3i116.44 (13)O3ix—K1—O4xi119.81 (7)
O2—As1—O1117.41 (13)O7vii—K1—O4xi93.40 (8)
O3i—As1—O1104.67 (13)O7ii—K1—O4xi86.60 (8)
O2—As1—O4105.16 (13)O1x—K1—O4xi82.64 (7)
O3i—As1—O4103.99 (12)O1—K1—O4xi97.36 (7)
O1—As1—O4108.17 (12)O3i—K1—O4v119.81 (7)
O2—As1—K1164.68 (9)O3ix—K1—O4v60.19 (7)
O3i—As1—K152.76 (9)O7vii—K1—O4v86.60 (8)
O1—As1—K161.84 (9)O7ii—K1—O4v93.40 (8)
O4—As1—K188.94 (9)O1x—K1—O4v97.36 (7)
O2—As1—K2ii94.71 (9)O1—K1—O4v82.64 (7)
O3i—As1—K2ii63.46 (9)O4xi—K1—O4v180.000 (1)
O1—As1—K2ii63.56 (9)O3i—K1—As1x151.32 (5)
O4—As1—K2ii159.92 (8)O3ix—K1—As1x28.68 (6)
K1—As1—K2ii71.00 (4)O7vii—K1—As1x112.10 (6)
O6—As2—O7113.63 (12)O7ii—K1—As1x67.90 (6)
O6—As2—O5106.34 (13)O1x—K1—As1x29.03 (5)
O7—As2—O5111.83 (13)O1—K1—As1x150.97 (5)
O6—As2—O4106.77 (13)O4xi—K1—As1x109.51 (6)
O7—As2—O4109.74 (12)O4v—K1—As1x70.49 (6)
O5—As2—O4108.26 (12)O3i—K1—As128.68 (5)
O6—As2—K253.92 (10)O3ix—K1—As1151.32 (6)
O7—As2—K2116.01 (10)O7vii—K1—As167.90 (6)
O5—As2—K254.99 (9)O7ii—K1—As1112.10 (6)
O4—As2—K2134.24 (8)O1x—K1—As1150.97 (5)
O6—As2—K1iii83.81 (10)O1—K1—As129.03 (5)
O7—As2—K1iii46.83 (9)O4xi—K1—As170.49 (6)
O5—As2—K1iii88.35 (10)O4v—K1—As1109.51 (6)
O4—As2—K1iii156.13 (8)As1x—K1—As1180.0
K2—As2—K1iii69.32 (4)O3i—K1—As2ii68.09 (6)
O6—As2—K1iv72.50 (10)O3ix—K1—As2ii111.91 (6)
O7—As2—K1iv85.22 (9)O7vii—K1—As2ii154.81 (5)
O5—As2—K1iv160.88 (9)O7ii—K1—As2ii25.19 (5)
O4—As2—K1iv55.72 (8)O1x—K1—As2ii78.58 (6)
K2—As2—K1iv126.37 (3)O1—K1—As2ii101.42 (6)
K1iii—As2—K1iv110.23 (3)O4xi—K1—As2ii94.66 (6)
O6v—Cr—O389.05 (12)O4v—K1—As2ii85.34 (6)
O6v—Cr—O188.35 (12)As1x—K1—As2ii87.50 (4)
O3—Cr—O1177.25 (11)As1—K1—As2ii92.50 (4)
O6v—Cr—O5177.24 (11)O3i—K1—As2vii111.91 (6)
O3—Cr—O590.41 (12)O3ix—K1—As2vii68.09 (6)
O1—Cr—O592.23 (12)O7vii—K1—As2vii25.19 (5)
O6v—Cr—O2vi85.28 (11)O7ii—K1—As2vii154.81 (5)
O3—Cr—O2vi88.97 (12)O1x—K1—As2vii101.42 (6)
O1—Cr—O2vi91.72 (12)O1—K1—As2vii78.58 (6)
O5—Cr—O2vi92.00 (11)O4xi—K1—As2vii85.34 (6)
O6v—Cr—O7vii91.24 (11)O4v—K1—As2vii94.66 (6)
O3—Cr—O7vii91.73 (11)As1x—K1—As2vii92.50 (4)
O1—Cr—O7vii87.42 (12)As1—K1—As2vii87.50 (4)
O5—Cr—O7vii91.49 (11)As2ii—K1—As2vii180.0
O2vi—Cr—O7vii176.44 (11)O6—K2—O6xii180.0
O6v—Cr—K2v48.75 (9)O6—K2—O556.38 (8)
O3—Cr—K2v60.75 (8)O6xii—K2—O5123.62 (8)
O1—Cr—K2v117.88 (8)O6—K2—O5xii123.62 (8)
O5—Cr—K2v128.80 (9)O6xii—K2—O5xii56.38 (8)
O2vi—Cr—K2v50.16 (8)O5—K2—O5xii180.0
O7vii—Cr—K2v127.49 (8)O6—K2—O2xiii54.52 (8)
O6v—Cr—K171.06 (9)O6xii—K2—O2xiii125.48 (8)
O3—Cr—K1128.54 (8)O5—K2—O2xiii109.72 (8)
O1—Cr—K149.56 (8)O5xii—K2—O2xiii70.28 (8)
O5—Cr—K1111.31 (9)O6—K2—O2viii125.48 (8)
O2vi—Cr—K1133.48 (9)O6xii—K2—O2viii54.52 (8)
O7vii—Cr—K144.10 (8)O5—K2—O2viii70.28 (8)
K2v—Cr—K1119.78 (4)O5xii—K2—O2viii109.72 (8)
As1—O1—Cr130.98 (15)O2xiii—K2—O2viii180.0
As1—O1—K189.13 (11)O6—K2—O3iv52.34 (7)
Cr—O1—K1101.64 (10)O6xii—K2—O3iv127.66 (7)
As1—O1—K2ii88.65 (10)O5—K2—O3iv93.39 (7)
Cr—O1—K2ii139.96 (11)O5xii—K2—O3iv86.61 (7)
K1—O1—K2ii81.57 (8)O2xiii—K2—O3iv52.42 (7)
As1—O2—Alvi136.04 (15)O2viii—K2—O3iv127.58 (7)
As1—O2—Crvi136.04 (15)O6—K2—O3xiv127.66 (7)
Alvi—O2—Crvi0.00 (8)O6xii—K2—O3xiv52.34 (7)
As1—O2—K2i125.20 (12)O5—K2—O3xiv86.61 (8)
Alvi—O2—K2i98.57 (10)O5xii—K2—O3xiv93.39 (7)
Crvi—O2—K2i98.57 (10)O2xiii—K2—O3xiv127.58 (7)
As1viii—O3—Cr131.84 (14)O2viii—K2—O3xiv52.42 (7)
As1viii—O3—K1viii98.56 (11)O3iv—K2—O3xiv180.0
Cr—O3—K1viii129.00 (12)O6—K2—O1vi80.54 (8)
As1viii—O3—K2v88.74 (10)O6xii—K2—O1vi99.46 (8)
Cr—O3—K2v87.81 (9)O5—K2—O1vi64.92 (8)
K1viii—O3—K2v85.79 (7)O5xii—K2—O1vi115.08 (8)
As2—O4—As1118.50 (14)O2xiii—K2—O1vi92.66 (7)
As2—O4—K1iv97.65 (10)O2viii—K2—O1vi87.34 (7)
As1—O4—K1iv131.00 (12)O3iv—K2—O1vi131.29 (7)
As2—O5—Cr127.51 (15)O3xiv—K2—O1vi48.71 (7)
As2—O5—K296.06 (11)O6—K2—O1iii99.46 (8)
Cr—O5—K2135.48 (11)O6xii—K2—O1iii80.54 (8)
As2—O6—Aliv145.71 (15)O5—K2—O1iii115.08 (8)
As2—O6—Criv145.71 (15)O5xii—K2—O1iii64.92 (8)
Aliv—O6—Criv0.00 (5)O2xiii—K2—O1iii87.34 (7)
As2—O6—K297.63 (11)O2viii—K2—O1iii92.66 (7)
Aliv—O6—K2100.69 (11)O3iv—K2—O1iii48.71 (7)
Criv—O6—K2100.69 (11)O3xiv—K2—O1iii131.29 (7)
As2—O7—Alvii143.23 (16)O1vi—K2—O1iii180.00 (7)
As2—O7—Crvii143.23 (16)O6—K2—As2xii151.55 (5)
Alvii—O7—Crvii0.00 (8)O6xii—K2—As2xii28.45 (5)
As2—O7—K1iii107.98 (11)O5—K2—As2xii151.05 (5)
Alvii—O7—K1iii107.62 (10)O5xii—K2—As2xii28.95 (5)
Crvii—O7—K1iii107.62 (10)O2xiii—K2—As2xii97.04 (6)
O3i—K1—O3ix180.000 (1)O2viii—K2—As2xii82.96 (6)
O3i—K1—O7vii95.75 (8)O3iv—K2—As2xii112.12 (6)
O3ix—K1—O7vii84.25 (8)O3xiv—K2—As2xii67.88 (6)
O3i—K1—O7ii84.25 (8)O1vi—K2—As2xii104.03 (6)
O3ix—K1—O7ii95.75 (8)O1iii—K2—As2xii75.97 (6)
O7vii—K1—O7ii180.0O6—K2—As228.45 (5)
O3i—K1—O1x126.04 (8)O6xii—K2—As2151.55 (5)
O3ix—K1—O1x53.96 (8)O5—K2—As228.95 (5)
O7vii—K1—O1x126.18 (8)O5xii—K2—As2151.05 (5)
O7ii—K1—O1x53.82 (8)O2xiii—K2—As282.96 (6)
O3i—K1—O153.96 (8)O2viii—K2—As297.04 (6)
O3ix—K1—O1126.04 (8)O3iv—K2—As267.88 (6)
O7vii—K1—O153.82 (8)O3xiv—K2—As2112.12 (6)
O7ii—K1—O1126.18 (8)O1vi—K2—As275.97 (6)
O1x—K1—O1180.0O1iii—K2—As2104.03 (6)
O3i—K1—O4xi60.19 (7)As2xii—K2—As2180.0
Symmetry codes: (i) x+1, y, z; (ii) x+1, y+1, z; (iii) x1, y1, z; (iv) x, y1, z; (v) x, y+1, z; (vi) x+1, y+1, z+1; (vii) x+1, y+1, z+2; (viii) x1, 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.
 

Acknowledgements

Special acknowledgements are dedicated to Besma Bouzemi Friaa, a member of the Laboratoire de Materiaux et Cristallochimie, Faculte des Sciences de Tunis from 1997 to 2003.

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