Crystal structure of CsCrAs₂O₇, a new member of the diarsenate family

The structure of CsCrAs₂O₇ can be described as a three-dimensional [CrAs₂O₇]⁻ anionic framework in which the Cs⁺ cations are located in empty channels running along [001].

Abstract

Caesium chromium(III) diarsenate(V), CsCrAs₂O₇, was prepared by solid-state reactions. The title structure consists of isolated CrO₆ octa­hedra and As₂O₇ diarsenate groups, sharing corners to build up a three-dimensional [CrAs₂O₇]⁻ anionic framework. In this framework, channels extending parallel to [001] are present in which the ten-coordinate Cs⁺ ions reside. CsCrAs₂O₇ is isotypic with the monoclinic A ^I M ^III X ₂O₇ (A ^I = alkali metal; M ^III = Al, Cr, Fe; X = As, P) type I family of compounds crystallizing in the space group P2₁/c.

Chemical context  

In recent years, inorganic metal phosphates and arsenates with formula A ^I M ^III X ₂O₇ (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 X ₂O₇ 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 corresponding diphosphates have been studied extensively, in contrast to the scarcely studied chromium diarsen­ates. Herein the preparation and crystal structure of CsCrAs₂O₇ is reported, one of a series of new cesium chromium(III) arsenate compounds recently isolated by our group.

Structural commentary  

The structure of CsCrAs₂O₇ can be described as a three-dimensional [CrAs₂O₇]⁻ anionic framework (Fig. 1 ▸) with channels extending parallel to [001] that are occupied by ten-coordinate Cs⁺ cations (Fig. 2 ▸).

Figure 1.

The coordination polyhedra around Cr and As atoms in the title structure. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) x,  − y,  + z; (ii) 2 − x,  + y,  − z; (iii) x,  − y, − + z; (iv) 1 − x,  + y,  − z.]

Figure 2.

Projection of the CsCrAs₂O₇ structure showing the channels parallel to [001] in which the Cs⁺ cations are located.

The two independent arsenic atoms form AsO₄ tetra­hedra and are connected via the bridging O4 atom into a diarsenate As₂O₇ anion. Like in the related 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 118.7 (2)° in the title structure is somewhat smaller than that of 125.9 (2)° reported for the isotypic structure of CsCrP₂O₇ (Linde & Gorbunova, 1982 ▸). The O—As—O bond angles span a range between 103.8 (2) and 116.2 (2)° and 105.5 (2) and 115.6 (2)°, respectively, for As1 and As2, reflecting the distortion of each of the AsO₄ tetra­hedra. The Cr^III cations are in a slightly distorted octa­hedral oxygen coordination with Cr—O distances ranging from 1.944 (4) to 2.010 (4) Å (Table 1 ▸), and with O—Cr—O angles ranging from 82.96 (18) to 95.94 (17)° and from 172.37 (19) to 173.72 (17)°. Each CrO₆ octa­hedron shares its corners with five As₂O₇ anions, one of which is chelating and the others belonging to four different As₂O₇ groups (Fig. 3 ▸). On the other hand, each As₂O₇ anion is surrounded by five CrO₆ octa­hedra as depicted in Fig. 4 ▸. The environment of the ten-coordinate Cs⁺ cation situated in the cavities of the resulting [CrAs₂O₇]⁻ framework is shown in Fig. 5 ▸.

Table 1. Selected bond lengths ().

CrO5i	1.944(4)	As1O2	1.664(4)
CrO7	1.954(4)	As1O3	1.681(4)
CrO1ii	1.978(4)	As1O4	1.763(4)
CrO3	1.982(4)	As2O5	1.641(4)
CrO2iii	2.007(4)	As2O6	1.661(4)
CrO6iv	2.010(4)	As2O7	1.669(4)
As1O1	1.651(4)	As2O4	1.750(4)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

Figure 3.

The environment of the CrO₆ octa­hedron in the structure of CsCrAs₂O₇.

Figure 4.

The environment of the diarsenate group in the structure of CsCrAs₂O₇.

Figure 5.

The surrounding of the ten-coordinated Cs⁺ cation in the structure of CsCrAs₂O₇.

It is worth mentioning that in the related aluminium diarsenate family A ^IAlAs₂O₇ (A ^I= K, Rb, Tl, Cs) (Boughzala & Jouini, 1992 ▸) that crystallizes isotypically in space group P and is classified as type II, the diarsenate groups have a different conformational orientation as those of the title structure. In the title structure, belonging to the type I family of A ^I M ^III X ₂O₇ diarsenates, the diarsenate tetra­hedra are in a nearly eclipsed conformation with a torsion angle O3—As1—As2—O7 of 39.8 (2)°, as shown in Fig. 6 ▸. The corresponding angle is 158.8 (2)° for KAlAs₂O₇ (Boughzala & Jouini, 1995 ▸).

Figure 6.

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

Using the bond-valence method (Brown, 2002 ▸), the calculated bond-valence-sum values (in valence units) of 5.08, 4.97, 3.01 and 1.35, respectively, for As1, As2, Cr and Cs are in good agreement with the expected oxidation states.

Database survey  

The structure of KAlP₂O₇ (Ng & Calvo, 1973 ▸) was the first published of the A ^I M ^III X ₂O₇ family. Afterwards, based on different substitutions and combinations, a large number of different phases were isolated and crystallographically characterized. Replacement of one of the cations can improve the structural and physical properties but also affects the coordin­ation numbers, the degree of distortion of the coord­ination polyhedra and the conformation of the X ₂O₇ groups. Also, 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 diphosphates are generally monoclinic. The isotypic A ^ICrP₂O₇ phases crystallize in space group P2₁/c for A ^I = Na (Bohaty et al., 1982 ▸), K (Gentil et al., 1997 ▸), Rb (Zhao & Li, 2011 ▸) and Cs (Linde & Gorbunova, 1982 ▸). The same counts 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 ▸).

Synthesis and crystallization  

The crystals of the title compound were obtained from heating a mixture of Cs₂CO₃, Cr₂O₃ and NH₄H₂AsO₄, with a Cs:Cr:As molar ratio of 1:1:2. 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 at that temperature for 24 h. In a second step, the crucible was progressively heated at 1023 K for 4 days and then slowly cooled down at a rate of 5 K/24 h to 923 K and finally quenched to room temperature. The product was washed with water and rinsed with an aqueous solution of HCl. Two phases could be isolated. The major phase forms regular cube-shaped dark-green crystals of yet unknown composition. The second phase represents the title compound and was obtained in the form of pink crystals.

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. The maximum and minimum electron density in the final difference Fourier map is located at 0.95 Å, 0.87 Å, respectively, from the Cs atom.

Table 2. Experimental details.

Crystal data
Chemical formula	CsCrAs2O7
M r	446.75
Crystal system, space group	Monoclinic, P21/c
Temperature (K)	293
a, b, c ()	7.908(1), 10.0806(10), 8.6371(10)
()	105.841(1)
V (3)	662.38(13)
Z	4
Radiation type	Mo K
(mm1)	17.05
Crystal size (mm)	0.20 0.20 0.10
Data collection
Diffractometer	EnrafNonius CAD-4
Absorption correction	scan (North et al., 1968 ▸)
T min, T max	0.132, 0.281
No. of measured, independent and observed [I > 2(I)] reflections	1530, 1433, 1205
R int	0.051
(sin /)max (1)	0.637
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.027, 0.075, 1.13
No. of reflections	1433
No. of parameters	101
max, min (e 3)	1.60, 1.23

Computer programs: CAD-4 EXPRESS (EnrafNonius, 1994 ▸), XCAD4 (Harms Wocadlo, 1995 ▸), SHELXS97 and SHELXL97 (Sheldrick, 2008 ▸), DIAMOND (Brandenburg, 2008 ▸) and publCIF (Westrip, 2010 ▸).

Supplementary Material

CCDC reference: 1400446

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

supplementary crystallographic information

Crystal data

CsCrAs2O7	F(000) = 804
Mr = 446.75	Dx = 4.480 Mg m−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 25 reflections
a = 7.908 (1) Å	θ = 3.8–27°
b = 10.0806 (10) Å	µ = 17.05 mm−1
c = 8.6371 (10) Å	T = 293 K
β = 105.841 (1)°	Monoclinic, pink
V = 662.38 (13) Å3	0.20 × 0.20 × 0.10 mm
Z = 4

Data collection

Enraf–Nonius CAD-4 diffractometer	1205 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.051
Graphite monochromator	θmax = 26.9°, θmin = 2.7°
ω/2θ scans	h = −10→9
Absorption correction: ψ scan (North et al., 1968)	k = 0→12
Tmin = 0.132, Tmax = 0.281	l = 0→11
1530 measured reflections	2 standard reflections every 120 min
1433 independent reflections	intensity decay: 1.1%

Refinement

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.027	w = 1/[σ2(Fo2) + (0.043P)2 + 1.1495P] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.075	(Δ/σ)max < 0.001
S = 1.13	Δρmax = 1.60 e Å−3
1433 reflections	Δρmin = −1.23 e Å−3
101 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraints	Extinction coefficient: 0.0018 (4)

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
As1	0.93130 (7)	0.13138 (6)	0.68578 (6)	0.00527 (16)
As2	0.63567 (7)	0.08924 (5)	0.84021 (6)	0.00495 (16)
Cs	0.31839 (5)	0.19797 (4)	0.45751 (4)	0.01429 (15)
Cr	0.73911 (11)	0.39967 (8)	0.76602 (10)	0.0043 (2)
O1	0.8001 (6)	0.1049 (5)	0.5039 (5)	0.0168 (10)
O2	1.1355 (5)	0.0742 (4)	0.7202 (5)	0.0122 (9)
O3	0.9413 (5)	0.2889 (4)	0.7514 (5)	0.0099 (8)
O4	0.8363 (5)	0.0371 (4)	0.8122 (5)	0.0093 (8)
O5	0.6538 (6)	0.0802 (5)	1.0338 (5)	0.0199 (10)
O6	0.4833 (5)	−0.0082 (4)	0.7244 (5)	0.0085 (8)
O7	0.5967 (5)	0.2403 (4)	0.7594 (5)	0.0109 (8)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
As1	0.0033 (3)	0.0057 (3)	0.0069 (3)	0.0003 (2)	0.0014 (2)	−0.0003 (2)
As2	0.0034 (3)	0.0055 (3)	0.0059 (3)	−0.0009 (2)	0.0013 (2)	−0.0002 (2)
Cs	0.0106 (2)	0.0165 (2)	0.0136 (2)	−0.00201 (14)	−0.00033 (14)	0.00299 (14)
Cr	0.0026 (4)	0.0045 (4)	0.0055 (4)	0.0004 (3)	0.0005 (3)	−0.0002 (3)
O1	0.017 (2)	0.022 (2)	0.010 (2)	−0.0029 (19)	0.0010 (17)	−0.0022 (18)
O2	0.0044 (19)	0.007 (2)	0.025 (2)	−0.0020 (16)	0.0035 (16)	−0.0024 (17)
O3	0.0040 (18)	0.0045 (19)	0.020 (2)	0.0006 (15)	0.0012 (16)	−0.0032 (16)
O4	0.0077 (19)	0.0067 (19)	0.015 (2)	0.0038 (15)	0.0049 (15)	0.0035 (16)
O5	0.016 (2)	0.035 (3)	0.007 (2)	−0.009 (2)	0.0016 (17)	−0.0009 (18)
O6	0.0065 (19)	0.008 (2)	0.011 (2)	−0.0061 (16)	0.0024 (15)	−0.0007 (16)
O7	0.0074 (19)	0.0047 (19)	0.020 (2)	−0.0012 (16)	0.0018 (15)	0.0030 (17)

Geometric parameters (Å, º)

Cs—O7	2.948 (4)	Cr—O1v	1.978 (4)
Cs—O3i	3.030 (4)	Cr—O3	1.982 (4)
Cs—O6	3.113 (4)	Cr—O2vi	2.007 (4)
Cs—O6ii	3.152 (4)	Cr—O6vii	2.010 (4)
Cs—O2i	3.155 (4)	As1—O1	1.651 (4)
Cs—O7iii	3.196 (4)	As1—O2	1.664 (4)
Cs—O1ii	3.237 (5)	As1—O3	1.681 (4)
Cs—O2iv	3.253 (4)	As1—O4	1.763 (4)
Cs—O4ii	3.314 (4)	As2—O5	1.641 (4)
Cs—O5iii	3.393 (5)	As2—O6	1.661 (4)
Cr—O5iii	1.944 (4)	As2—O7	1.669 (4)
Cr—O7	1.954 (4)	As2—O4	1.750 (4)
O1—As1—O2	116.2 (2)	O6ii—Cs—O5iii	91.60 (11)
O1—As1—O3	115.7 (2)	O2i—Cs—O5iii	80.89 (11)
O2—As1—O3	108.3 (2)	O7iii—Cs—O5iii	50.19 (10)
O1—As1—O4	103.8 (2)	O1ii—Cs—O5iii	146.54 (11)
O2—As1—O4	105.1 (2)	O2iv—Cs—O5iii	126.15 (10)
O3—As1—O4	106.8 (2)	O4ii—Cs—O5iii	136.61 (10)
O5—As2—O6	115.3 (2)	O5iii—Cr—O7	91.2 (2)
O5—As2—O7	115.6 (2)	O5iii—Cr—O1v	172.37 (19)
O6—As2—O7	105.5 (2)	O7—Cr—O1v	89.22 (18)
O5—As2—O4	107.1 (2)	O5iii—Cr—O3	92.97 (19)
O6—As2—O4	105.98 (19)	O7—Cr—O3	90.21 (17)
O7—As2—O4	106.69 (19)	O1v—Cr—O3	94.65 (19)
O7—Cs—O3i	152.96 (12)	O5iii—Cr—O2vi	89.8 (2)
O7—Cs—O6	51.76 (11)	O7—Cr—O2vi	173.72 (17)
O3i—Cs—O6	127.61 (10)	O1v—Cr—O2vi	88.99 (19)
O7—Cs—O6ii	100.13 (11)	O3—Cr—O2vi	95.94 (17)
O3i—Cs—O6ii	106.10 (11)	O5iii—Cr—O6vii	86.15 (18)
O6—Cs—O6ii	78.42 (11)	O7—Cr—O6vii	82.96 (18)
O7—Cs—O2i	124.64 (11)	O1v—Cr—O6vii	86.33 (18)
O3i—Cs—O2i	51.93 (11)	O3—Cr—O6vii	173.08 (17)
O6—Cs—O2i	172.88 (11)	O2vi—Cr—O6vii	90.92 (17)
O6ii—Cs—O2i	108.67 (11)	As1—O1—Criii	154.6 (3)
O7—Cs—O7iii	89.34 (11)	As1—O1—Csii	100.29 (19)
O3i—Cs—O7iii	112.83 (12)	Criii—O1—Csii	95.14 (16)
O6—Cs—O7iii	108.42 (11)	As1—O2—Crviii	139.0 (2)
O6ii—Cs—O7iii	48.86 (11)	As1—O2—Csix	96.55 (17)
O2i—Cs—O7iii	76.75 (11)	Crviii—O2—Csix	117.92 (17)
O7—Cs—O1ii	102.27 (11)	As1—O2—Csx	109.71 (19)
O3i—Cs—O1ii	80.54 (11)	Crviii—O2—Csx	94.09 (15)
O6—Cs—O1ii	50.85 (11)	Csix—O2—Csx	87.81 (10)
O6ii—Cs—O1ii	71.11 (11)	As1—O3—Cr	126.1 (2)
O2i—Cs—O1ii	131.26 (11)	As1—O3—Csix	100.80 (17)
O7iii—Cs—O1ii	119.98 (11)	Cr—O3—Csix	128.28 (18)
O7—Cs—O2iv	78.78 (11)	As2—O4—As1	118.7 (2)
O3i—Cs—O2iv	82.70 (11)	As2—O4—Csii	97.91 (16)
O6—Cs—O2iv	53.39 (11)	As1—O4—Csii	95.06 (16)
O6ii—Cs—O2iv	119.42 (10)	As2—O5—Crv	162.6 (3)
O2i—Cs—O2iv	121.35 (13)	As2—O5—Csv	85.37 (19)
O7iii—Cs—O2iv	161.82 (10)	Crv—O5—Csv	99.46 (18)
O1ii—Cs—O2iv	50.97 (11)	As2—O6—Crxi	138.4 (2)
O7—Cs—O4ii	140.21 (11)	As2—O6—Cs	98.02 (17)
O3i—Cs—O4ii	60.20 (10)	Crxi—O6—Cs	98.31 (15)
O6—Cs—O4ii	92.45 (11)	As2—O6—Csii	106.32 (18)
O6ii—Cs—O4ii	49.76 (10)	Crxi—O6—Csii	107.45 (16)
O2i—Cs—O4ii	92.76 (11)	Cs—O6—Csii	101.58 (11)
O7iii—Cs—O4ii	86.49 (10)	As2—O7—Cr	134.3 (2)
O1ii—Cs—O4ii	48.42 (10)	As2—O7—Cs	104.25 (18)
O2iv—Cs—O4ii	93.83 (10)	Cr—O7—Cs	115.48 (17)
O7—Cs—O5iii	51.51 (11)	As2—O7—Csv	91.67 (16)
O3i—Cs—O5iii	132.61 (11)	Cr—O7—Csv	107.45 (17)
O6—Cs—O5iii	98.57 (11)	Cs—O7—Csv	92.57 (11)

Symmetry codes: (i) x−1, −y+1/2, z−1/2; (ii) −x+1, −y, −z+1; (iii) x, −y+1/2, z−1/2; (iv) x−1, y, z; (v) x, −y+1/2, z+1/2; (vi) −x+2, y+1/2, −z+3/2; (vii) −x+1, y+1/2, −z+3/2; (viii) −x+2, y−1/2, −z+3/2; (ix) x+1, −y+1/2, z+1/2; (x) x+1, y, z; (xi) −x+1, y−1/2, −z+3/2.