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].

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](As-O) = 0.004 Å
• R factor = 0.027
• wR factor = 0.075
• Data-to-parameter ratio = 14.2

checkCIF/PLATON results

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Alert level C
PLAT213_ALERT_2_C Atom O6              has ADP max/min Ratio .....        3.7 oblate
PLAT906_ALERT_3_C Large K value in the Analysis of Variance ......      2.686 Check
PLAT934_ALERT_3_C Number of (Iobs-Icalc)/SigmaW > 10 Outliers ....          1 Check
PLAT975_ALERT_2_C Check Calcd Residual Density  1.06A From      O7       0.63 eA-3
PLAT975_ALERT_2_C Check Calcd Residual Density  0.44A From      O4       0.59 eA-3

Alert level G
PLAT004_ALERT_5_G Polymeric Structure Found with Maximum Dimension          3 Info
PLAT042_ALERT_1_G Calc. and Reported MoietyFormula Strings  Differ     Please Check
PLAT199_ALERT_1_G Reported _cell_measurement_temperature ..... (K)        293 Check
PLAT200_ALERT_1_G Reported   _diffrn_ambient_temperature ..... (K)        293 Check
PLAT794_ALERT_5_G Tentative Bond Valency for Cr      (IX)    .....       3.45 Note
PLAT899_ALERT_4_G SHELXL97   is Deprecated and Succeeded by SHELXL       2014 Note
PLAT955_ALERT_1_G Reported (CIF) and Actual (FCF) Lmax Differ by .          1 Units

   0 ALERT level A = Most likely a serious problem - resolve or explain
   0 ALERT level B = A potentially serious problem, consider carefully
   5 ALERT level C = Check. Ensure it is not caused by an omission or oversight
   7 ALERT level G = General information/check it is not something unexpected

   4 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   3 ALERT type 2 Indicator that the structure model may be wrong or deficient
   2 ALERT type 3 Indicator that the structure quality may be low
   1 ALERT type 4 Improvement, methodology, query or suggestion
   2 ALERT type 5 Informative message, check

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S1. Chemical context

In recent years, inorganic metal phosphates and 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 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.

S2. 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).

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.

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 P1 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^IM^IIIX₂O₇ 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).

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.

S3. Database survey

The structure of KAlP₂O₇ (Ng & Calvo, 1973) was the first published of the A^IM^IIIX₂O₇ 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 X₂O₇ groups. Also, the crystal symmetry can be affected. The structures are triclinic, in space group P1 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).

S4. 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.

S5. 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.

Related literature

For related literature, see: Bohaty et al. (1982); Boughzala & Jouini (1992, 1995); Boughzala et al. (1993); Brown (2002); Calvo (1973); Gabelica-Robert, Goreaud, Labbe & Raveau (1982); Gentil et al. (1997); Ivashkevich et al. (2007); Linde & Gorbunova (1982); Riou et al. (1988, 1990); Zhao & Li (2011).

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).

Figures

	Fig. 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, 1/2 - y, 1/2 + z; (ii) 2 - x, 1/2 + y, 3/2 - z; (iii) x, 1/2 - y, -1/2 + z; (iv) 1 - x, 1/2 + y, 3/2 - z.]
	Fig. 2. Projection of the CsCrAs2O7 structure showing the channels parallel to [001] in which the Cs^+ cations are located.
	Fig. 3. The environment of the CrO6 octahedron in the structure of CsCrAs2O7.
	Fig. 4. The environment of the diarsenate group in the structure of CsCrAs2O7.
	Fig. 5. The surrounding of the ten-coordinated Cs^+ cation in the structure of CsCrAs2O7.
	Fig. 6. View parallel to the As1—As2 direction, emphasizing the nearly eclipsed conformation of the diarsenate anion.

Caesium chromium (III) diarsenate

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) Å^3	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 F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.027	w = 1/[σ^2(Fo^2) + (0.043P)^2 + 1.1495P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 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+0.001xFc^2λ^3/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)

	U^11	U^22	U^33	U^12	U^13	U^23
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—O1^v	1.978 (4)
Cs—O3^i	3.030 (4)	Cr—O3	1.982 (4)
Cs—O6	3.113 (4)	Cr—O2^vi	2.007 (4)
Cs—O6^ii	3.152 (4)	Cr—O6^vii	2.010 (4)
Cs—O2^i	3.155 (4)	As1—O1	1.651 (4)
Cs—O7^iii	3.196 (4)	As1—O2	1.664 (4)
Cs—O1^ii	3.237 (5)	As1—O3	1.681 (4)
Cs—O2^iv	3.253 (4)	As1—O4	1.763 (4)
Cs—O4^ii	3.314 (4)	As2—O5	1.641 (4)
Cs—O5^iii	3.393 (5)	As2—O6	1.661 (4)
Cr—O5^iii	1.944 (4)	As2—O7	1.669 (4)
Cr—O7	1.954 (4)	As2—O4	1.750 (4)
O1—As1—O2	116.2 (2)	O6^ii—Cs—O5^iii	91.60 (11)
O1—As1—O3	115.7 (2)	O2^i—Cs—O5^iii	80.89 (11)
O2—As1—O3	108.3 (2)	O7^iii—Cs—O5^iii	50.19 (10)
O1—As1—O4	103.8 (2)	O1^ii—Cs—O5^iii	146.54 (11)
O2—As1—O4	105.1 (2)	O2^iv—Cs—O5^iii	126.15 (10)
O3—As1—O4	106.8 (2)	O4^ii—Cs—O5^iii	136.61 (10)
O5—As2—O6	115.3 (2)	O5^iii—Cr—O7	91.2 (2)
O5—As2—O7	115.6 (2)	O5^iii—Cr—O1^v	172.37 (19)
O6—As2—O7	105.5 (2)	O7—Cr—O1^v	89.22 (18)
O5—As2—O4	107.1 (2)	O5^iii—Cr—O3	92.97 (19)
O6—As2—O4	105.98 (19)	O7—Cr—O3	90.21 (17)
O7—As2—O4	106.69 (19)	O1^v—Cr—O3	94.65 (19)
O7—Cs—O3^i	152.96 (12)	O5^iii—Cr—O2^vi	89.8 (2)
O7—Cs—O6	51.76 (11)	O7—Cr—O2^vi	173.72 (17)
O3^i—Cs—O6	127.61 (10)	O1^v—Cr—O2^vi	88.99 (19)
O7—Cs—O6^ii	100.13 (11)	O3—Cr—O2^vi	95.94 (17)
O3^i—Cs—O6^ii	106.10 (11)	O5^iii—Cr—O6^vii	86.15 (18)
O6—Cs—O6^ii	78.42 (11)	O7—Cr—O6^vii	82.96 (18)
O7—Cs—O2^i	124.64 (11)	O1^v—Cr—O6^vii	86.33 (18)
O3^i—Cs—O2^i	51.93 (11)	O3—Cr—O6^vii	173.08 (17)
O6—Cs—O2^i	172.88 (11)	O2^vi—Cr—O6^vii	90.92 (17)
O6^ii—Cs—O2^i	108.67 (11)	As1—O1—Cr^iii	154.6 (3)
O7—Cs—O7^iii	89.34 (11)	As1—O1—Cs^ii	100.29 (19)
O3^i—Cs—O7^iii	112.83 (12)	Cr^iii—O1—Cs^ii	95.14 (16)
O6—Cs—O7^iii	108.42 (11)	As1—O2—Cr^viii	139.0 (2)
O6^ii—Cs—O7^iii	48.86 (11)	As1—O2—Cs^ix	96.55 (17)
O2^i—Cs—O7^iii	76.75 (11)	Cr^viii—O2—Cs^ix	117.92 (17)
O7—Cs—O1^ii	102.27 (11)	As1—O2—Cs^x	109.71 (19)
O3^i—Cs—O1^ii	80.54 (11)	Cr^viii—O2—Cs^x	94.09 (15)
O6—Cs—O1^ii	50.85 (11)	Cs^ix—O2—Cs^x	87.81 (10)
O6^ii—Cs—O1^ii	71.11 (11)	As1—O3—Cr	126.1 (2)
O2^i—Cs—O1^ii	131.26 (11)	As1—O3—Cs^ix	100.80 (17)
O7^iii—Cs—O1^ii	119.98 (11)	Cr—O3—Cs^ix	128.28 (18)
O7—Cs—O2^iv	78.78 (11)	As2—O4—As1	118.7 (2)
O3^i—Cs—O2^iv	82.70 (11)	As2—O4—Cs^ii	97.91 (16)
O6—Cs—O2^iv	53.39 (11)	As1—O4—Cs^ii	95.06 (16)
O6^ii—Cs—O2^iv	119.42 (10)	As2—O5—Cr^v	162.6 (3)
O2^i—Cs—O2^iv	121.35 (13)	As2—O5—Cs^v	85.37 (19)
O7^iii—Cs—O2^iv	161.82 (10)	Cr^v—O5—Cs^v	99.46 (18)
O1^ii—Cs—O2^iv	50.97 (11)	As2—O6—Cr^xi	138.4 (2)
O7—Cs—O4^ii	140.21 (11)	As2—O6—Cs	98.02 (17)
O3^i—Cs—O4^ii	60.20 (10)	Cr^xi—O6—Cs	98.31 (15)
O6—Cs—O4^ii	92.45 (11)	As2—O6—Cs^ii	106.32 (18)
O6^ii—Cs—O4^ii	49.76 (10)	Cr^xi—O6—Cs^ii	107.45 (16)
O2^i—Cs—O4^ii	92.76 (11)	Cs—O6—Cs^ii	101.58 (11)
O7^iii—Cs—O4^ii	86.49 (10)	As2—O7—Cr	134.3 (2)
O1^ii—Cs—O4^ii	48.42 (10)	As2—O7—Cs	104.25 (18)
O2^iv—Cs—O4^ii	93.83 (10)	Cr—O7—Cs	115.48 (17)
O7—Cs—O5^iii	51.51 (11)	As2—O7—Cs^v	91.67 (16)
O3^i—Cs—O5^iii	132.61 (11)	Cr—O7—Cs^v	107.45 (17)
O6—Cs—O5^iii	98.57 (11)	Cs—O7—Cs^v	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.