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].


Chemical context
In recent years, inorganic metal phosphates and arsenates with formula A I M III X 2 O 7 (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 2 O 7 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 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 À y, 1 2 + z; (ii) 2 À x, 1 2 + y, 3 2 À z; (iii) x, 1 2 À y, À 1 2 + z; (iv) 1 À x, 1 2 + y, 3 2 À z.] CsCrAs 2 O 7 is reported, one of a series of new cesium chromium(III) arsenate compounds recently isolated by our group.

Structural commentary
The structure of CsCrAs 2 O 7 can be described as a threedimensional [CrAs 2 O 7 ] À anionic framework ( Fig. 1) with channels extending parallel to [001] that are occupied by tencoordinate Cs + cations (Fig. 2). The two independent arsenic atoms form AsO 4 tetrahedra and are connected via the bridging O4 atom into a diarsenate As 2 O 7 anion. Like in the related structures of KAlAs 2 O 7 (Boughzala & Jouini, 1995) and RbAlAs 2 O 7 (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 CsCrP 2 O 7 (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 AsO 4 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) (17) . Each CrO 6 octahedron shares its corners with five As 2 O 7 anions, one of which is chelating and the others belonging to four different As 2 O 7 groups (Fig. 3). On the other hand, each As 2 O 7 anion is surrounded by five CrO 6 octahedra as depicted in Fig. 4. The environment of the ten-coordinate Cs + cation situated in the cavities of the resulting [CrAs 2 O 7 ] À framework is shown in Fig. 5.

Figure 2
Projection of the CsCrAs 2 O 7 structure showing the channels parallel to [001] in which the Cs + cations are located.

Figure 3
The environment of the CrO 6 octahedron in the structure of CsCrAs 2 O 7 .

Figure 4
The environment of the diarsenate group in the structure of CsCrAs 2 O 7 .
structure. In the title structure, belonging to the type I family of A I M III X 2 O 7 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 KAlAs 2 O 7 (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.

Database survey
The structure of KAlP 2 O 7 (Ng & Calvo, 1973) was the first published of the A I M III X 2 O 7 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 2 O 7 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 I AlAs 2 O 7 (A I = K, Rb, Tl, Cs) (Boughzala & Jouini, 1992;Boughzala et al., 1993;, whereas diphosphates are generally monoclinic. The isotypic A I CrP 2 O 7 phases crystallize in space group P2 1 /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 I FeP 2 O 7 phases for A I = Na (Gabelica-Robert et al., 1982) and K (Riou et al., 1988). However, the two Li-containing phases LiMP 2 O 7 show a symmetry reduction to space group P2 1 (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 Cs 2 CO 3 , Cr 2 O 3 and NH 4 H 2 AsO 4 , 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  Computer programs: CAD-4 EXPRESS (Enraf-Nonius, 1994), XCAD4 (Harms & Wocadlo, 1995), SHELXS97 and SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 2008) and publCIF (Westrip, 2010).

Figure 5
The surrounding of the ten-coordinated Cs + cation in the structure of CsCrAs 2 O 7 .

Figure 6
View parallel to the As1-As2 direction, emphasizing the nearly eclipsed conformation of the diarsenate anion.
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. program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.