New mixed aluminium–chromium diarsenate

The title compound 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.


Chemical context
In recent years, inorganic metal phosphates/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 the corresponding diphosphates have been studied extensively, in contrast to the scarcely studied chromium diarsenates. The title structure is isostructural with the A I Cr III As 2 O 7 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 1/4 Al 3/4 As 2 O 7 is reported. It is one of a series of new potassium chromiumaluminum diarsenate compounds recently isolated by our group.

Structural commentary
The structure of KCr 1/4 Al 3/4 As 2 O 7 can be described as a threedimensional framework of [(Cr/Al)As 2 O 7 ] À anions built up from corner-sharing (Cr/Al)O 6 octahedra and As 2 O 7 groups. The (Cr/Al)O 6 octahedron shares its six corners with five diarsenate groups while the As 2 O 7 anion shares all of its six corners with five octahedra; the interconnection between the polyhedra results in centrosymmetric (Cr/Al)As 2 O 11 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).
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 ISSN 2056-9890 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 AsO 4 polyhedra connected via the bridging O4 atom into a diarsenate As 2 O 7 anion. Like in the related triclinic 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 (  (12), respectively, reflecting a slight distortion of each AsO 4 tetrahedron.

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.

Figure 2
Projection of the KCr 1/4 Al 3/4 As 2 O 7 structure showing the channels parallel to [001] in which the K + cations are located.

Figure 3
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 I M III X 2 O 7 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 KAlAs 2 O 7 (Boughzala & Jouini, 1995). 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 (

Database survey
The structure of KAlP 2 O 7 (Ng & Calvo, 1973) was the first to be published for the A I M III X 2 O 7 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 X 2 O 7 groups. In addition, 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 the diphosphates are generally monoclinic. The isotypic A I CrP 2 O 7 phases crystallize in space group P2 1 /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 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). CsCrAs 2 O 7 (Bouhassine & Boughzala, 2015) is the first phase of the A I CrAs 2 O 7 family to crystallize in the P2 1 /c space group.

Synthesis and crystallization
The crystals of the title compound were obtained from heating a mixture of KNO 3 , Cr 2 O 3 and NH 4 H 2 AsO 4 , 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 View parallel to the As1-As2 direction, emphasizing the nearly eclipsed conformation of the diarsenate anion.

Figure 4
The environment of the diarsenate group As 2 O 7 in the title structure. 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.

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(F 2 ) = 0.053. Trying to improve the convergence factor, the occupation rate of the 2i site was refined, leading to R(F 2 ) = 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 1/4 Al 3/4 As 2 O 7. The presence of both aluminium and chromium in the structure was confirmed by TEM as shown in   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 (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 1/4 Al 3/4 As 2 O 7
Crystal data 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 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 > 2sigma(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.