Na7Al3(As2O7)4

The title compound, heptasodium trialuminium tetrakis(diarsenate), has been isolated as single crystals from a solid-state reaction. Its structure, which is isotypic with that of the Na7Fe3(X 2O7)4 (X = As, P) family of compounds, consists of AlO6 octahedra sharing their vertices with As2O7 groups, forming a three-dimensional [Al3(As2O7)4]∞ framework incorporating channels occupied by the sodium ions. One of the aluminium ions lies on a crystallographic twofold axis. The sodium ions are situated over ten positions (one with site symmetry 2), all but one of which are partially occupied.

The title compound, heptasodium trialuminium tetrakis-(diarsenate), has been isolated as single crystals from a solid-state reaction. Its structure, which is isotypic with that of the Na 7 Fe 3 (X 2 O 7 ) 4 (X = As, P) family of compounds, consists of AlO 6 octahedra sharing their vertices with As 2 O 7 groups, forming a three-dimensional [Al 3 (As 2 O 7 ) 4 ] 1 framework incorporating channels occupied by the sodium ions. One of the aluminium ions lies on a crystallographic twofold axis. The sodium ions are situated over ten positions (one with site symmetry 2), all but one of which are partially occupied.

Noura Fakhar Bourguiba and Ahmed Driss Comment
A new family of compounds Na 7 M 3 (X 2 O 7 ) 4 (M = Fe, Al, Cr, Ga; X = P, As) studied by Masquelier et al. (1994) present interesting ion conductivity properties. In this family line, a structural study was carried out only for Na 7 Fe 3 (As 2 O 7 ) 4 and Na 7 Fe 3 (P 2 O 7 ) 4 compounds by Masquelier et al.(1990Masquelier et al.( , 1991. In addition, another structural study was conducted to isotypes compounds Na 5 Ag 2 Fe 3 (As 2 O 7 ) 4 and Na 2 Ag 5 Fe 3 (P 2 O 7 ) 4 by Quarez et al. (2009). Also, electrical measurements were also made by Quarez et al. (2009) for Ag 7 Fe 3 (X 2 O 7 ) 4 (X = As, P) compounds. However, as for the Na 7 Al 3 (As 2 O 7 ) 4 compound no structural analysis has been reported so far.
The structure of compound Na 7 Al 3 (As 2 O 7 ) 4 shows a three-dimensional anionic framework [Al 3 (As 2 O 7 ) 4 ] ∞ composed of octahedron AlO 6 sharing vertices with As 2 O 7 groups. This framework defines the interstitial spaces in which the Na + ions are located in a partially disordered distribution to ensure the electrical neutrality. The single unit [Al 3 (As 2 O 7 ) 4 ] 7consists of an octahedron Al1O 6 and two octahedra Al2O 6 related to As 2 O 7 group by sharing vertices (Fig. 1). The two aluminium atoms occupy two crystallographically independent sites: the aluminium Al1 is in special position 4e of a symmetry equal to 2; whereas the Al2 is in general position of 8f. In this unit, the connection between the octahedra Al1O 6 and Al2O 6 is provided by the group As1As2O 7 through oxygen atom O14. In this structure, the units [Al 3 (As 2 O 7 ) 4 ] 7are inter-related thanks to composite bridges of types Al-O-As and As-O-Al respectively. These bridges are located between the octahedra AlO 6 and As 2 O 7 diarsenates groups in order to handle two types of parallel layers to the (a, b) plan. According to (010) (Fig. 2), a projection of the structure shows that the anionic framework consists of a succession of layers as follow: A: [Al1(As2As3O 7 ) 2 ] ∞ and B: [(Al2As3As4O 7 ) 2 ] ∞ perpendicular to c alternately. In the layers A, the only components are Octahedra Al1O 6 , whereas other aluminium octahedra Al2O 6 shape layers B.
On the one hand, in the layer A, each octahedron Al1O 6 is connected to two groups As1As2O 7 by sharing both a vertex and two other groups As1As2O 7 such as the Al1O 6 octahedron shares two of its vertices with the same group As 2 O 7 . By rotation around the fold axis, these groups are equivalent (Fig. 3). This mode of connection is found in phosphates: ARu 2 (P 2 O 7 ) 2 (A = Li, Na, Ag) (Fukuoka et al., 2003.), NaMo 2 (P 2 O 7 ) 2 (Hwu et al., 1991), and SrV 2 (P 2 O 7 ) 2 (Lii et al., 1989).
On the other hand, in the layer B, each octahedron Al2O 6 is connected to four As 2 O 7 groups through the formation of composite bridges Al-O-As and the division of two vertices with a fifth group As 2 O 7 (Fig. 3). This layer is formed by  (Boughzala et al., 1993), KAlAs 2 O 7 (Boughzala et al., 1995), KGaAs 2 O 7 (Lin et al., 1996) and TlFe 0.22 Al 0.78 As 2 O 7 (Ouerfelli et al., 2007).
The O14 oxygen atom guarantees the bond between the layers A and B through the bridges Al2-O14-As2.
In this structure, the sodium ions are distributed through ten crystallographic sites in the interstices of the anionic network. The first type of the site corresponding to 8f position is totally occupied by the Na1 cation which is localized in the layer B. As for the nine other sites, they are partially occupied. And these can be arranged into groups of three sets: (Na2A-Na2B-Na2C), (Na3A-Na3B) and (Na4A-Na4B-Na4C-Na4D). Different sites of the same group are too close to be occupied simultaneously. As a result, the total number of Na + ions for each group is less than or equal to one.

Experimental
The crystals related to the Na 7 Al 3 (As 2 O 7 ) 4 phase were obtained from reactants: NaHCO 3 (Prolabo, 27778), Al 2 O 3 (Riedelde Haen, 167305) NH 4 H 2 AsO 4 (prepared in the laboratory, JCPDS-775), taken in molar proportions: Na: A l: As = 10: 1: 9. Finely ground, the mixture was put in a porcelain crucible, placed in an oven and preheated in air at 673 K for 24 h to remove volatiles. After that, the mixture is raised to a synthesis temperature close to the melting 968 K through stages of 100 degrees followed by grinding. The mixture is then left at this temperature for a week to promote germination and growth of crystals. The final residue undergoes a first slow cooling (5 ° / half day, at 800 K) and a second fast (50 ° / h) to room temperature. Transparent crystals of prismatic form, clear outline and sufficient size for measurements of intensities, have been separated from the stream by successive hot water washes.

Refinement
The collection was carried out in the monoclinic system of C2 / c space group. During the final refinement and for electrical neutrality reasons, the occupancy rate of Na + cations were conducted using the SUMP condition authorized by the SHELXL program. The refinement of all variable parameters leads to well defined ellipsoids. The maximum and minimum densities of electrons remaining in the Fourier-difference are respectively situated at 0.86 Å from the As4 site and at 1.35 Å from the NA3B site.

Figure 2
Projection of Na 7 Al 3 (As 2 O 7 ) 4 structure along [010]. The Na sites are numbered. The picture indicates the location of the A and B layers and the π planes at z ≈ 0.15 and 0.35.   Alternate stacking of octahedral AlO 6 layers and As 2 O 7 groups layers parallel to (-101). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.27 e Å −3 Δρ min = −0.98 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00057 (6) 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.