Crystal structure of Na4Co7−xAl0.67x(As1−yPyO4)6 (x = 1.60; y = 0.116)

Tetrasodium hepta(cobalt/aluminium) hexa(arsenate/phosphate) is a new member of the isostructural family of compounds with the general formula A 4 M 7(XO4)6 (A: Na, K; M: Ni, Co; X: P, As). The proposed structural model is based both on a careful investigation of the crystal data, as well as validation tools by means of bond-valence-sum (BVS) and charge-distribution (CHARDI) calculations.


Chemical context
Metal-substituted aluminophosphates and aluminoarsenates form an important group of materials with many interesting properties such as molecular sieves, catalysts, etc. Li et al. (2012) reported the progress in heteroatom-containing aluminophosphate molecular sieves. With regard to their As homologues, one can cite AlAsO 4 -5 and AlAsO 4 -6, two aluminoarsenates with occluded ethylenediamine (Chen et al. 1990). The analogous cobalt compounds, such as ammoniumtemplated cobalt aluminophosphates with zeolite-like structures (Bontchev & Sevov, 1999), possess similar structural properties.
The title compound, Na 4 Co 7Àx Al 0.67x (As 1Ày P y O 4 ) 6 (x = 1.60; y = 0.116), was obtained during the exploration of the Na-Co-P-As-O system by solid-state reaction; as for many aluminophosphates, aluminum was incorporated from the reaction container. The chemical composition and crystal structure were determined by energy-dispersive X-ray spectroscopy (EDX) analysis ( Fig. 1) and single-crystal X-ray diffraction; the proposed structural model is supported by validation tools by means of bond-valence-sum (BVS) calculations and charge-distribution (CHARDI) analysis (Brown, 2002;Adams, 2003, Nespolo, 2015, 2016Eon & Nespolo, 2015). The correlation between the experimental and the validation results is discussed.

Structural commentary
The title compound is a new member of the isostructural compounds family with the general formula A 4 M 7 (XO 4 ) 6 (A: Na, K; M: Ni, Co; X: P, As) (Moring & Kostiner, 1986;Kobashi et al., 1998;Ben Smail et al., 1999;Marzouki et al., 2010Marzouki et al., , 2013. The asymmetric unit of the title compound (I) (Fig. 2) contains seven metallic sites of which four are occupied by Na + cations (occupancies ranging from 0.23 to 0.50) with eight cations per unit cell, two others (denoted M A and M B ) are simultaneously shared by Co 2+ and Al 3+ ions, and one is fully occupied by Co 2+ ions: the same distribution is observed in the homologous arsenate Na 4 Co 7Àx Al 0.67x (AsO 4 ) 6 (x = 1.37) (II) (Marzouki et al., 2010).

Validation of the structural model using BVS and CHARDI
Two validation tools, BVS and CHARDI, are used to support and analyse the proposed structural model. Briefly, for a properly refined structure, the valences V according to the BVS model and charges Q from the CHARDI analysis should agree with the oxidation states of the atoms (Brown, 2002;Adams, 2003, Nespolo, 2015, 2016Eon & Nespolo, 2015).
The M A site, with an octahedral environment by oxygen atoms, is fully occupied by the two cations with overall occupancy Co 0.189 Al 0.811 . This distribution scheme is confirmed by the validation tools, with a better convergence with the CHARDI model (Table 1). If compared to the homologous site in (II) with overall occupancy Co 0.286 Al 0.714 (Marzouki et al., 2010), the average arithmetic distance in (I) (1.91 Å ) is smaller than in (II) (1.96 Å ) due to the higher fraction of the small cation (Al 3+ ) in (I).
For the M B site with a tetrahedral coordination, the Co 2+ / Al 3+ distribution is based on the same observations as in (II), mainly if it is refined as partially occupied by just Co 2+ , the charge neutrality is not achieved, and then a fraction of Al 3+ was introduced in the M B site yielding an overall occupancy distribution of Co 0.605 Al 0.135 & 0.260 , with & expressing the vacancy. The validation results for this particular distribution are: V(M B ) = 1.31 and Q(M B ) = 1.58, the theoretical value is 1.61 (Table 1). Finally, with regard to P and As atoms, the P/As substitutional disorder is observed in one of the two sites (M C ): P/As = 0.35/0.65; V = 5.21 and Q = 5.00.
The final result corresponds to the formula Na 4 Co 5.40 Al 1.07 (As 0.883 P 0.116 O 4 ) 6 . It is the first case in its   Table 1 BVS and CHARDI analysis of cation polyhedra in the title compound (the structure described as being built of cation-centred polyhedra).  (2016)].

Figure 1
The EDX spectrum of the title compound. The inset shows the morphology of one crystal.

Figure 3
The structure of the title compound viewed appoximately along [100], showing the tunnels and the Na + cations.
homologous family which contains such a number of elements. The similarity to (II) (Marzouki et al., 2010) is clear, the cell parameters of (I) are smaller than those of (II) as it contains more small elements than (II). The CHARDI method is extended, as for (II), to analyse the coordination polyhedra by means of the Effective Coordination Numbers (ECoN): the polyhedron distortion is more pronounced if the ECoN deviates more from the classical coordination number (CN). The framework of the title compound is of an open character (Fig. 3). Its aptitude for sodium conduction through the tunnels appears to be possible, as shown in experimental and theoretical studies for the similar compound (II) (Marzouki et al., 2013). These studies will be the subject of future works.

Synthesis and crystallization
A mixture of sodium nitrate, cobalt nitrate hexahydrate, NH 4 H 2 XO 4 (X: P, As) in the molar ratio Na:Co:P:As = 2:1:0.5:1 was dissolved in deionized water and then heated at 373 K to dehydration. After grinding, it was placed in a porcelain boat and first heated at 673 K in air for 24 h and then heated gradually to 1123 K for 1 d. Some pink parallelepipedshaped crystals were isolated from the sample. A qualitative EDX analysis confirmed the presence of Na, Co, Al, As and O ( Fig. 1), with the aluminium diffusing from the reaction container.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The Co and Al atoms occupying the M A and M B sites, as well as the P and As atoms occupying the M C site, were constrained using the EXYZ and EADP instructions of SHELXL97 (Sheldrick, 2008). Three linear free variable restraints (SUMP) were required to restrain the sum of their occupation factors. The Na1 and Na2 cations are at half-occupancy sites and the two others (Na31 and Na32) with isotropic refinement have a total occupancy of 0.50 because, when refined freely, their occupations converged to these values. Computer programs: CAD-4 EXPRESS (Duisenberg, 1992;Macíček & Yordanov, 1992), XCAD4 (Harms & Wocadlo, 1995), SHELXS97 and SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 2006), WinGX (Farrugia, 2012) and publCIF (Westrip, 2010 Crystal structure of Na 4 Co 7−x Al 0.67x (As 1−y P y O 4 ) 6 (x = 1.60; y = 0.116)

Crystal data
Na 4