Synthesis and crystal structure of Na4Ni7(AsO4)6

The structure of Na4Ni7(AsO4)6 is made of layers of Ni octahedra and As tetrahedra assembled in sheets parallel to the bc plane. These layers are interconnected by corner-sharing between Ni octahedra and As tetrahedra. This linkage creates tunnels running along the c axis in which the Na atoms are located.

The title compound, tetrasodium heptanickel hexaarsenate, was obtained by ceramic synthesis and crystallizes in the monoclinic space group C2/m. The asymmetric unit contains seven Ni atoms of which two have site symmetry 2/m and three site symmetry 2, four As atoms of which two have site symmetry m and two site symmetry 2, three Na atoms of which two have site symmetry 2, and fifteen O atoms of which four have site symmetry m. The structure of Na 4 Ni 7 (AsO 4 ) 6 is made of layers of Ni octahedra and As tetrahedra assembled in sheets parallel to the bc plane. These layers are interconnected by cornersharing between NiO 6 octahedra and AsO 4 tetrahedra. This linkage creates tunnels running along the c axis in which the Na atoms are located. This arrangement is similar to the one observed in Na 4 Ni 7 (PO 4 ) 6 , but the layers of the two compounds are slightly different because of the disorder of one of the Ni sites in the structure of the title compound.

Chemical context
Although the structures of transition metal phosphates have been widely investigated during the last decades, very little work has been done on comparable arsenates due to the toxicity of arsenic. The latter phases can exhibit, however, peculiar properties. BaCo 2 (AsO 4 ) 2 is a good example of a quasi-2D system with a magnetically frustrated honeycomb lattice (Regnault et al., 1977). BaCoAs 2 O 7 appears as the first example of a magnetization step promoted by a structural modulation (David et al., 2013a). LiCoAsO 4 shows reversible electrochemical activity at high potential (Satya Kishore & Varadaraju, 2006). Moreover, a recent study reveals the interest of arsenate groups in playing the role of efficient disconnecting units in the magnetic compound BaCo 2 (As 3 O 6 ) 2 ÁH 2 O, being the first pure inorganic compound with slow spin dynamics (David et al., 2013b). From the crystal chemistry point of view, substitution of phosphate by arsenate gives the possibility of stabilizing new phases. For example, NaNiPO 4 crystallizes with the maricite structure (Senthilkumar et al., 2014), whereas NaNiAsO 4 has a honeycomb layer structure (Range & Meister, 1984). In this study, we describe the structure of Na 4 Ni 7 (AsO 4 ) 6 and compare it with its phosphate analogue.

Structural commentary
The structure of the title compound Na 4 Ni 7 (AsO 4 ) 6 is quite similar to the one of the phosphate homologue Na 4 Ni 7 (PO 4 ) 6 . Both are made of interconnected Ni 7 (XO 4 ) 6 layers with tunnels in between where the Na atoms are located, as shown in Fig. 1a. The arrangement of NiO 6 and XO 4 in the layer is, however, slightly different, as evidenced in Fig. 2. As described ISSN 2056-9890 by Moring & Kostiner (1986), Na 4 Ni 7 (PO 4 ) 6 layers are made of parallel ribbons (called ribbon 1) containing Ni1, Ni2, P3 and P4 polyhedra. These ribbons 1 are interconnected by another kind of ribbon (called ribbon 2) made of dimers consisting of edge-sharing NiO 6 octahedra (Ni3 and Ni4). The latter are linked to PO 4 tetrahedra (P1 and P2) by edge-and corner-sharing. The difference between the two compounds is associated with the possibility of the Ni2 atom in Description of the layers (top) and the ribbon 2 (bottom) of (a) Na 4 Ni 7 (AsO 4 ) 6 and (b) Na 4 Ni 7 (PO 4 ) 6 . The dotted lines show the cell edges.

Figure 1
Description of the crystal structure of Na 4 Ni 7 (AsO 4 ) 6 with (a) a view of the stacking and (b) a view of the Na layers. The dotted lines show the cell edges. Displacement ellipsoids are drawn at the 50% probability level. Na 4 Ni 7 (PO 4 ) 6 occupying two octahedral sites. The first site, belonging to ribbon 1, is equivalent to the Ni2a site in Na 4 Ni 7 (AsO 4 ) 6 . The other, equivalent to the Ni2b and Ni5 sites in Na 4 Ni 7 (AsO 4 ) 6 , belongs to ribbon 2, forming pentamers of edge-sharing NiO 6 octahedra. The layers of the title compound Na 4 Ni 7 (AsO 4 ) 6 can thus be described with three kinds of ribbons, as shown in Fig. 2. The linkage between the layers is done by corner-sharing between NiO 6 and AsO 4 units of two consecutive ribbons 2 along the stacking axis ( Fig. 1a). This linkage is identical to the one of the phosphorus homologue. However, since in Na 4 Ni 7 (AsO 4 ) 6 layers are made of three different kinds of ribbons, two adjacent layers are shifted to align ribbon 1 with ribbon 1 0 . That is why in Na 4 Ni 7 (AsO 4 ) 6 the stacking axis is roughly doubled compared to Na 4 Ni 7 (PO 4 ) 6 [c = 6.398 (2) Å versus a = 14.5383 (11) Å in the title structure]. It implies two different kinds of Na layers, as shown in Fig. 1b.

Synthesis and crystallization
Sodium carbonate (>99.5%), arsenic oxide (99%) and nickel sulfate hexahydrate (>99.9%) were purchased from Sigma-Aldrich. They were used as received without further purification. Reagents were ground together in stoichiometric ratio in an agate mortar. The obtained mixture was pelletized, placed in an alumina boat and annealed at 573 K for 1h. The obtained mixture was reground, pelletized and heated at 1073 K (5 K min À1 ) for 48 h, after which the alumina boat was removed from the furnace and cooled to room temperature. The brown crystals of the title compound were isolated by hand.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 1. The (001) reflection, affected by the beamstop, has been removed from the refinement. Another reflection (201), flagged as potentially affected by the beamstop, was in fact not and was kept in the refinement. After positioning and refining all the atom positions except Ni2b, the difference Fourier map revealed residual density ('8 e Å À3 ) near Ni2a (at '0.6 Å ). It was refined introducing a second position Ni2b with complementary occupation. The occupancy ratio was refined to 0.80 (4):0.20 (4) for the Ni2a/ Ni2b site, constraining the sum to be equal to 1.   (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).