Disilver(I) trinickel(II) hydrogenphosphate bis(phosphate), Ag2Ni3(HPO4)(PO4)2

The title compound, Ag2Ni3(HPO4)(PO4)2, has been synthesized by the hydrothermal method. Its structure is formed by two types of chains running along the b axis. The first chain results from a linear and continuous succession of NiO6 octahedra linked to PO4 tetrahedra by a common vertex. The second chain is built up from two adjacent edge-sharing octahedra (dimers) whose ends are linked to two PO4 tetrahedra by a common edge. Those two types of chains are linked together by the phosphate groups to form polyhedral sheets parallel to the (001) plane. The three-dimensional framework delimits two types of hexagonal tunnels parallel to the a-axis direction, at (x, 1/2, 0) and (x, 0, 1/2), where the Ag atoms are located. Each silver cation is surrounded by eight O atoms. The same Ag+ coordination is found in other phosphates with the alluaudite structure, for example, AgMn3(PO4)(HPO4)2. Moreover, O—H⋯O hydrogen bonds link three PO4 tetrahedra so as to build a three-dimensional network.

The title compound, Ag 2 Ni 3 (HPO 4 )(PO 4 ) 2 , has been synthesized by the hydrothermal method. Its structure is formed by two types of chains running along the b axis. The first chain results from a linear and continuous succession of NiO 6 octahedra linked to PO 4 tetrahedra by a common vertex. The second chain is built up from two adjacent edge-sharing octahedra (dimers) whose ends are linked to two PO 4 tetrahedra by a common edge. Those two types of chains are linked together by the phosphate groups to form polyhedral sheets parallel to the (001) plane. The threedimensional framework delimits two types of hexagonal tunnels parallel to the a-axis direction, at (x, 1/2, 0) and (x, 0, 1/2), where the Ag atoms are located. Each silver cation is surrounded by eight O atoms. The same Ag + coordination is found in other phosphates with the alluaudite structure, for example, AgMn 3 (PO 4 )(HPO 4 ) 2 . Moreover, O-HÁ Á ÁO hydrogen bonds link three PO 4 tetrahedra so as to build a three-dimensional network.   The structure of this compound is formed by two types of chains running along the b axis. The first chain (Ni2P2HO 9 )∞ is built up from Ni2 and P2 atoms in special Wyckoff position 4 b (m) of the space group Ima2. This chain results from linear and continuous succession of octahedron (Ni2O 6 ) and P2O 3 OH tetrahedron which share a vertex. The second chain (Ni 2 P 2 O 14 )n is built up from two adjacent edge sharing octahedra ((Ni1) 2 O 10 dimmers) whose ends are linked to two P1O 4 tetrahedra by a common edge (Fig.1). Those chains are linked together by the phosphate groups to form polyhedral sheets parallel to the (0 0 1) plane as shown in Fig.2.

Crystal data
The three dimensional framework delimits two types of hexagonal tunnels running along the a direction, at x 1/2 0 and x 0 1/2 (Fig.3). The Ag2 atom is located at centre of tunnels, this explains the high value of its anisotropic displacement U 11 , whereas Ag1 is slightly shifted from this center (Wyckoff positions: Ag2 at 2a: 0, 0, z and Ag1 at 2 b: 1/4, y, z). However, each Ag + ion is surrounded by 8 O atoms with different Ag-O distances. Indeed, the first coordination environment of Ag2 + is almost square planar with four short Ag2-O distances between 2.373 (4) and 2.421 (4) Å and the other four larger distances are in the range of 2.869 (4) to 3.133 (4) Å. A similar coordination surrounding Ag1 + is observed with Ag1-O bond lengths in the range of 2.537 (4)-2.616 (4) Å and the longest bonds are situated between 2.661 (4) and 2.963 (4) Å. The same coordination for this cation is found in other phosphate with alluaudite structure like AgMn 3 (PO 4 )(HPO 4 ) 2 (Leroux et al. (1995)) and AgNi 3 (PO 4 )(HPO 4 ) 2 (Ben Smail & Jouini (2002)).

Experimental
By means of hydrothermal synthesis, we have isolate a new silver nickel phosphate from the reaction mixture of silver nitrate (AgNO 3 ; 0.1699 g), metallic nickel (Ni; 0.0881 g), 85%wt phosphoric acid (H 3 PO 4 ; 0,10 ml) and water (12 ml). The hydrothermal treatment was conducted in a 23 ml Teflon-lined autoclave under autogeneous pressure at 468 K for two days.
After being filtered off, washed with deionized water and air dried, the reaction product consists of a monophasic green powder and some green parallelepipedic crystals corresponding to the title compound.

Refinement
The structure is solved by direct method technique and refined by full-matrix least-squares using SHELXS97 and SHELXL97 program packages. The structure refinement in the centrosymmetric space group was unsuccessful. Infact the crystal is a racemic twinned with a refined ratio of 0.479 (26), which explains the ambiguity in the Flack parameter. The space group is not centro symmetric and the polar axis restraint is generated automatically by SHELXL program. Friedel opposites reflections are not merged. The O-bound H atom is initially located unambiguously in a difference map and refined with O-H distance restraints of 0.86 (1). In a the last cycle ther is refined in the riding model approximation with U iso (H) set to 1.2U eq (O). The highest and deepest hole residual peak in the final difference Fourier map are located at 0.72 Å and 0.62 Å, from Ag1. Fig. 1. Partial plot of Ag 2 Ni 3 (HPO 4 )(PO 4 ) 2 crystal structure. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: (i) -x, -y + 1, z; (ii) x + 1/2, -y + 1, z; (iii) x, -y + 3/2, z -1/2; (iv) -x + 1/2, -y + 3/2, z -1/2; (v) -x + 1/2, -y + 1/2, z -1/2; (vi) -x, y + 1/2, z -1/2; (vii) x + 1/2, y + 1/2, z -1/2; (viii) x, -y + 1/2, z -1/2; (ix) -x, -y, z; (x) -x, y + 1/2, z + 1/2; (xi) x, -y + 1/2, z + 1/2; (xii) -x + 1/2, y, z.   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 Rfactors(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.