The γ-polymorph of AgZnPO4 with an ABW zeolite-type framework topology

The γ-polymorph of the title compound, silver zinc orthophosphate, was synthesized under hydrothermal conditions. The structure consists of ZnO4, PO4 and AgO4 units. The coordination spheres of ZnII and PV are tetrahedral, whereas the AgI atom is considerably distorted from a tetrahedral coordination. Each O atom is linked to each of the three cations. An elliptic eight-membered ring system is formed by corner-sharing of alternating PO4 and ZnO4 tetrahedra, leading to a framework with an ABW-type zeolite structure. The framework encloses channels running parallel to [100] in which the Ag cations are located, with Ag⋯Ag contacts of 3.099 (3) Å. This short distance results from d 10⋯d 10 interactions, which play a substantial role in the crystal packing. The structure of γ-AgZnPO4 is distinct from the two other polymorphs α-AgZnPO4 and β-AgZnPO4, but is isotypic with NaZnPO4-ABW, NaCoPO4-ABW and NH4CoPO4-ABW.

Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: WinGX (Farrugia, 1999).  Elouadi and Elammari (1990) who used the combination of the coordination number and the correlative cationic radii r(A) and r(B) as basic parameters to predict the structural evolution versus the nature of both A and B elements. In fact, the appearance of a structural variety does not depend only on the size and the nature of both cations A and B, but could also be favored by specific parameters such as the Jahn-Teller effect, which mainly characterizes compounds containing Cu(II). In addition, the structural stability is also expected to be both temperature-and pressure-dependent. Therefore, the thermodynamic conditions for the preparation of all phases considered are of prime importance. This is also corroborated by the fact that most of the compounds A I B II PO 4 undergo at least one phase transition (Elammari et al., 1988). For instance, it has been found that the thermal treatment (quenching, sintering, etc.) is a key parameter to foresee the structural variety to be stabilized at room temperature (Moring & Kostiner 1986;Bu et al., 1996). In addition to α-AgZnPO 4 and β-AgZnPO 4 characterized by Hammond et al.(1998), we report here on the crystal structure of a new form of silver zinc phosphate (γ-AgZnPO 4 ) that was hydrothermally synthesized.

Structure Reports Online
The structure of this monophosphate consists of zinc and phosphorus atoms tetrahedrally coordinated to oxygen atoms, whereas the silver atom is surrounded by four O atoms in a considerably distorted coordination, with Ag-O bond lengths between 2.2992 (13) and 2.4975 (14) Å. As shown in Fig. 1, the PO 4 and ZnO 4 tetrahedra share a vertex and are almost regular with P-O and Zn-O distances in the range 1.5283 (14)-1.5415 (13) Å and 1.9372 (13)-1.9516 (13) Å, respectively (Table 1). The expected +I, +II and +V oxidation states of the Ag, Zn and P atoms were confirmed by bond valence sum calculations (Brown & Altermatt, 1985) with 0.94, 2.09 and 4.93 valence units, respectively.
A three-dimensional polyhedral view of the crystal structure is represented in Fig. 2. It shows PO 4 tetrahedra linked to ZnO 4 tetrahedra by sharing corners in the way to build an eight-membered ring system surrounding the silver atoms. This arrangements give rise to eight-membered elliptical channels running parallel to [100] where the Ag I atoms are located with short Ag···Ag contacts of 3.099 (3) Å. This short distance is due to d 10 ···d 10 interactions (Jansen, 1987) that play an important role in the crystal structure.
It is particularly interesting to compare the crystal structures of the three different polymorphs of AgZnPO 4 : The hightemperature β-AgZnPO 4 polymorph adopts a monoclinic beryllonite-type structure similar to that of NaZnPO 4 (Elammari et al., 1987) whereas the low-temperature α-AgZnPO 4 polymorph crystallizes with a hexagonal structure like that of highp/low-T KZnPO 4 . In both α-and β-polymorphs, corner-sharing PO 4 and ZnO 4 tetrahedra form a fully ordered framework containing six-membered rings with distinct topologies around the Ag I atoms (Hammond et al., 1998). As noted above, in the case of γ-AgZnPO 4 they build up an elliptical eight-membered ring system of alternating PO 4 and ZnO 4 tetrahedra around the Ag I atoms with an ABW zeolite topology UUUUDDDD, where U and D represent tetrahedra pointing up and down, respectively (Baerlocher et al., 2007).

Experimental
The hydrothermal exploration of the Ag 2 O-ZnO-P 2 O 5 system, in order to search for new phases, in particular with alluaudite-like structure, has allowed to isolate a new form of silver zinc orthophosphate. The reaction mixture contained silver nitrate (AgNO 3 ; 0.1699 g), zinc oxide (ZnO; 0.1221 g), 85 % wt phosphoric acid (H 3 PO 4 ; 0,10 ml) and water (12 ml) and was hydrothermally treated 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 white powder and some colorless parallelepipedic crystals corresponding to the title compound.