RbSn2(PO4)3, a NASICON-type phosphate

The title compound, rubidium ditin(IV) tris(phosphate), RbSn2(PO4)3, belongs to the NASICON-type family of phosphates and crystallizes in the space group R . The structure is composed of PO4 tetrahedra (1 symmetry) and two slightly distorted SnO6 octahedra, both with 3. symmetry, which are interlinked through corner-sharing O atoms to form a 3 ∞[Sn2(PO4)3]− framework. The Rb+ cations are located on threefold inversion axes in the voids of this framework and exhibit a coordination number of 12. The crystal studied was twinned by merohedry with a component ratio of 0.503:0.497.

The title compound, rubidium ditin(IV) tris(phosphate), RbSn 2 (PO 4 ) 3 , belongs to the NASICON-type family of phosphates and crystallizes in the space group R3. The structure is composed of PO 4 tetrahedra (1 symmetry) and two slightly distorted SnO 6 octahedra, both with 3. symmetry, which are interlinked through corner-sharing O atoms to form a 3 1 [Sn 2 (PO 4 ) 3 ] À framework. The Rb + cations are located on threefold inversion axes in the voids of this framework and exhibit a coordination number of 12. The crystal studied was twinned by merohedry with a component ratio of 0.503:0.497.

Comment
In recent years, the AM 2 (PO 4 ) 3 (A = alkali metal; M = Ti, Zr, Ge, Sn) family with NASICON (Na 3 Zr 2 Si 2 PO 12 ; Boilot et al., 1987) -type structures attracted a growing interest due to their intriguing properities, e.g. ionic conductivity of the A cations located in the voids of the three-dimensional NASICON-type framework. This framework is composed of isolated PO 4 tetrahedra sharing corners with MO 6 octahedra ( Fig. 1), and is amenable to a wide variety of chemical substitutions at the various crystallographic positions, thus yielding a large number of closely related compounds, such as NaFeNb(PO 4 ) 3 (Zatovskii et al., 2006), Rb 2 Ca 2 (SO 4 ) 3 (Boujelben et al., 2007) or Al 0.5 Nb 1.5 (PO 4 ) 3 (Zhao et al., 2011). In order to augment this family of compounds, we prepared crystals of the compound RbSn 2 (PO 4 ) 3 using a solid state reaction route. Unlike the analogous Ti compound RbTi 2 (PO 4 ) 3 which crystallises in space group R3c (Duhlev, 1994), RbSn 2 (PO 4 ) 3 crystallises in space group R3.
A projection of the crystal structure of RbSn 2 (PO 4 ) 3 is given in Fig. 2. It is characterized by the presence of isolated PO 4 tetrahedra (1 symmetry) and two different SnO 6 octahedra (both 3. symmetry), linked by sharing corner O atoms, to establish a three-dimensional 3 ∞ [Sn 2 (PO 4 ) 3 ]framework. This framwork delimits two types of channels in which the twelve-coordinate Rb + atoms (site symmetry 3.) are located to compensate the negative charges. The PO 4 tetrahedra are quite regular, with P-O distances ranging from 1.523 (4) to 1.537 (4) Å. The two SnO 6 octahedra exhibit Sn-O distances ranging from 2.015 (4) to 2.033 (4) Å.

Experimental
Single crystals of RbSn 2 (PO 4 ) 3 have been prepared by a high-temperature method in air. A powder mixture of RbNO 3 , SnO 2 and NH 4 H 2 PO 4 in the molar ratio of Rb: Sn: P = 10: 1: 15 was first ground in an agate mortar and then transferred to a platinum crucible. The sample was gradually heated in air at 1173 K for 24 h. After that, the intermediate product was slowly cooled to 673 K at the rate of 2 K h -1 . It was kept at 673 K for another 10 h and then quenched to room temperature.
The obtained crystals were colorless with a prismatic shape.

Refinement
The RbSn 2 (PO 4 ) 3 crystal studies was twinned by merohedry. For refinement the twin law (0 1 0 1 0 0 0 0 1) was used; the twin component ratio refined to 0.503: 0.497. The highest peak in the difference electron density map is at a distance of