RbYb(PO3)4

Rubidium ytterbium(III) tetrakis(polyphosphate), RbYb(PO3)4, was synthesized by solid-state reaction. It adopts structure type IV of the MRE(PO3)4 (M = alkali metal and RE = rare earth metal) family of compounds. The structure is composed of a three-dimensional framework made up from double spiral polyphosphate chains parallel to [10-1] and irregular [YbO8] polyhedra. There are eight PO4 tetrahedra in the repeat unit of the polyphosphate chains. The Rb+ cation is located in channels extending along [100] that are delimited by the three-dimensional framework. It is surrounded by 11 O atoms, defining an irregular polyhedron.


Jing Zhu and Hui Chen Comment
Extensive studies on structures and properties of condensed rare earth phosphates have been carried out in the past owing to their potential application in the optics domain (Miyazawa et al., 1979;Malinowski et al., 1989). Furthermore, their chemical and thermal stability ensures the feasibility of possible applications. Numerous rare-earth phosphates with ytterbium, such as YbP 3 O 9 (Hong, 1974), CsYbP 2 O 7 (Jansen et al., 1991), and K 2 CsYb(PO 4 ) 2 (Rghioui et al., 2002) (Fang et al., 2008)]. The reason for this situation is probably the difficulty in obtaining crystals of high quality. Accordingly, our research group is paying attention to the preparation of new polyphosphates MYb(PO 3 ) 4 .
We successfully grew single crystals of rubidium ytterbium polyphosphate, RbYb(PO 3 ) 4 , the structure of which is reported here.
The crystal structure of RbYb(PO 3 ) 4 is shown in Figs. 1 and 2. It is isostructural with CsEu(PO 3 ) 4 (Zhu et al., 2009) and belongs to type IV of the MRE(PO 3 ) 4 (M = alkali metal and RE = rare earth) family of compounds. The structure can be described as a three-dimensional framework made up from polyphosphate double spiral chains extending parallel to [101] and YbO 8 polyhedra. The Rb + cations are located in infinite tunnels along [100] delimited by the three-dimensional framework. As illustrated in Fig. 3, the P atom is four-coordinated, and four crystallographically distinct PO 4 tetrahedra form the (PO 3 ) ∞double spiral chains by corner-sharing. The P-O bond lengths and O-P-O bond angles show normal values for catena-polyphosphates. There are eight PO 4 tetrahedra in the repeating unit of the double spiral chain. The Yb III cation is eight-coordinated in form of a distorted polyhedron with Y-O bond lengths ranging from 2.253 (5) to 2.412 (5) Å, which are consistent with those reported previously (Fang et al., 2008). The shortest Yb···Yb contact is 6.3540 (8) Å.
The Rb + cation are located in the intersecting channels and are surrounded by eleven O atoms, with Rb-O bond lengths in the range of 2.915 (5)-3.504 (5) Å. Neighboring two RbO 11 polyhedra are connected by corner-sharing.

Experimental
Single crystals of RbYb(PO 3 ) 4 were grown by solid state reactions. All reagents were purchased commercially and used without further purification. The starting materials RbNO 3 , Yb 2 O 3 and NH 4 H 2 PO 4 were weighed in the molar ratio of Rb/Yb/P = 7/1/18 and finely ground in an agate mortar to ensure the best homogeneity and reactivity, and were then placed in a corundum crucible and preheated at 373 K for 6 h. Afterwards, the material was reground and heated to 723 K for 36 h and then cooled to 393 K at a rate of 6 K/h and finally air-quenched to room temperature. A few colourless block-shaped crystals were obtained from the reaction product. Refinement EDX spectrometry using a JSM6700F scanning electron microscope confirmed the composition. The remaining maximum and minimum electron densities are located 0.87 Å and 0.81 Å, respectively, from the Rb atom.

Figure 1
View of a part of the title structure. Ellipsoids are shown at the 50% probability level. [Symmetry code: (i) -0.5 + x, 0.5 -

Figure 2
The packing of the structure of RbYb(PO 3 ) 4 , viewed along the a axis. Polyhedra represents PO 4 tetrahedra.

Figure 3
The polyphosphate double spiral chains

Rubidium ytterbium(III) tetrakis(polyphosphate)
Crystal data 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 R-factors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq