Caesium europium(III) polyphosphate, CsEu(PO3)4

Caesium europium polyphosphate, CsEu(PO3)4, was synthesized by a high-temperature solution reaction. Its structure is charaterized by a three-dimensional framework made up of double PO4 spiral chains and EuO8 and CsO11 polyhedra.

Crystallographic data and structural refinement of CsEu(PO 3 ) 4 are summarized in Table 1. The atomic coordinates and thermal parameters are listed in Table 2. Selected bond lengths and angles are given in Table 3.
The structure of crystal CsEu(PO 3 ) 4 is shown in Fig. 1. It belongs to the monoclinic space group P2 1 /n, which is isostructural with CsGd(PO 3 ) 4 (Naïli et al., 2005). The crystallographically distinct atoms of the asymmetric unit in the structure are one caesium, one europium, four phosphorus, and twelve oxygen atoms. It is described as a threedimensional framework made up from double PO 4 spiral chains and Cs-and Eu-polyhedra. As illustrated in Fig. 2, the double PO 4 spiral chains have the same repeating unit (eight PO 4 tetrahedra) as single one of CsNd(PO 3 ) 4 (Koizumi et al., 1978). These spiral chains are linked by EuO 8 and CsO 11 polyhedra. In addition, comparing with CsNd(PO 3 ) 4 , the different characteristics are noted in the coordination environments of the cations. The Eu cation is eight-coordinated with the Eu-O band distances ranging from 2.344 (3) to 2.471 (3) Å. Each EuO 8 polyhedron is corner-and faceconnected with two and two CsO 11 polyhedra, respectively (Fig. 3). The isolation of EuO 8 polyhedra gives rise to the large Eu-Eu distances, the shortest of which is 5.7415 (3) Å. The Cs cation is coordinated by eleven oxygen atoms, and the large range of the Cs-O bond distances implies that CsO 11 polyhedra are distorted. Neighboring two CsO 11 polyhedra are linked by corner-sharing (Fig. 4).

S2. Experimental
The title compound was prepared by the high temperature solution reaction, using analytical reagents Cs 2 CO 3 , Eu 2 O 3 , and NH 4 H 2 PO 4 in the molar ratio of Cs/Eu/P = 7:1:12. Starting mixtures were finely ground in an agate mortar to ensure the best homogeneity and reactivity, then placed in a platinum crucible and heated at 373 K for 4 h. Afterwards, the mixtures were reground and heated to 973 K for 24 h. Finally, the temperature was cooled to 773 K at a rate of 2 K/h and airquenched to room temperature. A few colorless and block-shaped crystals were obtained from the melt of the mixture.

S3. Refinement
A single-crystal of the compound was selected for X-ray Diffraction determination. The structure was solved using direct methods and refined on F2 by the full-matrix least-squares method with the SHELXL97 program package (Sheldrick, 2008). The position of the Eu atom was refined by the application of the direct method, and the remaining atoms were located in succeeding difference Fourier synthesis. In order to confirm the chemical composition of the the compound, the single-crystal investigated on the diffractometer was analyzed by Energy-dispersive X-ray spectrometry (EDX) using a JSM6700F scanning electron microscope. The obtained result is in good agreement with that obtained by the refinement of the crystal structure. No impurity elements have been detected.

Figure 1
Displacement ellipsoid plot (50% probability) of CsEu(PO 3 ) 4 . 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   (9)