An orthorhombic polymorph of cerium(III) ultraphosphate, CeP5O14

Cerium(III) ultraphosphate, CeP5O14, was synthesized by a high-temperature solution reaction between CeO2 and NH4H2PO4 in a Ce–P molar ratio of 1:12. Colourless crystals of the orthorhombic polymorph were obtained by cooling the melt of the mixture. The structure contains (P5O14)3− anionic ribbons linked by distorted CeO8 polyhedra.

These compounds can be generally classified into four structure types: monoclinic (P2 1 /a), monoclinic (C2/c), orthorhombic (Pnma), and triclinic (P1) (Averbuch-Pouchot & Durif, 1992). In this chemical system, many of the compounds are isotypic, and some are polymorphic. However, many polymorphs of ultraphosphates LnP 5 O 14 have not been realised to date. Herein, we present the synthesis and crystal structure of an orthorhombic polymorph of CeP 5 O 14 .
In the structure (Figs. 1 and 2), the Ce 3+ cation plays an important bridging role, connecting neighbouring (P 5 O 14 ) 3anionic ribbons. The CeO 8 polyhedron is corner-sharing with eight PO 4 tetrahedra, with the Ce-O bond distances ranging from 2.436 (5) to 2.534 (8) Å. The shortest Ce-Ce distance is 5.2271 (9) Å. The (P 5 O 14 ) 3anionic ribbon may be described as two PO 4 infinite chains linked by P(2)O 4 tetrahedra, as shown in Fig. 3. P(1)O 4 , P(3)O 4 , and P(4)O 4 tetrahedra are corner-shared to form screwed infinite chains along the b axis. P(2)O 4 tetrahedra are corner-shared with two surrounding PO 4 infinite chains along the a axis. Thus, a (P 5 O 14 ) 3anionic ribbon is observed parallel to b.

Experimental
The title compound was prepared by a high-temperature solution reaction, using analytical reagent CeO 2 and NH 4 H 2 PO 4 in a molar ratio corresponding to Ce/P = 1:12. Starting mixtures were finely ground in an agate mortar to ensure optimal 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 air-quenched to room temperature. A few colourless, block-shaped crystals were obtained from the melt of the mixture.

Refinement
The position of the Ce atom was obtained using direct methods, and the remaining atoms were located in succesive difference Fourier syntheses. The chemical composition of the single crystal was confirmed by energy-dispersive X-ray (EDX) analysis, and no impurity elements were detected. Fig. 1. Asymmetric unit with displacement ellipsoids shown at 50% probability.

Special details
Geometry. All e.s. 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.