7. Conclusion

The cyclotriphosphate BaCsP3O9.2H2O was obtained as a monocrystal by the resin exchange method. It crystallizes in the monoclinic system, space group P21/n, Z = 4, and is an isotype of BaNH4P3O9.2H2O and BaTlP3O9.2H2O.

The crystal structure of BaCsP3O9.2H2O was solved from 2448 independent reflections. The final value of the unweighted reliability factor is R = 0.0329. The unit cell of BaCsP3O9.2H2O contains four P3O9 <sup>3</sup>� rings, each of them consists of three crystallographically independent P (1)O4, P(2)O4, and P(3)O4 tetrahedra. The three tetrahedra have no special characteristics. The P3O9 cycle observed in the structure of BaCsP3O9.2H2O has no internal symmetry. The cohesion between the cycles P3O9 <sup>3</sup>� is ensured via the associated cations Cs<sup>+</sup> and Ba2+. The main geometrical characteristics of the three P(1)O4, P(2)O4, and P(3)O4 tetrahedra of the P3O9 cycle are quite similar to those observed generally in cyclotriphosphates.

The thermogram (TG) of BaCsP3O9.2H2O shows that dehydration takes place in two distinct steps between 70 and 560�C.

The total removal of the water at 560�C is accompanied by a total destruction of the BaCsP3O9.2H2O structure, probably leading to a mixture of amorphous oxidesin X-ray diffraction BaO + 3/2 P2O5 + ½ Cs2O. The product resulting from calcinations of BaCsP3O9.2H2O between 300 and 560�C is the long chain polyphosphate BaCs4(PO3)6.
