Vibrational Study and Crystal Structure of Barium Cesium Cyclotriphosphate Dihydrate

Chemical preparation, crystal structure, thermal behavior, and IR studies are reported for the barium cesium cyclotriphosphate dihydrate BaCsP3O9.2H2O and its anhydrous form BaCs4(PO3)6. BaCsP3O9.2H2O, isotypic to BaTlP3O9.2H2O and BaNH4P3O9.2H2O, is monoclinic P21/n with the following unit cell dimensions: a = 7.6992(2)Å, b = 12.3237(3)Å, c = 11.8023(3)Å, α = 90 (2) , β = 101.18(5) , γ = 90. (3) , and Z = 4. The total dehydration of BaCsP3O9.2H2O is between 100 C and 580 C. The IR absorption spectroscopy spectrum for the crystal confirms that most of the vibrational modes are comparable to similar cyclotriphosphates and to the calculated frequencies. The thermal properties reveal that the compound is stable until 90 C.


Experimental parameters 2.1. Chemical preparation
Single crystals of BaCsP 3 O 9 .2H 2 O were prepared by slowly adding dilute cyclotriphosphoric acid, H 3 P 3 O 9 , to an aqueous solution of barium carbonate, BaCO 3 , and cesium carbonate, Cs 2 CO 3 , with a stoichiometric ratio of Ba-Cs = 1:1, according to the following chemical reaction: The solution was then slowly evaporated at room temperature for 45 days until single crystals of BaCsP 3 O 9 .2H 2 O were obtained. The cyclotriphosphoric acid, H 3 P 3 O 9 , used in this reaction was prepared from an aqueous solution of Na 3 P 3 O 9 passed through an ion-exchange resin "Amberlite IR120" [3]. Na 3 P 3 O 9 was obtained by thermal treatment of sodium dihydrogen monophosphate, NaH 2 PO 4 , at 530 C for 5 h in the air, according to the following chemical reaction [4]: 2.2. XRD, crystal data, intensity data collection, and structure A single-crystal X-ray structure determination of BaCsP 3 O 9 .2H 2 O was performed by using an Oxford Xcalibur S diffractometer at 293 K.
The structure was solved by direct methods using SHELXS [5] implemented in the Olex2 program [6]. The refinement was then carried out with SHELXL by full-matrix least squares minimization and difference Fourier methods. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were generated in idealized positions, riding on the carrier atoms, with isotropic thermal parameters.
The final R1 value is 0.0401 for 1782 reflections with I > 2σ (I), and full X-crystal data is presented in Table 1. The main geometrical features, bond distances, and angles are reported in Table 6.

Fourier transform infrared spectroscopy (FTIR)
A Nicolet Magna IR 560 spectrometer (resolution 1 cm À1 , 200 scans) and an OMNIC software were used to characterize the stretching and bending bands between 400 and 4000 cm À1 .

Structural analysis
The final atomic positions and anisotropic thermal parameters for the non-hydrogen atoms in the BaCsP 3 O 9 .2H 2 O structure are given in Tables 2 and 3, respectively. A projection of the BaCsP 3 O 9 .2H 2 O atomic arrangement along the c axis is given in Figure 1. It shows that all the components of the atomic arrangements are located around the two axes in order to form arrays delimiting large channels parallel to the c direction.

Barium and cesium arrangement in the structure
The barium atom, located on the twofold axis, is coordinated by two water molecules and six oxygen atoms (Figure 2), forming an almost regular dodecahedron. The Ba-O distances spread between 2.298(6) and 2.349(6) Å. Each BaO 8 dodecahedron shares six oxygen atoms with two anionic rings belonging to two phosphoric layers, thus providing the cohesion between these layers ( Figure 2). BaO 8 dodecahedra do not share any edge or corner and form layers alternating with P 3 O 9 ones. The shortest Ba-Ba distance is found to be 4.70731 Å ( Table 4).
The cesium atom occupies a general position and is coordinated to 10 external oxygen atoms and one water molecule ( Figure 3). The Cs-O distances spread between 3.0278(2) and 3.5982(9) Ǻ.

Characterization by infrared spectroscopy
Crystals were ground in a mortar with dry KBr powder in a ratio of 2:200 and pelleted in a press (8*10 3 kg, 30 s). Then, they were stored at 95 C for 1 d to dry before use.
The IR spectrum of BaCsP 3 O 9 .2H 2 O illustrated in Figure 4 reveals the presence of three bands due to water molecules in the domain 4000-1600 cm À1 . This confirms the existence of nonequivalent positions of water molecules in the BaCsP 3 O 9 .2H 2 O atomic arrangement: 3449 cm À1 attributed to O-H valence vibration, around 3270 cm À1 to hydrogen bonds and 1637 cm À1 to δHOH deformation. The valence vibration bands related to the P 3 O 9 cycles are expected in the domain 1400-650 cm À1 , as well as possible bands due to interactions between P 3 O 9 cycles and water molecules and also of water vibration modes.  (7) P (1)  The vibration modes of the phosphate anions usually occur in the 1400-650 cm À1 area. The two IR bands observed at 1384 and 1286 cm À1 can be attributed to the νas (PO 2 ) stretching vibration ( Table 5). The shouldered band at 1157 cm À1 and the doublet observed at 1100 and  983 cm À1 can be assigned to νs(PO 2 ) and νas(POP), respectively. The most characteristic feature of the P 3 O 9 ring anions is the occurrence of a strong intensity band near 767 cm À1 in addition to 747 cm À1 due to the νs(POP) stretching vibration. The weak peak appearing at 685 cm À1 can be assigned to νs(POP) [9]. The broad bands observed at 519 cm À1 and the weak peak at 637 cm À1 can be due to the deformation vibrations of the anionic group.
In the spectral domain 650-400 cm À1 , the spectrum of BaCsP 3 O 9 .2H 2 O (Figure 4) shows bending vibration band characteristic of phosphates with ring anions.

Vibrational study
The percentage of participation of each group was determined ( Table 6). The geometrical parameters of the P3O9 3-ring with D3h symmetry, optimized by the MNDO [10] programs, are comparable with those obtained, by X-ray diffraction for the compounds with known structures.  [12,13], M II K 4 (P 3 O 9 ) 2 .7H 2 O (M II = Ni,Co), C 1 in M II (NH 4 ) 4 (P 3 O 9 ) 2 .4H 2 O (M II = Cu, Co, Ni) [14], and NiNa 4 (P 3 O 9 ) 2 .6H 2 O [15] are characterized by three intense bands situated between 1153 and 1180, 640-680, and 297-313 cm À1 , which confirm the results of our calculations ( Table 6). Indeed, the theory predicts on the whole four bands with A' 1 modes for the P 3 O 9 ring with D 3h symmetry which are situated, according to our results, at 1169 cm À1 for νs P-Oe, 671 cm À1 for δs P-Oi, 559 cm À1 for δsPOiP, and 302 cm À1 for δs PO 2 . These four frequencies are predicted to be characteristic in any Raman spectrum of a cyclotriphosphate (with cycle of symmetry, C 3 , C 2 , Cs, or C 1 ). These four IR fundamental frequencies have a null calculated intensity and are non-observable for D 3h or C 3h symmetries, and their appearance in any IR spectrum indicates a symmetry lower than C 3h . Table 7. Attribution of the observed valence IR frequencies (cm À1 ) of the P 3 O 9 ring (C 1 ) in BaCsP 3 O 9 .2H 2 O. This allowed us an attribution of the 30 fundamental frequencies of the cycle D 3h on valid theoretical bases including 12 valence vibration frequencies and 18 bending vibration frequencies. The correlation between the D 3h group and the site group C 1 shows that the simple normal modes (A' 1 , A' 2 , A" 1 , and A" 2 ), of the D 3h group, are resolved each into the mode A of the C 1 group and the doubly degenerate E' and E" modes are resolved into two modes and are active in IR and Raman. The factor group analysis predicts for four cycles of the unit cells of BaCsP 3 O 9 .2H 2 O (C 2h ), respectively, 24 and 36 valence vibration bands active in IR. But, we observe in the IR spectra of BaCsP 3 O 9 .2H 2 O (C 2h ) only six or seven bands and one inflection ( Figure 4). It seems that the vibrational couplings between the P 3 O 9 cycles of the unit cell are absent or very weak; thus, we will be able to interpret the IR spectrum, in the range 1400-650 cm À1 , of BaCsP 3 O 9 .2H 2 O according to the vibrations of an isolated cycle with local symmetry C 1 . The values of the calculated frequencies, for the D 3h symmetry, are close to those observed for BaCsP 3 O 9 .2H 2 O ( Table 6). Table 7 gives the attribution of the observed valence frequencies, 1400-650 cm À1 , of the P 3 O 9 ring, with D 3h symmetry of BaCsP 3 O 9 .2H 2 O.

Thermal analysis
The curve corresponding to the TG analyses in an air atmosphere and at a heating rate of 10 C. min À1 of BaCsP3O9.2H2O is given in Figure 5. The dehydration of the barium cyclotriphosphate and of cesium dihydrate BaCsP3O9.2H2O is carried out in two steps in two temperature ranges from 105 to 180 C and from 180 to 580 C ( Figure 5). In the thermogravimetric (TG) curve, the first step between 95 and 180 C corresponds to the elimination of 1.14 water molecules; the second step from 180 to 580 C is due to the removal of 0.86 water molecules.   [16], which leads to the anhydrous barium and thallium BaTlP 3 O 9 cyclotriphosphate at 280 C. After amorphous X-ray state, BaTlP 3 O 9 remains stable till its melting point at 670 C.

Conclusion
The cyclotriphosphate BaCsP 3 O 9 .2H 2 O was obtained as a monocrystal by the resin exchange method. It crystallizes in the monoclinic system, space group P2 1 /n, Z = 4, and is an isotype of BaNH 4 P 3 O 9 .2H 2 O and BaTlP 3 O 9 .2H 2 O.
The crystal structure of BaCsP 3 O 9 .2H 2 O was solved from 2448 independent reflections. The final value of the unweighted reliability factor is R = 0.0329. The unit cell of BaCsP 3 O 9 .2H 2 O contains four P 3 O 9 3À 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 P 3 O 9 cycle observed in the structure of BaCsP 3 O 9 .2H 2 O has no internal symmetry. The cohesion between the cycles P 3 O 9 3À is ensured via the associated cations Cs + and Ba 2+ . The main geometrical characteristics of the three P(1)O4, P(2)O4, and P(3)O4 tetrahedra of the P 3 O 9 cycle are quite similar to those observed generally in cyclotriphosphates.
The thermogram (TG) of BaCsP 3 O 9 .2H 2 O 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 BaCsP 3 O 9 .2H 2 O structure, probably leading to a mixture of amorphous oxidesin X-ray diffraction BaO + 3/2 P 2 O 5 + ½ Cs 2 O. The product resulting from calcinations of BaCsP 3 O 9 .2H 2 O between 300 and 560 C is the long chain polyphosphate BaCs 4 (PO 3 ) 6 .