Vibrational Study and Crystal Structure of Barium Cesium Cyclotriphosphate Dihydrate

Abstract

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.

1. Introduction

During a systematic investigation of cyclophosphates, types BaCsP₃O₉.xH₂O, BaCs₄(P₃O₉)₂.xH₂O, BaCs₂P₄O₁₂.2H₂O, Ba₃Cs₂(P₄O₁₂)₂.2H₂O, and BaCsP₃O₉.2H₂O were obtained. Barium and cesium cyclotriphosphate dihydrate, BaCsP₃O₉.2H₂O, was prepared for the first time by using Boulle’s process [1] by Masse and Averbuch-Pouchot [2], who described it as a monohydrate. The literature provides BaCsP₃O₉.2H₂O crystallizing in the monoclinic system, space group P2₁/n, Z = 4 with the following unit cell parameters, a = 7.6992(2) Å, b = 12.3237(3) Å, c = 11.8023 (3) Å, and β = 101.181 (5)° with a brief report of the structural refinement based on single-crystal XRD data. In the present work, we report the chemical preparation, crystalline structure, thermogravimetric analysis, and infrared study of this crystal barium and cesium cyclotriphosphate dihydrate, BaCsP₃O₉.2H₂O, in order to have maximum information about structure and reactivity of the solids.

2. Experimental parameters

2.1. Chemical preparation

Single crystals of BaCsP₃O₉.2H₂O were prepared by slowly adding dilute cyclotriphosphoric acid, H₃P₃O₉, to an aqueous solution of barium carbonate, BaCO₃, and cesium carbonate, Cs₂CO₃, with a stoichiometric ratio of Ba-Cs = 1:1, according to the following chemical reaction:

$${\mathrm{H}}_3{\mathrm{P}}_3{\mathrm{O}}_9+{\text{BaCO}}_3+1/{\text{2Cs}}_2{\mathrm{CO}}_3\to {\text{BaCsP}}_3{\mathrm{O}}_9.{\text{2H}}_2\mathrm{O}+3/{\text{2CO}}_2$$

The solution was then slowly evaporated at room temperature for 45 days until single crystals of BaCsP₃O₉.2H₂O were obtained. The cyclotriphosphoric acid, H₃P₃O₉, used in this reaction was prepared from an aqueous solution of Na₃P₃O₉ passed through an ion-exchange resin “Amberlite IR120” [3]. Na₃P₃O₉ was obtained by thermal treatment of sodium dihydrogen monophosphate, NaH₂PO₄, at 530°C for 5 h in the air, according to the following chemical reaction [4]:

$${\text{3NaH}}_2{\mathrm{PO}}_4\to {\mathrm{Na}}_3{\mathrm{P}}_3{\mathrm{O}}_9+{\text{3H}}_2\mathrm{O}$$

2.2. XRD, crystal data, intensity data collection, and structure

A single-crystal X-ray structure determination of BaCsP₃O₉.2H₂O 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.

2.3. Fourier transform infrared spectroscopy (FTIR)

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

3. Results and discussion

3.1. Structural analysis

The final atomic positions and anisotropic thermal parameters for the non-hydrogen atoms in the BaCsP₃O₉.2H₂O structure are given in Tables 2 and 3, respectively. A projection of the BaCsP₃O₉.2H₂O 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.

Compound	2
Empirical formula	BaCs H4O11P3
Formula weight	543.20 g.mol^−1
Crystal system/space group	Monoclinic/P 21/n
a/Å	7.6992(2) Å
b/Å	12.3237(3) Å
c/Å	11.8023(3) Å
α/°	90°
β/°	101.18(3)°
γ/°	90°
V/Å^3	1098.57(5)Å^3
Z	4
D calc (g/cm^3)	3.284 g/cm^3
μ (mm^−1)	7.362
Crystal size (mm)	0.3296 × 0.1602 × 0.0957 mm^3
Color/shape	Colorless/prism
Temp (K)	293(2)K
Theta range for collection	3.50°/27.59°
Reflections collected	9176
Independent reflections	2448
Data/restraints/parameters	2448/0/147
Goodness of fit on F^2	1.113
Final R indices [I > 2σ(I)]	R1 = 0.0285, wR2 = 0.0611
R indices (all data)	R1 = 0.0329, wR2 = 0.0638
Largest difference peak/hole	0.78/−1.40 Å^−3

Table 1.

Crystal data and experimental parameters for the X-ray intensity data collection for BaCsP₃O₉.2H₂O.

Atoms	X	Y	Z	Ueq
Ba	0.24946(3)	0.06963(2)	0.37463(2)	0.01486(9)
Cs	1.23531(4)	0.37670(3)	0.60501(3)	0.02500(10)
P(1)	0.49653(15)	0.33939(9)	0.34729(10)	0.0135(2)
P(2)	0.75498(15)	0.17362(10)	0.42595(10)	0.0140(2)
P(3)	0.72984(16)	0.35936(10)	0.57392(11)	0.0185(3)
O(1i)	0.8311(4)	0.2619(3)	0.5238(3)	0.0194(7)
O(2i)	0.6424(4)	0.2510(2)	0.3278(2)	0.0159(7)
O(3i)	0.6022(4)	0.4024(2)	0.4588(3)	0.0178(7)
O(4e)	0.8606(5)	0.4456(3)	0.6136(4)	0.0393(10)
O(5e)	0.6256(5)	0.3191(3)	0.6572(3)	0.0312(9)
O(6e)	0.4740(4)	0.4168(2)	0.2497(3)	0.0212(7)
O(7e)	0.9053(4)	0.1308(3)	0.3805(3)	0.0223(8)
O(8e)	0.6306(4)	0.0994(3)	0.4691(3)	0.0201(7)
O(9e)	0.3428(4)	0.2843(3)	0.3783(3)	0.0205(7)
O(10w)	0.2195(4)	0.1274(3)	0.5953(3)	0.0233(8)
O(11w)	0.5532(5)	0.0975(3)	0.7172(3)	0.0315(9)
H(1)	0.5738	0.0941	0.7926	0.047
H(2)	0.5846	0.1617	0.6363	0.047
H(3)	0.1274	0.1017	0.6242	0.035
H(4)	0.3191	0.0980	0.6366	0.035

Table 2.

Final atomic coordinates and U-equivalent temperature factors for BaCsP₃O₉.2H₂O.

i, internal; e, external; w, water.

Atom	U11(s)	U22	U33	U23	U13	U12
Ba	0.01483(15)	0.01281(16)	0.01595(15)	0.00044(10)	0.00056(11)	−0.00029(10)
Cs	0.02411(18)	0.0258(2)	0.02600(18)	0.00406(13)	0.00715(14)	0.00042(13)
P(1)	0.0146(6)	0.0132(6)	0.0122(5)	0.0016(4)	0.0015(5)	0.0020(5)
P(2)	0.0150(6)	0.0130(6)	0.0134(5)	0.0004(5)	0.0016(5)	0.0028(5)
P(3)	0.0171(6)	0.0193(7)	0.0171(6)	−0.0062(5)	−0.0015(5)	0.0022(5)
O(1i)	0.0173(16)	0.0172(17)	0.0207(17)	−0.0054(14)	−0.0035(14)	0.0032(14)
O(2i)	0.0195(17)	0.0166(17)	0.0113(15)	0.0007(13)	0.0023(13)	0.0071(14)
O(3i)	0.0192(17)	0.0140(17)	0.0176(17)	−0.0035(13)	−0.0026(14)	0.0027(14)
O(4e)	0.025(2)	0.027(2)	0.058(3)	−0.0226(19)	−0.0086(19)	−0.0009(17)
O(5e)	0.037(2)	0.037(2)	0.0201(18)	0.0038(16)	0.0079(17)	0.0101(18)
O(6e)	0.0241(18)	0.0194(17)	0.0210(18)	0.0080(14)	0.0067(15)	0.0069(15)
O(7e)	0.0173(17)	0.0245(19)	0.0247(18)	−0.0051(15)	0.0031(15)	0.0064(14)
O(8e)	0.0201(17)	0.0147(17)	0.0253(18)	0.0064(14)	0.0042(15)	0.0009(14)
O(9e)	0.0177(17)	0.0185(18)	0.0255(18)	0.0026(14)	0.0050(15)	−0.0004(14)
O(10w)	0.0190(17)	0.027(2)	0.0236(18)	−0.0012(15)	0.0043(15)	0.0010(15)
O(11w)	0.030(2)	0.034(2)	0.028(2)	−0.0008(17)	−0.0009(18)	−0.0001(18)

Table 3.

Anisotropic thermal parameters (Ų) for BaCsP₃O₉.2H₂O.

i, internal; e, external; w, water.

Figure 1.

Projection along the c axis of the atomic arrangement in BaCsP₃O₉.2H₂O.

3.2. 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 BaO₈ dodecahedron shares six oxygen atoms with two anionic rings belonging to two phosphoric layers, thus providing the cohesion between these layers (Figure 2). BaO₈ dodecahedra do not share any edge or corner and form layers alternating with P₃O₉ ones. The shortest Ba-Ba distance is found to be 4.70731 Å (Table 4).

Figure 2.

The coordination of the barium atom in BaCsP₃O₉.2H₂O.

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.

Figure 3.

ORTEP representation of BaCsP₃O₉.2H₂O (H-bonds are represented by dashed lines). Thermal ellipsoids are scaled to enclose 50% probability.

The water group, its environment, established by strong hydrogen bonds, is depicted in (Figure 3) as an ORTEP representation [7].

3.3. 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*10³ kg, 30 s). Then, they were stored at 95°C for 1 d to dry before use.

The IR spectrum of BaCsP₃O₉.2H₂O illustrated in Figure 4 reveals the presence of three bands due to water molecules in the domain 4000–1600 cm⁻¹. This confirms the existence of nonequivalent positions of water molecules in the BaCsP₃O₉.2H₂O atomic arrangement: 3449 cm⁻¹ attributed to O-H valence vibration, around 3270 cm⁻¹ to hydrogen bonds and 1637 cm⁻¹ to δHOH deformation. The valence vibration bands related to the P₃O₉ cycles are expected in the domain 1400–650 cm⁻¹, as well as possible bands due to interactions between P₃O₉ cycles and water molecules and also of water vibration modes.

Figure 4.

FTIR spectrum of BaCsP₃O₉.2H₂O crystal.

The vibration modes of the phosphate anions usually occur in the 1400–650 cm⁻¹ area. The two IR bands observed at 1384 and 1286 cm⁻¹ can be attributed to the νas (PO₂) stretching vibration (Table 5). The shouldered band at 1157 cm⁻¹ and the doublet observed at 1100 and 983 cm⁻¹ can be assigned to νs(PO₂) and νas(POP), respectively. The most characteristic feature of the P₃O₉ ring anions is the occurrence of a strong intensity band near 767 cm⁻¹ in addition to 747 cm⁻¹ due to the νs(POP) stretching vibration. The weak peak appearing at 685 cm⁻¹ can be assigned to νs(POP) [9]. The broad bands observed at 519 cm⁻¹ and the weak peak at 637 cm⁻¹ can be due to the deformation vibrations of the anionic group.

Tetrahedron around P(1)
P(1)	O(2i)	O(3i)	O(6e)	O(9e)
O(2i)	1.6126(5)	100.5(9)	107.8(1)	109.7(7)
O(3i)	2.4804(3)	1.6065(6)	106.9(7)	108.7(6)
O(6e)	2.4983(3)	2.4803(1)	1.4795(3)	120.9(4)
O(9e)	2.5254(9)	2.5046(5)	2.5662(1)	1.4708(8)
Tetrahedron around P(2)
P(2)	O(1i)	O(2i)	P(2)	O(1i)
O(1i)	1.6120(2)	100.7(7)	107.4(8)	109.7(3)
O(2i)	2.4853(7)	1.6164(2)	107.3(7)	108.6(1)
O(7e)	2.4843(1)	2.4879(6)	1.4650(7)	120.7(8)
O(8e)	2.5346(1)	2.5175(5)	2.5637(8)	1.4838(6)
Tetrahedron around P(3)
P(3)	(O1i)	(O3i)	(O4e)	(O5e)
O(1i)	1.6058(7)	101.4(4)	107.8(7)	111.2(4)
O(3i)	2.4836(6)	1.6045(7)	107.4(7)	110.5(9)
O(4e)	2.4914(1)	2.48390	1.4762(3)	117.3(4)
O(5e)	2.5386(2)	2.5308(4)	2.5158(1)	1.4705(7)
P(1)–P(2)	2.8773(2)	P(2)–O(1i)–P(3)	129.4(1)
P(1)–P(3)	2.9289(6)	P(1)–O(2i)–P(3)	131.6(6)
P(2)–P(3)	2.9081(6)	P(1)–O(3i)–P(2)	125.8(9)
P(2)–P(1)–P(3)	60.2(1)
P(1)–P(2)–P(3)	60.7(2)
P(1)–P(3)–P(2)	59.1(6)

Table 4.

Main interatomic distances (A°) and bond angles (°) in the P₃O₉ ring [8].

In the spectral domain 650–400 cm⁻¹, the spectrum of BaCsP₃O₉.2H₂O (Figure 4) shows bending vibration band characteristic of phosphates with ring anions.

4. 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.

ν (cm^−1)	Vibration
3449	ν OH
1637
	δ HOH
1637
1384	νas OPO^−
1286
1157	νs OPO^−
1100
983	νas POP
767	νs POP
747
685
	δ OPO^−
637	+
519	ρ OPO^−

Table 5.

Frequencies (cm⁻¹) of IR absorption bands for BaCsP₂O₉.2H₂O.

All the Raman spectra available in the literature of compounds with the P₃O₉³⁻ cycle of C_3h symmetry, in LnP₃O₉.3H₂O [11] and M^IIM^IP₃O₉ with benitoite structure 4, and cycle of Cs symmetry in NiRb₄(P₃O₉)₂.6H₂O [17, 18], ZnM^I₄(P₃O₉)₂.6H₂O (M^I = K, Rb) [12, 13], M^IIK₄(P₃O₉)₂.7H₂O (M^II = Ni,Co), C₁ in M^II(NH₄)₄(P₃O₉)₂.4H₂O (M^II = Cu, Co, Ni) [14], and NiNa₄(P₃O₉)₂.6H₂O [15] are characterized by three intense bands situated between 1153 and 1180, 640–680, and 297–313 cm⁻¹, which confirm the results of our calculations (Table 6). Indeed, the theory predicts on the whole four bands with A’₁ modes for the P₃O₉ ring with D_3h symmetry which are situated, according to our results, at 1169 cm⁻¹ for νs P-Oe, 671 cm⁻¹ for δs P-Oi, 559 cm⁻¹ for δsPOiP, and 302 cm⁻¹ for δs PO₂. These four frequencies are predicted to be characteristic in any Raman spectrum of a cyclotriphosphate (with cycle of symmetry, C₃, C₂, Cs, or C₁). 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.

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 C₁ 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 C₁ 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 BaCsP₃O₉.2H₂O (C_2h), respectively, 24 and 36 valence vibration bands active in IR. But, we observe in the IR spectra of BaCsP₃O₉.2H₂O (C_2h) only six or seven bands and one inflection (Figure 4). It seems that the vibrational couplings between the P₃O₉ 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⁻¹, of BaCsP₃O₉.2H₂O according to the vibrations of an isolated cycle with local symmetry C₁. The values of the calculated frequencies, for the D_3h symmetry, are close to those observed for BaCsP₃O₉.2H₂O (Table 6). Table 7 gives the attribution of the observed valence frequencies, 1400–650 cm⁻¹, of the P₃O₉ ring, with D_3h symmetry of BaCsP₃O₉.2H₂O.

Table 6.

IR frequencies and displacements (Δν in cm⁻¹) calculated for the P₃O₉ (D_3h symmetry).

Table 7.

Attribution of the observed valence IR frequencies (cm⁻¹) of the P₃O₉ ring (C₁) in BaCsP₃O₉.2H₂O.

5. Thermal analysis

The curve corresponding to the TG analyses in an air atmosphere and at a heating rate of 10°C.min⁻¹ 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.

Figure 5.

TG curves of BaCsP₃O₉.2H₂O at rising temperature (10°C min⁻¹).

6. Comparison of the thermal behavior of BaCsP₃O₉.2H₂O with BaNH₄P₃O₉.2H₂O and BaTlP₃O₉.2H₂O

The thermal behavior of BaNH₄P₃O₉.2H₂O and BaTlP₃O₉.2H₂O [16] was studied (Laboratory of Physical Chemistry of Materials, Ben M’sik faculty of Sciences, Casablanca, Morocco). It would be useful to compare the thermal behavior of BaCsP₃O₉.2H₂O with that of its isotypic compounds BaNH₄P₃O₉.2H₂O and BaTlP₃O₉.2H₂O.

The thermal behavior of BaCsP₃O₉.2H₂O is different from that obtained in the case of BaTlP₃O₉.2H₂O [16], which leads to the anhydrous barium and thallium BaTlP₃O₉ cyclotriphosphate at 280°C. After amorphous X-ray state, BaTlP₃O₉ remains stable till its melting point at 670°C.

$${\mathit{\text{BaTlP}}}_3{O}_9.2H_{2}O{\displaystyle \stackrel{70-270°C}{\to }}\mathit{\text{Amorphous}}X-\mathit{ray}\mathrm{P}\mathit{\text{hase}}+2H_{2}O{\displaystyle \stackrel{300°C}{\to }}{\mathit{\text{BaTlP}}}_3{O}_9$$

The total dehydration of BaCsP₃O₉.2H₂O, after passing through an amorphous X-ray state, leads to monobarium polyphosphate and tetracerium BaCs₄(PO₃)₆ at 500°C [17, 18].

$$\begin{array}{c}{\mathit{\text{BaCsP}}}_3{O}_9.2H_{2}O{\displaystyle \stackrel{100-250°C}{\to }}\mathit{\text{Amorphous}}X-\mathit{ray}\mathit{\text{Phase}}+2H_{2}O{\displaystyle \stackrel{450°C}{\to }}\mathrm{¼}{\mathit{\text{BaCs}}}_4{\left({\mathit{PO}}_3\right)}_6\mathit{\text{crystallized}}\\ +¾\left[\mathit{Ba}{\left({\mathit{PO}}_3\right)}_2\right]\infty \left({\mathit{BaP}}_2{O}_6\mathit{\text{amorphous in}}X-\mathit{ray}\mathit{\text{diffraction}}\right).\end{array}$$

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 P₃O₉³⁻ 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 P₃O₉³⁻ is ensured via the associated cations Cs⁺ 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.