1 Introduction

During the investigation of ternary diagrams Na₂CO₃–YbF₃–H₂O [1] and Na₂CO₃–YF₃–H₂O [2] at T = 190 °C, we have found four new fluoride carbonate families, Na₂REE(CO₃)₂F (REE = Y, Yb), Na₃Y(CO₃)₂F₂, Na₃Yb(CO₃)₂F₂, Na₄Y(CO₃)₂F₃$\dot{}$H₂O, together with one carbonate hydrate Na₅Yb(CO₃)₄$\dot{}$2 H₂O [3]. At high carbonate concentration and T = 220 °C, two other carbonates appear, Na₅Y(CO₃)₄ [3] and Na₃Y(CO₃)₃$\dot{}$6 H₂O (this publication). Five sodium yttrium carbonates have been previously reported: four minerals, NaY(CO₃)F₂ horvathite [4], Na₃(Y,REE)(CO₃)₃$\dot{}$3 H₂O shomiokite [5], Na(Y,REE)(HCO₃)(OH)₃$\dot{}$4 H₂O thomasclarkite [6], NaY(CO₃)₂$\dot{}$6 H₂O adamsite [7,8], and one synthetic phase, NaY(CO₃)₂ [9].

In this paper, the synthesis, the crystal structure, the thermal behaviour and the vibrational analysis of Na₃Y(CO₃)₃$\dot{}$6 H₂O are reported. A structural relationship is found with Na₃(Y,REE)(CO₃)₃$\dot{}$3 H₂O shomiokite.

2 Experimental

Crystals of Na₃Y(CO₃)₃$\dot{}$6 H₂O grow in a Teflon lined Parr autoclave at 220 °C during 48 h from Na₂CO₃/YF₃/H₂O in the molar ratio 25:1:55. They are washed with water and acetone and dried in air.

Volume weight was measured with a pycnometer AccuPyc 1330 V3.03. The experimental value ρ_exp = 2.25 (5) g cm^–3 is consistent with the calculated value ρ_calc = 2.24 g cm^–3.

Thermal analyses were performed with a DTA–TGA TA Instrument 2960 (heating rate 10 °C min^–1, argon atmosphere) in the temperature range 25–1000 °C.

Single crystal diffraction data were obtained on a Siemens AED2 four-circle diffractometer. The conditions of intensity measurement are reported in Table 1.

The infrared spectrum of Na₃Y(CO₃)₃$\dot{}$6 H₂O was obtained using a Bomem Michelson MB120 FTIR spectrometer with a diamond-anvil cell as a microsampling device. Infrared data were collected in the range 650–4000 cm^–1. Spectral resolution was 4 cm^–1.

The Raman scattering study was performed in the backscattering geometry, under microscope (×50 objective), on a powder (without polarisation analysis). The spectrum was collected with a T64000 Jobin-Yvon multichannel spectrometer (in triple substractive configuration with 1800 lines/mm grating), using the 514.5 nm radiation of an argon–krypton laser as excitation, with 8 mW power on the sample in the range 40–4000 cm^–1. The typical spectral resolution was better than 0.7 cm^–1.

3 Structure determination

The systematic existing conditions of reflections 000l (l =2n) led to the P6₃ and P6₃/m space groups. Owing to a positive second harmonic generation test, the non-centrosymmetric P6₃ space group was chosen. The positions of yttrium atoms were obtained using the Patterson method (option PATT in SHELXS-86 [10]). Analysis of successive Fourier difference maps allowed the location of the remaining atoms. Na, C, and O were distinguished from distance criteria and from valence bond analysis. The refinement (SHELXL-97 [11]) of the atomic coordinates and anisotropic displacement parameters after absorption correction (Gauss method in SHELX-76 [12]), decreased the reliability factors to R = 0.058 and R_w = 0.161.

The final atomic coordinates with isotropic displacement parameters and bond valence analysis, the anisotropic displacement parameters and selected bond distances and angles are given in Tables 2, 3 and 4, respectively.

4 Structure description

In the structure of Na₃Y(CO₃)₃$\dot{}$6H₂O, yttrium atoms are surrounded by six oxygen atoms and three water molecules to form tricaped triangular prisms [YO₃₊₃(H₂O)₃] similar to that found in shomiokite (Fig. 1 left and right). The mean Y–O distance, 2.41 Å, is very close to that encountered in Na₃(Y,REE)(CO₃)₃$\dot{}$3 H₂O shomiokite (2.42 Å) and NaY(CO₃)₂$\dot{}$6 H₂O adamsite (2.42 Å). Sodium cations form distorted [NaO₄(H₂O)₂] octahedra. Na–O distances range from 2.33 to 2.61 Å, with an average of 2.44 Å; these values and the coordination number are comparable to that found in Na₃Y(CO₃)₃$\dot{}$3 H₂O and NaY(CO₃)₂$\dot{}$6 H₂O. The Ow(2) atom is bonded to one Y³⁺ (2.37 Å) and 1Na⁺ (2.61 Å). Ow(1) is only bonded to one Na⁺ (2.41 Å); consequently, the Ow(1) atom presents an abnormally high anisotropic displacement parameter (U₃₃ = 0.16(1)).

The distorted [NaO₄(H₂O)₂] octahedra build infinite [001] corrugated chains [NaO₄O_2/2]_∞ (Fig. 2, left). Each [YO₃₊₃(H₂O)₃] polyhedron is connected by edges to six [NaO₄(H₂O)₂] octahedra that belong to three independent [NaO₄O_2/2]_∞ chains (Fig. 2 right). Two adjacent [YO₃₊₃(H₂O)₃] polyhedra located at z = 0.4 and z = 0.9 share a unique [NaO₄O_2/2]_∞ chain. Six chains form cavities in which hydrogen atoms of water molecules point (Fig. 3). Water molecules H₂O_w(1) and H₂O_w(2) establish hydrogen bonds with O(1) and O(2), respectively (Table 5).

Carbon atoms lie at z = 0.176 and z = 0.676 (Fig. 4); with O(1), O(2) and O(3) atoms, they form infinite layers of carbonate groups in which the shortest C–C distance is 4.37 Å. In one layer, it is remarkable that carbon atoms are approximately located at the nodes of a two-dimensional hexagonal network with $a_{2\mathrm{D}}=4.01(\mathrm{a}=11.347=2{\mathrm{a}}_{2\mathrm{D}}\sqrt{2})$ (Fig. 5); three over seven nodes are occupied (Fig. 5). According to Grice et al. [13], this hexagonal network is characteristic of ‘flat-lying’ layers of carbonate groups (a_2D ≈ 5.1 Å) found in numerous carbonates (CaCO₃ calcite, Na₅Y(CO₃)₄ [3]). The smaller a_2D parameter observed in Na₃Y(CO₃)₃$\dot{}$6 H₂O is due to the out of plane tilt of the CO₃^2– ions.

A relationship between the structures of Na₃Y(CO₃)₃$\dot{}$6 H₂O and of Na₃(Y,REE)(CO₃)₃$\dot{}$3 H₂O shomiokite can be easily established: the c lattice parameters are similar because of the existence of [Na(CO₃)$\dot{}$2 H₂O] and [Na(CO₃)$\dot{}$H₂O] layers with a comparable thickness (Fig. 6 and Table 6). Moreover, Y³⁺ or Na⁺ coordinations are identical in both phases, as was previously indicated.

5 Characterization

5.2 Vibration analysis

In Na₃Y(CO₃)₃$\dot{}$6 H₂O, the Y atom is on the threefold axis (2 b) and all the other atoms lie in general positions (6 c). Using the standard correlation method [14] in C₆⁶ (P6₃), the normal modes of vibration can be classified according to the irreducible representations of C₆ (Table 7):

$${\Gamma }^M=34\mathrm{A}\oplus 34\mathrm{B}\oplus 34{\mathrm{E}}_1\oplus 34{\mathrm{E}}_2$$

Table 7

Vibrational analysis of Na₃Y(CO₃)₃$\dot{}$6 H₂O

Factor group C6		External modes	Internal modes	Activity
CO3	H2O
N	A	TCO3	RCO3	TY	TNa	TH2O	RH2O	ν1	ν2	ν3	ν4	ν	δ	γ	IR	Raman
A	34	–1	3	3	1	3	6	6	1	1	2	2	2	2	2	z	αxx + αyy ; αzz
B	34	0	3	3	1	3	6	6	1	1	2	2	2	2	2	–	–
E1	34	–1	3	3	1	3	6	6	1	1	2	2	2	2	2	(x, y)	(αxx, αyz)
E2	34	0	3	3	1	3	6	6	1	1	2	2	2	2	2	–	(αxx – αyy, αxy)

The acoustic modes are: Γ^acous = A ⊕ E₁. The A and E₁ modes are Raman and IR actives, while E₂ modes are only Raman actives. B modes are inactive.

Free CO₂^2– anion has the D_3h symmetry and exhibits four normal vibrations ν₁(A₁′ ), ν₂(A₂′ ′ ), ν₃(E′) and ν₄(E′). E′ modes are IR and Raman actives, while A₁′ is only Raman active and A₂′ ′ is only IR active. ν₁ (1000–1100 cm^–1) and ν₃ (1400–1500 cm^–1) represent the symmetric and asymmetric stretching vibrations, respectively. ν₂ (650–800 cm^–1) and ν₄ (800–1000 cm^–1) correspond to out of plane and in plane deformation vibrations, respectively. The crystal modes (and the macroscopic geometry of observation) can be related to the previous modes by the so-called site method. In Na₃Y(CO₃)₃$\dot{}$6 H₂O, the carbon site symmetry is C₁; the correlation analysis is schematised in Table 8.

Table 8

Correlation scheme for the internal vibration modes of CO₃ groups in Na₃Y(CO₃)₃$\dot{}$6 H₂O

Fig. 7 shows the Raman spectrum. One line at 1065 cm^–1 and two lines at 715 and 680 cm^–1 are attributed to the ν₁ and to the ν₄ modes, respectively. Theoretically, three lines are expected for the ν₁ mode and six lines for the ν₄ mode (Table 7). Though no polarisation analysis was possible, these lines can be unambiguously assigned to the A mode as a reason of the diagonal form of the polarisability tensor. The ν₂ and ν₃ modes around 820 cm^–1 and 1500 cm^–1 are very weak. For the ν₂ mode, this feature can be explained by the fact that ν₂ derives from (A₂′ ′ ), Raman inactive in D_3h symmetry.

The observed bands at low frequencies are very large. They are attributed to external modes of Na⁺ and Y³⁺ translations and H₂O or CO₃^2– translations and librations.

The IR spectrum is given in Fig. 8. The band around 1065 cm^–1 and the shoulder observed at 1045 cm^–1 are assigned to the symmetric stretching modes ν₁ of CO₃^2–. The two bands located at 890 and 875 cm^–1 are attributed to the out-of-plane deformation modes ν₂. In the region 1200–1550 cm^–1, two large bands, around 1375 and 1485 cm^–1, correspond to the asymmetric ν₃ stretching. For the out-of-plane ν₄ deformation, four bands predicted in the region 800–650 cm^–1 are observed at frequencies 680, 715, 755 cm^–1 and 760 cm^–1 (shoulder). Then, it is noted that the number of theoretical and observed bands is identical for ν₂ and ν₄.

H₂O vibration modes, attributed to antisymmetric stretching, symmetric stretching, in plane β-OH deformation and out of plane γ-OH deformation, are expected in the intervals 3600–2000, 1700–1550, 1300–1200 and 800–700 cm^–1, respectively. Experimentally, only five very large bands in the region 3600–2000 cm^–1, together with one small band at 1635 cm^–1, are observed.

6 Conclusion

The crystal structure of a new hydrated carbonate, Na₃Y(CO₃)₃$\dot{}$6 H₂O, is determined and can be described from lacunar layers of carbonate groups. A structural relationship is found with Na₃(Y,REE)(CO₃)₃$\dot{}$3 H₂O shomiokite and both hydrates are acentric. IR and Raman spectroscopy studies of Na₃Y(CO₃)₃$\dot{}$6 H₂O confirm the structural determination. It is observed that O–H vibration lines are very large and also weak, in spite of the presence of six water molecules for three carbonate groups.

It must be noted that shomiokite-(Y) mineral was found in association with an alteration yttrium carbonate. This silky white carbonate was too fine grained and the material quantity was too small; consequently, the structure is still unknown (J.D. Grice, personal communication). It should be interesting to collect X-ray diffraction and/or spectroscopic data of this carbonate, which could be compared with Na₃Y(CO₃)₃$\dot{}$6 H₂O.

Acknowledgements

The authors are very indebted to Pr. A. Bulou (LPEC, Univ. Le Mans) for his help in the analysis of vibrational spectra and to Dr P. Aschehoug (LCAES, ENSC Paris) for the SHG test.