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Anion and cation disorder in [CN3H6]⋅(TaF6)
Solid State Sciences 7 (2005) 1070–1073 www.elsevier.com/locate/ssscie
Anion and cation disorder in [CN3 H6]·(TaF6) M.A. Saada a,b , A. Hémon-Ribaud a , M. Leblanc a , V. Maisonneuve a,∗ a Laboratoire des oxydes et fluorures, UMR 6010 CNRS, faculté des sciences et techniques, Université du Maine, Avenue O. Messiaen,
72085 Le Mans Cedex 9, France b Laboratoire de chimie inorganique et structurale, faculté des sciences de Bizerte, 7021 Jarzouna, Tunisie
Received 2 April 2004; accepted 25 April 2005 Available online 1 June 2005
Abstract A new guanidinium fluoride tantalate, [CN3 H6 ]·(TaF6 ), crystallises at 50 ◦ C from a solution of Ta2 O5 in 40% aqueous HF and of ¯ space group, with the equivalent hexagonal cell aH = 8.647(1) Å, cH = guanidinium chloride. The structure is rhombohedral, R3m 8.507(2) Å, Z = 3 and R = 0.029, Rw = 0.077 for 312 reflections. The three-dimensional network is built up from parallel (0001)H layers 5− between which tantalum atoms are inserted. These tantalum atoms, in 3a sites, adopt an octahedral coordination with ∞ ([CN3 H6 ]F6 ) dTa–F = 1.894(7) Å. (TaF6 )− anions are disordered over (0001)H mirror plane related positions while guanidinium cations are disordered over centrosymmetric positions; site occupancy of fluoride and nitrogen sites is fifty per cent. 2005 Elsevier SAS. All rights reserved. Keywords: Hybrid fluoride; Guanidine; Tantalate; X-ray diffraction
1. Introduction In the search of open structures in fluorides, high valence cations, Zr+IV and Ta+V , were recently examined and associated with amine cations in fluoride solutions [1–3]. Only one 3D open framework was evidenced in a guanidinium zirconate, (H3 O)·[CN3 H6 ]5 ·(ZrF5 )6 [4], while numerous phases with isolated fluoride polyanions were obtained [5– 8]. Then, the study of the Ta2 O5 -guanidine-aqueous HF system was undertaken over large concentration domains of the starting materials at low crystallisation temperatures. One new phase, which exhibits anion and cation disorder, was evidenced; it is reported here.
2. Experimental Ta2 O5 powder (1 g) was first dissolved under stirring in 40% aqueous HF (40 ml) at 80 ◦ C during 4–5 h. After cool* Corresponding author. Tel.: +33 2 43 83 35 61; Fax: +33 2 43 83 35 06.
E-mail address: [email protected] (V. Maisonneuve). 1293-2558/$ – see front matter 2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.04.008
ing to room temperature, guanidinium chloride (0.191 g) was added. Single crystals of [CN3 H6 ]·(TaF6 ), which are moisture sensitive, were grown by the slow evaporation of the solution at 50 ◦ C over 2 days. Truncated (0001)H platelets were selected by optical examination and single crystal diffraction data were obtained on a Siemens AED2 four-circle diffractometer.
3. Structure determination Crystal data and the conditions of the intensity measurements are reported in Table 1 for [CN3 H6 ]·(TaF6 ). Absorption effects were corrected by the Gauss method. The structure was determined in the equivalent hexagonal cell by direct methods using SHELXS-86 [9] and refined with SHELXL-97 [10]. The rhombohedral cell parameters are 3 aR = 5.742(1) Å, αR = 97.17(2)◦ , VR = 183.7(1) Å . One heavy atom position was found in 3a special position ¯ space group with TREF option of SHELXS(0, 0, 0) of R3m 86 and was attributed to Ta. Then, successive refinements and Fourier difference maps allowed to locate F(1), F(2), N in 18h positions with a fifty per cent site occupancy and C in
M.A. Saada et al. / Solid State Sciences 7 (2005) 1070–1073
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Fig. 1. View of the disordered environment of tantalum (top left) or carbon (top right) atoms in [CN3 H6 ]·(TaF6 ); possible orientations of the resulting (TaF6 )− octahedra (bottom left and center) and representation of one possible orientation of a triangular (TaF6 )− prism (bottom right).
Table 1 Crystallographic data of [CN3 H6 ]·(TaF6 ) Molar weight (g·mol−1 ) Crystal size (mm3 ) Crystal system, space group aH (Å) cH (Å) V (Å3 ), Z ρcalc (g·cm−3 ) Temperature (K) Four circle diffractometer Monochromator 2θ range (◦ ) Reflections measured/unique/used (I > 2σ (I )) (hkl) limits (two centric ¯ independent sets in 3) Rint Scan mode Absorption correction, Amin , Amax Parameters refined (on F2 ) a R/b R w Goodness of fit Weighting scheme (p = [F02 + 2Fc2 ]/3) Residues of Fourier difference (e Å−3 ) a R = ||F | − |F ||/ |F |; o c o w(Fo2 )2 ]1/2 .
355.02 0.19 × 0.19 × 0.09 ¯ Rhombohedral, R3m 8.647(1) 8.507(2) 550.9(3), 3 3.21 298 Siemens AED2 graphite 2–70 1080/316/312 |h| 13; |k| 13; |l| 13 0.056 ω − 2θ Gaussian, 0.137, 0.299 22 0.029/0.077 1.13 1/[σ 2 (F02 + (0.0515p)2 + 0.38P ]
Table 2 Atomic coordinates, site occupancy and equivalent atomic displacement parameters in [CN3 H6 ]·(TaF6 ) Atom
Site
τ
x
y
z
Beq (Å2 )
Ta F(1) F(2) C N
3a 18h 18h 3b 18h
1 1/2 1/2 1 1/2
0 0.1030(5) x(F (1)) 0 0.0884(8)
0 −x −x 0 −x
0 0.1292(8) −z(F (1)) 1/2 0.492(2)
2.25(1) 4.2(1) B(F(1)) 2.3(1) 3.5(2)
Table 3 Anisotropic displacement parameters in [CN3 H6 ]·(TaF6 ) Atom
U11 = U22
U33
U23 = −U13
U12
Ta F(1), F(2) C N
0.0187(2) 0.048(3) 0.025(3) 0.037(4)
0.0480(3) 0.073(4) 0.038(5) 0.065(6)
0 −0.006(14) 0 0.002(2)
U11 /2 0.032(3) U11 /2 0.025(5)
Table 4 Selected inter-atomic distances (Å) and angles (◦ ) in [CN3 H6 ]·(TaF6 ) 6 × Ta–F(1,2)
1.894(7)
6 × F(1,2)–F(1,2) 6 × F(1,2)–F(1,2)
2.67(1) 2.69(2)
1.2, −1.5 b R = [w(|F |2 − |F |2 )2 / w o c
3b (R = 0.15). These atoms were differentiated from bond distance considerations. It was recognised here that a disorder affected the fluorine and nitrogen atom positions: the F(1)–F(2) distance was very short (2.18 Å) and it was found that F(1) and F(2) positions were related by a mirror plane at z = 0 (Fig. 1). Similarly, two CN3 groups were found to be related by a symmetry centre located on carbon atom.
3 × C–N 2 × N· · ·F(1) 2 × N· · ·F(2) 1 × N· · ·F(1) 1 × N· · ·F(1) 3 × N–C–N
1.33(1) 3.02(1) 3.21(1) 3.09(2) 3.23(2) 119.0(4)
Consequently, the atomic coordinates of F(1) and F(2) were constrained, xF(1) = xF(2) , zF(1) = −zF(2) (Table 2), together with their anisotropic thermal motion parameters (Table 3). Hydrogen atom position was found from a difference Fourier map but was not included in the refinement. The non-centrosymmetric R3m and R32 space groups were tested. The preceding disorder was maintained and
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M.A. Saada et al. / Solid State Sciences 7 (2005) 1070–1073
these space groups were excluded. Moreover, a test of optical Second Harmonic Generation, though non-conclusive, was negative. The results of the final refinements are given in Tables 2 and 3; the corresponding reliability factors are R = 0.029 and Rw = 0.077 for 312 independent reflections and 22 parameters. Selected bond distances and angles, consistent with the existence of only one out of two positions of the (TaF6 )− octahedra or CN3 groups, are given in Table 4. Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary pub-
lication no. CCDC 268906. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033 or [email protected]).
4. Structure description and discussion As a reason of fluorine and nitrogen disorder, the environments of tantalum and carbon atoms satisfy a 6/mmm ¯ point symmetry, respectively. Then, tan(Fig. 1) and a 3m talum atoms can be attributed either a regular octahedral
Table 5 Range of Ta–F and F–F distances (Å) in TaF6 , TaF7 and TaF8 polyhedra Polyhedron
Phase
Ta–F
F–F
Ref.
TaF6 (octahedron)
AgTaF6 Ag(TaF6 )2 [ClF2 ](TaF6 ) Hg4 (Ta2 F11 )2 [NH4 ]4 (Ni(TaF6 )6 ) [C5 H12 NO2 ]2 ·(TaF6 )·2H2 O [CN3 H6 ]·(TaF6 )
1.86–1.89 1.82–2.00 1.82–1.93 1.82–1.93, 2.05 1.81–1.92 1.86–1.86 1.894
2.59–2.65 2.57–2.76 2.59–2.69 2.56–2.79 2.55–2.66 2.61–2.65 2.67–2.69
[11] [12] [13] [14] [15] [16] This work
TaF7 (3 : 3 : 1) (3 : 3 : 1; 1 : 5 : 1)
K2 TaF7 [(C2 H4 NH3 )3 N]·[TaF7 ]·F [(C2 H4 NH3 )3 NH]·(TaF7 )2 ·H2 O [(C2 H4 NH3 )3 NH]·(TaF7 )2
1.92–1.98 1.93–1.97 1.89–2.02 1.86–2.01
2.34–2.91 2.38–2.69 2.32–2.94 2.28–2.95
[17] [18] [19] [19]
TaF8 (4 : 4)
Na3 TaF8
1.93–2.02
2.33–2.49
[20]
Fig. 2. [0001] projection of a layer of the structure of [CN3 H6 ]·(TaF6 ) at −0.129 z 0.462. The heights along c of Ta, F, C and N atoms are indicated. Only one of two symmetry related positions of (TaF6 )− and CN3 groups is represented in order to build a network of hydrogen bonds with 3.0 dN–F 3.21 Å. Hydrogen atom positions were found from Fourier difference maps.
M.A. Saada et al. / Solid State Sciences 7 (2005) 1070–1073
¯ symmetry) with dTa–F = 1.894(7) Å and coordination (3m dF–F ≈ 2.68 Å or a triangular prismatic coordination (3m symmetry). In this last case, three lateral F–F distances are excessively short (2.18 Å) and this coordination type is ruled out; moreover, such a coordination is never observed in fluorite tantalates (Table 5). F(1) or F(2) fluoride neighbours form regular triangles which are parallel to (0001)H . The configuration of carbon and nitrogen atoms in guanidinium cations is approximately planar with dC–N = 1.33(1) Å; the plane is parallel to (0001)H . Four hydrogen bonds establish between each NH2 group of [CN3 H6 ]+ cations and F(1) or F(2) fluoride ions of (TaF6 )− anions in the same layer (Fig. 2); they ensure the stability of the 3D network. Together with guanidinium cations, fluoride anions build 5− layers between which tantalum atoms are ∞ {[CN3 H6 ]F6 } inserted. Such a parallel arrangement of anion triangles and CN3 groups is expected to favour a high birefringence, as found in calcite CaCO3 . The analogy between isoelectronic (CO3 )2− anion and [CN3 H6 ]+ cation can be noted here. 5. Conclusion A guanidinium fluoride tantalate [CN3 H6 ]·(TaF6 ) is evidenced and the rhombohedral structure is determined by single crystal X-ray diffraction. Fluoride anions and nitrogen atoms are disordered over symmetry related positions with a fifty per cent site occupancy while tantalum and carbon atoms lie in fully occupied 3a and 3b sites. Regular (TaF6 )− octahedra and guanidinium cations can be recognized. Such a disorder can be probably attributed to microtwinning. Acknowledgements The authors wish to thank Dr. N. Chigarev and D. Mounier (ENSIM Le Mans) for the SHG test.
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References [1] C.R. Ross II, B.L. Paulsen, R.M. Nielson, S.C. Abrahams, Acta Cryst. B 54 (1998) 417. [2] M.R. Bauer, C.R. Ross II, R.M. Nielson, S.C. Abrahams, Inorg. Chem. 38 (1999) 1028. [3] N.G. Fumanova, I.A. Verin, I.E. Zanin, A.N. Zozulin, E.G. Il’in, Kristallografiya 36 (1991) 384 (in Russian). [4] A.V. Gerasimenko, B.V. Bukvetskii, V.B. Logvinova, R.L. Davidovich, Koord. Khim. 22 (1996) 584. [5] B.V. Bukvetskii, A.V. Gerasimenko, R.L. Davidovich, Koord. Khim. 16 (1990) 1479. [6] A.V. Gerasimenko, I.P. Kondratyuk, R.L. Davidovich, M.A. Medkov, B.V. Bukvetskii, Koord. Khim. 11 (1985) 566. [7] I.P. Kondratyuk, B.V. Bukvetskii, R.L. Davidovich, M.A. Medkov, Koord. Khim. 8 (1982) 218. [8] V.V. Tkachev, R.L. Davidovich, L.O. Atovmyan, Koord. Khim. 19 (1993) 288. [9] G.M. Sheldrick, in: G.M. Sheldrick, C. Krüger, R. Goddard (Eds.), “SHELXS-86” in Crystallographic Computing 3, Oxford Univ. Press, London, 1985, p. 175. [10] G.M. Sheldrick, “SHELXL-97”, A Program for Crystal Structure Determination, Göttingen University, Germany, 1997. [11] K. Matsumoto, R. Hagiwara, Y. Ito, O. Tamada, J. Fluo. Chem. 110 (2001) 117. [12] B.G. Mueller, Angew. Chem. (German edition) 99 (1987) 685. [13] A.M. Ellern, M.Yu. Antipin, A.V. Sharabarin, Yu.T. Struchkov, Zh. Neorg. Khim. 36 (1991) 2266. [14] I.D. Brown, R.J. Gillespie, K.R. Morgan, J.F. Sawyer, K.J. Schmidt, Z. Tun, P.K. Ummat, J.E. Vekris, Inorg. Chem. 26 (1987) 689. [15] R.-A. Schoening, G. Meyer, Z. Anorg. Allg. Chem. 623 (1997) 1759. [16] M. Ramos Silva, A. Matos Beja, J.A. Paixao, L. Alte da Veiga, J. Martin-Gil, Z. Kristallogr. New Cryst. Struct. 214 (1999) 477. [17] C.C. Torardi, L.H. Brixner, G. Blasse, J. Solid State Chem. 67 (1987) 21. [18] E. Goreshnik, M. Leblanc, V. Maisonneuve, J. Solid State Chem. 177 (2004) 4023. [19] M.A. Saada, A. Hémon-Ribaud, M. Leblanc, L.S. Smiri, V. Maisonneuve, J. Fluo. Chem. (2005), in press. [20] J.L. Hoard, W.J. Martin, M.E. Smith, J.F. Whitney, J. Amer. Chem. Soc. 76 (1954) 3820.
Anion and cation disorder in [CN3 H6]·(TaF6) M.A. Saada a,b , A. Hémon-Ribaud a , M. Leblanc a , V. Maisonneuve a,∗ a Laboratoire des oxydes et fluorures, UMR 6010 CNRS, faculté des sciences et techniques, Université du Maine, Avenue O. Messiaen,
72085 Le Mans Cedex 9, France b Laboratoire de chimie inorganique et structurale, faculté des sciences de Bizerte, 7021 Jarzouna, Tunisie
Received 2 April 2004; accepted 25 April 2005 Available online 1 June 2005
Abstract A new guanidinium fluoride tantalate, [CN3 H6 ]·(TaF6 ), crystallises at 50 ◦ C from a solution of Ta2 O5 in 40% aqueous HF and of ¯ space group, with the equivalent hexagonal cell aH = 8.647(1) Å, cH = guanidinium chloride. The structure is rhombohedral, R3m 8.507(2) Å, Z = 3 and R = 0.029, Rw = 0.077 for 312 reflections. The three-dimensional network is built up from parallel (0001)H layers 5− between which tantalum atoms are inserted. These tantalum atoms, in 3a sites, adopt an octahedral coordination with ∞ ([CN3 H6 ]F6 ) dTa–F = 1.894(7) Å. (TaF6 )− anions are disordered over (0001)H mirror plane related positions while guanidinium cations are disordered over centrosymmetric positions; site occupancy of fluoride and nitrogen sites is fifty per cent. 2005 Elsevier SAS. All rights reserved. Keywords: Hybrid fluoride; Guanidine; Tantalate; X-ray diffraction
1. Introduction In the search of open structures in fluorides, high valence cations, Zr+IV and Ta+V , were recently examined and associated with amine cations in fluoride solutions [1–3]. Only one 3D open framework was evidenced in a guanidinium zirconate, (H3 O)·[CN3 H6 ]5 ·(ZrF5 )6 [4], while numerous phases with isolated fluoride polyanions were obtained [5– 8]. Then, the study of the Ta2 O5 -guanidine-aqueous HF system was undertaken over large concentration domains of the starting materials at low crystallisation temperatures. One new phase, which exhibits anion and cation disorder, was evidenced; it is reported here.
2. Experimental Ta2 O5 powder (1 g) was first dissolved under stirring in 40% aqueous HF (40 ml) at 80 ◦ C during 4–5 h. After cool* Corresponding author. Tel.: +33 2 43 83 35 61; Fax: +33 2 43 83 35 06.
E-mail address: [email protected] (V. Maisonneuve). 1293-2558/$ – see front matter 2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.04.008
ing to room temperature, guanidinium chloride (0.191 g) was added. Single crystals of [CN3 H6 ]·(TaF6 ), which are moisture sensitive, were grown by the slow evaporation of the solution at 50 ◦ C over 2 days. Truncated (0001)H platelets were selected by optical examination and single crystal diffraction data were obtained on a Siemens AED2 four-circle diffractometer.
3. Structure determination Crystal data and the conditions of the intensity measurements are reported in Table 1 for [CN3 H6 ]·(TaF6 ). Absorption effects were corrected by the Gauss method. The structure was determined in the equivalent hexagonal cell by direct methods using SHELXS-86 [9] and refined with SHELXL-97 [10]. The rhombohedral cell parameters are 3 aR = 5.742(1) Å, αR = 97.17(2)◦ , VR = 183.7(1) Å . One heavy atom position was found in 3a special position ¯ space group with TREF option of SHELXS(0, 0, 0) of R3m 86 and was attributed to Ta. Then, successive refinements and Fourier difference maps allowed to locate F(1), F(2), N in 18h positions with a fifty per cent site occupancy and C in
M.A. Saada et al. / Solid State Sciences 7 (2005) 1070–1073
1071
Fig. 1. View of the disordered environment of tantalum (top left) or carbon (top right) atoms in [CN3 H6 ]·(TaF6 ); possible orientations of the resulting (TaF6 )− octahedra (bottom left and center) and representation of one possible orientation of a triangular (TaF6 )− prism (bottom right).
Table 1 Crystallographic data of [CN3 H6 ]·(TaF6 ) Molar weight (g·mol−1 ) Crystal size (mm3 ) Crystal system, space group aH (Å) cH (Å) V (Å3 ), Z ρcalc (g·cm−3 ) Temperature (K) Four circle diffractometer Monochromator 2θ range (◦ ) Reflections measured/unique/used (I > 2σ (I )) (hkl) limits (two centric ¯ independent sets in 3) Rint Scan mode Absorption correction, Amin , Amax Parameters refined (on F2 ) a R/b R w Goodness of fit Weighting scheme (p = [F02 + 2Fc2 ]/3) Residues of Fourier difference (e Å−3 ) a R = ||F | − |F ||/ |F |; o c o w(Fo2 )2 ]1/2 .
355.02 0.19 × 0.19 × 0.09 ¯ Rhombohedral, R3m 8.647(1) 8.507(2) 550.9(3), 3 3.21 298 Siemens AED2 graphite 2–70 1080/316/312 |h| 13; |k| 13; |l| 13 0.056 ω − 2θ Gaussian, 0.137, 0.299 22 0.029/0.077 1.13 1/[σ 2 (F02 + (0.0515p)2 + 0.38P ]
Table 2 Atomic coordinates, site occupancy and equivalent atomic displacement parameters in [CN3 H6 ]·(TaF6 ) Atom
Site
τ
x
y
z
Beq (Å2 )
Ta F(1) F(2) C N
3a 18h 18h 3b 18h
1 1/2 1/2 1 1/2
0 0.1030(5) x(F (1)) 0 0.0884(8)
0 −x −x 0 −x
0 0.1292(8) −z(F (1)) 1/2 0.492(2)
2.25(1) 4.2(1) B(F(1)) 2.3(1) 3.5(2)
Table 3 Anisotropic displacement parameters in [CN3 H6 ]·(TaF6 ) Atom
U11 = U22
U33
U23 = −U13
U12
Ta F(1), F(2) C N
0.0187(2) 0.048(3) 0.025(3) 0.037(4)
0.0480(3) 0.073(4) 0.038(5) 0.065(6)
0 −0.006(14) 0 0.002(2)
U11 /2 0.032(3) U11 /2 0.025(5)
Table 4 Selected inter-atomic distances (Å) and angles (◦ ) in [CN3 H6 ]·(TaF6 ) 6 × Ta–F(1,2)
1.894(7)
6 × F(1,2)–F(1,2) 6 × F(1,2)–F(1,2)
2.67(1) 2.69(2)
1.2, −1.5 b R = [w(|F |2 − |F |2 )2 / w o c
3b (R = 0.15). These atoms were differentiated from bond distance considerations. It was recognised here that a disorder affected the fluorine and nitrogen atom positions: the F(1)–F(2) distance was very short (2.18 Å) and it was found that F(1) and F(2) positions were related by a mirror plane at z = 0 (Fig. 1). Similarly, two CN3 groups were found to be related by a symmetry centre located on carbon atom.
3 × C–N 2 × N· · ·F(1) 2 × N· · ·F(2) 1 × N· · ·F(1) 1 × N· · ·F(1) 3 × N–C–N
1.33(1) 3.02(1) 3.21(1) 3.09(2) 3.23(2) 119.0(4)
Consequently, the atomic coordinates of F(1) and F(2) were constrained, xF(1) = xF(2) , zF(1) = −zF(2) (Table 2), together with their anisotropic thermal motion parameters (Table 3). Hydrogen atom position was found from a difference Fourier map but was not included in the refinement. The non-centrosymmetric R3m and R32 space groups were tested. The preceding disorder was maintained and
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M.A. Saada et al. / Solid State Sciences 7 (2005) 1070–1073
these space groups were excluded. Moreover, a test of optical Second Harmonic Generation, though non-conclusive, was negative. The results of the final refinements are given in Tables 2 and 3; the corresponding reliability factors are R = 0.029 and Rw = 0.077 for 312 independent reflections and 22 parameters. Selected bond distances and angles, consistent with the existence of only one out of two positions of the (TaF6 )− octahedra or CN3 groups, are given in Table 4. Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary pub-
lication no. CCDC 268906. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033 or [email protected]).
4. Structure description and discussion As a reason of fluorine and nitrogen disorder, the environments of tantalum and carbon atoms satisfy a 6/mmm ¯ point symmetry, respectively. Then, tan(Fig. 1) and a 3m talum atoms can be attributed either a regular octahedral
Table 5 Range of Ta–F and F–F distances (Å) in TaF6 , TaF7 and TaF8 polyhedra Polyhedron
Phase
Ta–F
F–F
Ref.
TaF6 (octahedron)
AgTaF6 Ag(TaF6 )2 [ClF2 ](TaF6 ) Hg4 (Ta2 F11 )2 [NH4 ]4 (Ni(TaF6 )6 ) [C5 H12 NO2 ]2 ·(TaF6 )·2H2 O [CN3 H6 ]·(TaF6 )
1.86–1.89 1.82–2.00 1.82–1.93 1.82–1.93, 2.05 1.81–1.92 1.86–1.86 1.894
2.59–2.65 2.57–2.76 2.59–2.69 2.56–2.79 2.55–2.66 2.61–2.65 2.67–2.69
[11] [12] [13] [14] [15] [16] This work
TaF7 (3 : 3 : 1) (3 : 3 : 1; 1 : 5 : 1)
K2 TaF7 [(C2 H4 NH3 )3 N]·[TaF7 ]·F [(C2 H4 NH3 )3 NH]·(TaF7 )2 ·H2 O [(C2 H4 NH3 )3 NH]·(TaF7 )2
1.92–1.98 1.93–1.97 1.89–2.02 1.86–2.01
2.34–2.91 2.38–2.69 2.32–2.94 2.28–2.95
[17] [18] [19] [19]
TaF8 (4 : 4)
Na3 TaF8
1.93–2.02
2.33–2.49
[20]
Fig. 2. [0001] projection of a layer of the structure of [CN3 H6 ]·(TaF6 ) at −0.129 z 0.462. The heights along c of Ta, F, C and N atoms are indicated. Only one of two symmetry related positions of (TaF6 )− and CN3 groups is represented in order to build a network of hydrogen bonds with 3.0 dN–F 3.21 Å. Hydrogen atom positions were found from Fourier difference maps.
M.A. Saada et al. / Solid State Sciences 7 (2005) 1070–1073
¯ symmetry) with dTa–F = 1.894(7) Å and coordination (3m dF–F ≈ 2.68 Å or a triangular prismatic coordination (3m symmetry). In this last case, three lateral F–F distances are excessively short (2.18 Å) and this coordination type is ruled out; moreover, such a coordination is never observed in fluorite tantalates (Table 5). F(1) or F(2) fluoride neighbours form regular triangles which are parallel to (0001)H . The configuration of carbon and nitrogen atoms in guanidinium cations is approximately planar with dC–N = 1.33(1) Å; the plane is parallel to (0001)H . Four hydrogen bonds establish between each NH2 group of [CN3 H6 ]+ cations and F(1) or F(2) fluoride ions of (TaF6 )− anions in the same layer (Fig. 2); they ensure the stability of the 3D network. Together with guanidinium cations, fluoride anions build 5− layers between which tantalum atoms are ∞ {[CN3 H6 ]F6 } inserted. Such a parallel arrangement of anion triangles and CN3 groups is expected to favour a high birefringence, as found in calcite CaCO3 . The analogy between isoelectronic (CO3 )2− anion and [CN3 H6 ]+ cation can be noted here. 5. Conclusion A guanidinium fluoride tantalate [CN3 H6 ]·(TaF6 ) is evidenced and the rhombohedral structure is determined by single crystal X-ray diffraction. Fluoride anions and nitrogen atoms are disordered over symmetry related positions with a fifty per cent site occupancy while tantalum and carbon atoms lie in fully occupied 3a and 3b sites. Regular (TaF6 )− octahedra and guanidinium cations can be recognized. Such a disorder can be probably attributed to microtwinning. Acknowledgements The authors wish to thank Dr. N. Chigarev and D. Mounier (ENSIM Le Mans) for the SHG test.
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