The ternary system SrBr
2-CdBr
2-H
2O was investigated at room temperature. The title phase, SrCd
2Br
6·8H
2O, has been isolated from this system and its structure determined by single-crystal X-ray diffraction. The structure consists of infinite double chains of CdBr
6 octahedra and chains of Sr(H
2O)
9 polyhedra packed along the
b axis. The interaction between these two isolated chains occurs through O-H
O and O-H
Br hydrogen bonds. The structure is compared with that of SrCd
2Cl
6·8H
2O.
Supporting information
Single crystals of SrCd2Br6·8H2O were prepared from a heated mixture of strontium carbonate and cadmium bromide in HBr in a molar ratio of 1/1 at 363 K. This solution was cooled to room temperature and allowed to evaporate. A few days later, colourless hygroscopic needle-shaped crystals were obtained. Characterizations of the compound were performed by X-ray powder patterns and elemental chemical analysis. The water content was determined by thermogravimetric analysis and the formula was confirmed by density measurements and refinement of the crystal structure. Differential thermal analysis and thermogravimetric analysis were performed on polycrystalline samples using a SETARAM TGDTA92 instrument between 303 and 573 K. Seven sharp weight losses were detected between 333 and 498 K, of 7.49, 5.77 and 1.92% This is only three values - are data missing here? of the original weight, and these were assigned to the loss of two water molecules and six water molecules of crystallization per unit formula.
The absolute structure parameter was calculated using SHELX97 (Sheldrick, 1997). The Flack parameter (Flack, 1983) is 0.336 (17). From the full text of Flack & Bernardinelli (2000), one can understand that the standard uncertainty of 0.017 indicates that the inversion-distinguishing power is strong and the domains around the Flack parameter should be well defined and clearly distinguishable from one another. The Flack parameter of 0.336 (17) indicates that the crystal is twinned by inversion (Flack & Bernardinelli, 1999) and it is not possible to determine the absolute structure of such a crystal, which is considered as constituted of a mixture of inverted structures. H atoms were located in geometrically idealized positions after their location, and were given riding constraints to their positional and thermal parameters, with O—H distances of 0.9 Å, O—H—O angles of 106°, Sr—O—H angles of 120° and Uiso(H) = 1.5Ueq(O).
Data collection: Please provide missing information; cell refinement: Please provide missing information; data reduction: JANA2000 (Petříček & Dušek, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Pennington, 1999); software used to prepare material for publication: WinGX (Farrugia, 1999).
Strontium hexabromodicadmate(II) octahydrate
top
Crystal data top
SrCd2Br6·8H2O | F(000) = 848 |
Mr = 936.01 | Dx = 3.441 Mg m−3 |
Orthorhombic, P21212 | Mo Kα radiation, λ = 0.71069 Å |
Hall symbol: P 2 2ab | Cell parameters from 20 reflections |
a = 25.247 (2) Å | θ = 2–11° |
b = 4.0827 (10) Å | µ = 18.56 mm−1 |
c = 8.764 (2) Å | T = 293 K |
V = 903.4 (3) Å3 | Needle, colourless |
Z = 2 | 0.20 × 0.06 × 0.04 mm |
Data collection top
Oxford Instruments point detector diffractometer | 1494 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.063 |
Graphite monochromator | θmax = 27.0°, θmin = 1.6° |
θ/2θ scans | h = 0→32 |
Absorption correction: gaussian (JANA2000; Petříček & Dušek, 2000) | k = −5→5 |
Tmin = 0.135, Tmax = 0.408 | l = −11→11 |
3930 measured reflections | 3 standard reflections every 100 reflections |
1982 independent reflections | intensity decay: 0.9% |
Refinement top
Refinement on F2 | Hydrogen site location: inferred from neighbouring sites |
Least-squares matrix: full | H-atom parameters constrained |
R[F2 > 2σ(F2)] = 0.029 | w = 1/[σ2(Fo2) + (0.0325P)2] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.070 | (Δ/σ)max = 0.001 |
S = 1.07 | Δρmax = 1.43 e Å−3 |
1982 reflections | Δρmin = −1.53 e Å−3 |
81 parameters | Extinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
0 restraints | Extinction coefficient: 0.0018 (2) |
Primary atom site location: structure-invariant direct methods | Absolute structure: Flack (1983), with 767 Friedel pairs |
Secondary atom site location: difference Fourier map | Absolute structure parameter: 0.336 (17) |
Crystal data top
SrCd2Br6·8H2O | V = 903.4 (3) Å3 |
Mr = 936.01 | Z = 2 |
Orthorhombic, P21212 | Mo Kα radiation |
a = 25.247 (2) Å | µ = 18.56 mm−1 |
b = 4.0827 (10) Å | T = 293 K |
c = 8.764 (2) Å | 0.20 × 0.06 × 0.04 mm |
Data collection top
Oxford Instruments point detector diffractometer | 1494 reflections with I > 2σ(I) |
Absorption correction: gaussian (JANA2000; Petříček & Dušek, 2000) | Rint = 0.063 |
Tmin = 0.135, Tmax = 0.408 | 3 standard reflections every 100 reflections |
3930 measured reflections | intensity decay: 0.9% |
1982 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.029 | H-atom parameters constrained |
wR(F2) = 0.070 | Δρmax = 1.43 e Å−3 |
S = 1.07 | Δρmin = −1.53 e Å−3 |
1982 reflections | Absolute structure: Flack (1983), with 767 Friedel pairs |
81 parameters | Absolute structure parameter: 0.336 (17) |
0 restraints | |
Special details top
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
Cd | 0.69331 (2) | −0.54268 (14) | −0.13705 (6) | 0.02159 (14) | |
Sr | 0.5000 | 0.0000 | 0.33421 (10) | 0.0172 (2) | |
Br1 | 0.58737 (3) | −0.5106 (2) | −0.16244 (9) | 0.02628 (19) | |
Br2 | 0.71132 (3) | −0.04889 (19) | −0.33916 (9) | 0.02064 (18) | |
Br3 | 0.69238 (3) | −0.0291 (2) | 0.08383 (8) | 0.02107 (17) | |
O1 | 0.5000 | −0.5000 | 0.5358 (8) | 0.0253 (16) | |
O2 | 0.5000 | 0.0000 | 0.0408 (8) | 0.0287 (18) | |
O3 | 0.56097 (17) | 0.5021 (16) | 0.2322 (6) | 0.0252 (11) | |
O4 | 0.5845 (2) | −0.9946 (16) | 0.5030 (6) | 0.0294 (12) | |
O5 | 0.6538 (2) | −0.5404 (16) | 0.3864 (6) | 0.0289 (13) | |
H1A | 0.5187 | −0.3844 | 0.6064 | 0.050* | |
H2A | 0.4724 | 0.0052 | −0.0247 | 0.050* | |
H3A | 0.5667 | 0.5671 | 0.1338 | 0.050* | |
H3B | 0.5854 | 0.6059 | 0.2899 | 0.050* | |
H4A | 0.5846 | −0.7735 | 0.5095 | 0.050* | |
H4B | 0.5901 | −1.0699 | 0.5999 | 0.050* | |
H5A | 0.6772 | −0.3873 | 0.3452 | 0.050* | |
H5B | 0.6758 | −0.6962 | 0.4362 | 0.050* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
Cd | 0.0232 (3) | 0.0157 (3) | 0.0259 (3) | 0.0003 (2) | −0.0028 (2) | 0.0008 (3) |
Sr | 0.0191 (4) | 0.0128 (4) | 0.0196 (4) | −0.0001 (4) | 0.000 | 0.000 |
Br1 | 0.0220 (4) | 0.0273 (4) | 0.0295 (4) | 0.0000 (3) | −0.0012 (3) | −0.0005 (5) |
Br2 | 0.0227 (3) | 0.0184 (4) | 0.0208 (4) | −0.0001 (3) | −0.0012 (3) | 0.0001 (4) |
Br3 | 0.0245 (4) | 0.0177 (4) | 0.0210 (4) | −0.0002 (4) | −0.0009 (3) | −0.0002 (3) |
O1 | 0.024 (4) | 0.029 (5) | 0.024 (4) | −0.004 (4) | 0.000 | 0.000 |
O2 | 0.033 (4) | 0.037 (5) | 0.016 (4) | 0.005 (4) | 0.000 | 0.000 |
O3 | 0.022 (3) | 0.029 (3) | 0.024 (3) | 0.004 (3) | 0.0002 (19) | −0.001 (3) |
O4 | 0.029 (3) | 0.026 (3) | 0.033 (3) | −0.002 (3) | −0.011 (2) | 0.002 (4) |
O5 | 0.022 (3) | 0.030 (3) | 0.035 (3) | 0.002 (2) | 0.000 (2) | 0.005 (3) |
Geometric parameters (Å, º) top
Cd—Br1 | 2.6870 (9) | Sr—Sri | 4.0827 (10) |
Cd—Br2 | 2.7218 (10) | Sr—H1A | 2.894 |
Cd—Br2i | 2.7596 (11) | Br2—Cdiii | 2.7596 (11) |
Cd—Br3i | 2.7734 (11) | Br3—Cdiii | 2.7734 (11) |
Cd—Br3 | 2.8537 (11) | Br3—Cdvii | 2.9240 (9) |
Cd—Br3ii | 2.9240 (9) | O1—Sri | 2.700 (4) |
Sr—O2 | 2.571 (7) | O1—H1A | 0.91 |
Sr—O4iii | 2.597 (5) | O2—H2A | 0.90 |
Sr—O4iv | 2.597 (5) | O3—Sriii | 2.702 (6) |
Sr—O1iii | 2.700 (4) | O3—H3A | 0.913 |
Sr—O1 | 2.700 (4) | O3—H3B | 0.904 |
Sr—O3i | 2.702 (6) | O4—Sri | 2.597 (5) |
Sr—O3v | 2.702 (6) | O4—H4A | 0.905 |
Sr—O3vi | 2.715 (6) | O4—H4B | 0.913 |
Sr—O3 | 2.715 (6) | O5—H5A | 0.932 |
Sr—Sriii | 4.0827 (10) | O5—H5B | 0.950 |
| | | |
Br1—Cd—Br2 | 94.38 (3) | O3vi—Sr—O3 | 141.5 (2) |
Br1—Cd—Br2i | 98.47 (3) | O2—Sr—Sriii | 90.0 |
Br2—Cd—Br2i | 96.29 (3) | O4iii—Sr—Sriii | 89.52 (15) |
Br1—Cd—Br3i | 94.84 (3) | O4iv—Sr—Sriii | 90.48 (15) |
Br2—Cd—Br3i | 170.40 (3) | O1iii—Sr—Sriii | 40.88 (11) |
Br2i—Cd—Br3i | 85.01 (3) | O1—Sr—Sriii | 139.12 (11) |
Br1—Cd—Br3 | 90.70 (3) | O3i—Sr—Sriii | 138.79 (12) |
Br2—Cd—Br3 | 84.18 (3) | O3v—Sr—Sriii | 41.21 (12) |
Br2i—Cd—Br3 | 170.74 (3) | O3vi—Sr—Sriii | 139.03 (11) |
Br3i—Cd—Br3 | 93.02 (3) | O3—Sr—Sriii | 40.97 (11) |
Br1—Cd—Br3ii | 174.12 (3) | O2—Sr—Sri | 90.0 |
Br2—Cd—Br3ii | 85.69 (3) | O4iii—Sr—Sri | 90.48 (15) |
Br2i—Cd—Br3ii | 87.36 (3) | O4iv—Sr—Sri | 89.52 (15) |
Br3i—Cd—Br3ii | 84.87 (3) | O1iii—Sr—Sri | 139.12 (11) |
Br3—Cd—Br3ii | 83.45 (3) | O1—Sr—Sri | 40.88 (11) |
O2—Sr—O4iii | 124.73 (12) | O3i—Sr—Sri | 41.21 (12) |
O2—Sr—O4iv | 124.73 (12) | O3v—Sr—Sri | 138.79 (12) |
O4iii—Sr—O4iv | 110.5 (2) | O3vi—Sr—Sri | 40.97 (11) |
O2—Sr—O1iii | 130.88 (11) | O3—Sr—Sri | 139.03 (11) |
O4iii—Sr—O1iii | 67.71 (15) | Sriii—Sr—Sri | 180.0 |
O4iv—Sr—O1iii | 68.50 (15) | O2—Sr—H1A | 145.5 |
O2—Sr—O1 | 130.88 (11) | O4iii—Sr—H1A | 53.2 |
O4iii—Sr—O1 | 68.50 (15) | O4iv—Sr—H1A | 70.1 |
O4iv—Sr—O1 | 67.71 (15) | O1iii—Sr—H1A | 82.6 |
O1iii—Sr—O1 | 98.2 (2) | O1—Sr—H1A | 18.3 |
O2—Sr—O3i | 70.68 (11) | O3i—Sr—H1A | 76.8 |
O4iii—Sr—O3i | 74.14 (17) | O3v—Sr—H1A | 140.7 |
O4iv—Sr—O3i | 130.57 (19) | O3vi—Sr—H1A | 87.40 (11) |
O1iii—Sr—O3i | 141.75 (11) | O3—Sr—H1A | 126.0 |
O1—Sr—O3i | 69.38 (15) | Sriii—Sr—H1A | 122.8 |
O2—Sr—O3v | 70.68 (11) | Sri—Sr—H1A | 57.2 |
O4iii—Sr—O3v | 130.57 (19) | Cd—Br2—Cdiii | 96.29 (3) |
O4iv—Sr—O3v | 74.14 (17) | Cdiii—Br3—Cd | 93.02 (3) |
O1iii—Sr—O3v | 69.38 (15) | Cdiii—Br3—Cdvii | 96.69 (3) |
O1—Sr—O3v | 141.75 (11) | Cd—Br3—Cdvii | 94.95 (3) |
O3i—Sr—O3v | 141.4 (2) | Sri—O1—Sr | 98.2 (2) |
O2—Sr—O3vi | 70.77 (11) | Sri—O1—H1A | 146.8 |
O4iii—Sr—O3vi | 131.29 (18) | Sr—O1—H1A | 93.0 |
O4iv—Sr—O3vi | 73.46 (16) | Sr—O2—H2A | 129.5 |
O1iii—Sr—O3vi | 141.85 (11) | Sriii—O3—Sr | 97.82 (15) |
O1—Sr—O3vi | 69.18 (14) | Sriii—O3—H3A | 100.6 |
O3i—Sr—O3vi | 69.27 (16) | Sr—O3—H3A | 128.3 |
O3v—Sr—O3vi | 97.82 (15) | Sriii—O3—H3B | 81.5 |
O2—Sr—O3 | 70.77 (11) | Sr—O3—H3B | 123.8 |
O4iii—Sr—O3 | 73.46 (16) | H3A—O3—H3B | 106.5 |
O4iv—Sr—O3 | 131.29 (18) | Sri—O4—H4A | 92.6 |
O1iii—Sr—O3 | 69.18 (14) | Sri—O4—H4B | 130.8 |
O1—Sr—O3 | 141.85 (11) | H4A—O4—H4B | 106.1 |
O3i—Sr—O3 | 97.82 (15) | H5A—O5—H5B | 104.9 |
O3v—Sr—O3 | 69.27 (16) | | |
Symmetry codes: (i) x, y−1, z; (ii) −x+3/2, y−1/2, −z; (iii) x, y+1, z; (iv) −x+1, −y−1, z; (v) −x+1, −y+1, z; (vi) −x+1, −y, z; (vii) −x+3/2, y+1/2, −z. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1A···O4iii | 0.91 | 2.47 | 2.982 (6) | 116 |
O1—H1A···Br1viii | 0.91 | 2.72 | 3.444 (5) | 138 |
O2—H2A···Br1iv | 0.90 | 2.80 | 3.468 (4) | 132 |
O2—H2A···Br1vi | 0.90 | 2.83 | 3.519 (4) | 134 |
O3—H3A···O2iii | 0.91 | 2.57 | 3.052 (7) | 113 |
O3—H3A···Br1iii | 0.91 | 2.67 | 3.522 (5) | 156 |
O3—H3B···O4ix | 0.90 | 2.48 | 3.195 (8) | 136 |
O3—H3B···O5iii | 0.90 | 2.01 | 2.712 (7) | 133 |
O4—H4A···O1 | 0.91 | 2.42 | 2.952 (6) | 118 |
O4—H4A···O5 | 0.91 | 2.26 | 2.747 (9) | 113 |
O4—H4B···Br1viii | 0.91 | 3.09 | 3.536 (6) | 112 |
O5—H5A···Br3 | 0.93 | 2.74 | 3.512 (6) | 140 |
O5—H5A···Br2ii | 0.93 | 2.89 | 3.430 (5) | 118 |
O5—H5B···Br2x | 0.95 | 2.60 | 3.493 (6) | 157 |
Symmetry codes: (ii) −x+3/2, y−1/2, −z; (iii) x, y+1, z; (iv) −x+1, −y−1, z; (vi) −x+1, −y, z; (viii) x, y, z+1; (ix) x, y+2, z; (x) x, y−1, z+1. |
Experimental details
Crystal data |
Chemical formula | SrCd2Br6·8H2O |
Mr | 936.01 |
Crystal system, space group | Orthorhombic, P21212 |
Temperature (K) | 293 |
a, b, c (Å) | 25.247 (2), 4.0827 (10), 8.764 (2) |
V (Å3) | 903.4 (3) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 18.56 |
Crystal size (mm) | 0.20 × 0.06 × 0.04 |
|
Data collection |
Diffractometer | Oxford Instruments point detector diffractometer |
Absorption correction | Gaussian (JANA2000; Petříček & Dušek, 2000) |
Tmin, Tmax | 0.135, 0.408 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 3930, 1982, 1494 |
Rint | 0.063 |
(sin θ/λ)max (Å−1) | 0.639 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.029, 0.070, 1.07 |
No. of reflections | 1982 |
No. of parameters | 81 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 1.43, −1.53 |
Absolute structure | Flack (1983), with 767 Friedel pairs |
Absolute structure parameter | 0.336 (17) |
Selected geometric parameters (Å, º) topCd—Br1 | 2.6870 (9) | Sr—O4iv | 2.597 (5) |
Cd—Br2 | 2.7218 (10) | Sr—O1iii | 2.700 (4) |
Cd—Br2i | 2.7596 (11) | Sr—O1 | 2.700 (4) |
Cd—Br3i | 2.7734 (11) | Sr—O3i | 2.702 (6) |
Cd—Br3 | 2.8537 (11) | Sr—O3v | 2.702 (6) |
Cd—Br3ii | 2.9240 (9) | Sr—O3vi | 2.715 (6) |
Sr—O2 | 2.571 (7) | Sr—O3 | 2.715 (6) |
Sr—O4iii | 2.597 (5) | | |
| | | |
Br1—Cd—Br2 | 94.38 (3) | O4iii—Sr—O4iv | 110.5 (2) |
Br2—Cd—Br3i | 170.40 (3) | O2—Sr—O1iii | 130.88 (11) |
Br2i—Cd—Br3i | 85.01 (3) | O4iii—Sr—O1iii | 67.71 (15) |
Br2i—Cd—Br3 | 170.74 (3) | O2—Sr—O3i | 70.68 (11) |
Br1—Cd—Br3ii | 174.12 (3) | O4iii—Sr—O3i | 74.14 (17) |
Br3—Cd—Br3ii | 83.45 (3) | O1iii—Sr—O3i | 141.75 (11) |
O2—Sr—O4iii | 124.73 (12) | O2—Sr—O3v | 70.68 (11) |
O2—Sr—O4iv | 124.73 (12) | | |
Symmetry codes: (i) x, y−1, z; (ii) −x+3/2, y−1/2, −z; (iii) x, y+1, z; (iv) −x+1, −y−1, z; (v) −x+1, −y+1, z; (vi) −x+1, −y, z. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1A···O4iii | 0.91 | 2.47 | 2.982 (6) | 116 |
O1—H1A···Br1vii | 0.91 | 2.72 | 3.444 (5) | 138 |
O2—H2A···Br1iv | 0.90 | 2.80 | 3.468 (4) | 132 |
O2—H2A···Br1vi | 0.90 | 2.83 | 3.519 (4) | 134 |
O3—H3A···O2iii | 0.91 | 2.57 | 3.052 (7) | 113 |
O3—H3A···Br1iii | 0.91 | 2.67 | 3.522 (5) | 156 |
O3—H3B···O4viii | 0.90 | 2.48 | 3.195 (8) | 136 |
O3—H3B···O5iii | 0.90 | 2.01 | 2.712 (7) | 133 |
O4—H4A···O1 | 0.91 | 2.42 | 2.952 (6) | 118 |
O4—H4A···O5 | 0.91 | 2.26 | 2.747 (9) | 113 |
O4—H4B···Br1vii | 0.91 | 3.09 | 3.536 (6) | 112 |
O5—H5A···Br3 | 0.93 | 2.74 | 3.512 (6) | 140 |
O5—H5A···Br2ii | 0.93 | 2.89 | 3.430 (5) | 118 |
O5—H5B···Br2ix | 0.95 | 2.60 | 3.493 (6) | 157 |
Symmetry codes: (ii) −x+3/2, y−1/2, −z; (iii) x, y+1, z; (iv) −x+1, −y−1, z; (vi) −x+1, −y, z; (vii) x, y, z+1; (viii) x, y+2, z; (ix) x, y−1, z+1. |
MCl2—CdCl2—H2O systems (where M is Mg, Ca and Ba) have been intensively investigated by several authors (Bassett & Strain, 1952; Bernath & Lechner, 1940; Moshinskii & Tikhomirova, 1975). Structural studies of the phases obtained from these systems were undertaken to compare the coordination of the cations present together with the water molecules, and to study the arrangement of the polyhedra and particular properties (pseudo-symmetry and twinning) (Ledésert & Monier, 1981; Leligny & Monier, 1978, 1982, 1983, 1989a,b; Ledésert, 1985; Ledésert & Raveau, 1987). In our previous work (Yahyaoui et al., 2002) investigating the SrCl2—CdCl2—H2O ternary system, we have isolated the hydrated phase SrCd2Cl6·8H2O and determined its structure. The room-temperature phase of this compound has a triclinic structure, space group P1, and is characterized by a very persistent occurrence of twinning by pseudo-monoclinic symmetry; the twin element was found to be a twofold axis, [001]. At high temperature, SrCd2Cl6·8H2O exhibits a structural phase transition at 323 K related to a higher symmetry, accompanied by a disappearance of the twin. In the present study, our interest in the SrBr2—CdBr2—H2O system, which has not been previously reported in the literature, is mainly based on the structure determination of the new double-salt hydrate SrCd2Br6·8H2O, in order to understand the influence of the substitution of the Cl− anion by the Br− anion on the structural properties. We report here the synthesis of SrCd2Br6·8H2O and X-ray diffraction measurements, accompanied by thermogravimetric and differential thermal analyses. Please confirm the two acronyms have been expanded correctly.
The structural arrangement in SrCd2Br6·8H2O at room temperature (Fig. 1) seems to be the same as that of SrCd2Cl6·8H2O, with an increase in the unit-cell volume due to the large size of the Br− ions. In the structure of SrCd2Br6·8H2O, two types of polyhedra around the Cd2+ and Sr2+ cations are present. The Cd2+ atoms are each bonded to six Br neighbours to form irregular CdBr6 octahedra (Table 1), and these CdBr6 octahedra are held together to generate endless double chains running along the [010] direction (Fig. 2), with four edges (Br3—Br3 × 2 and Br2—Br3 × 2) shared by four adjacent octahedra and an average Cd—Br distance of 2.7865 Å. Comparing the CdBr6 octahedra in SrCd2Br6·8H2O with those in SrCd2Cl6·8H2O, we note that the Cd—Br distances are longer than the Cd—Cl distances, leading to an elongated octahedron in SrCd2Br6·8H2O, as opposed to the compressed one in SrCd2Cl6·8H2O (mean Cd—Cl 2.6339 Å).
The coordination sphere of the Sr consists only of O atoms, namely two O1, one O2, four O3 and two O4 atoms belonging to nine water molecules. Thus the coordination number is nine. The average Sr—O distance is 2.6665 Å. However, we noted seven O and two Cl atoms surrounding the Sr2+ cations in SrCd2Cl6·8H2O, with the mean Sr—O distance being 2.6234 Å. The Sr polyhedron can be described as a nearly regular triangular prism, with three faces capped by three O atoms, one O2 and two O4 [Sr—O2 2.571 (7) Å and Sr—O4 2.597 (5) Å × 2; Table 1, Fig. 3]. The two triangular bases of this polyhedron are each formed by one O1 and two O3 atoms [O1—O3 3.074 (6) Å × 2, O3—O3 3.079 (9) Å]. The Sr(H2O)9 polyhedra are connected together to form simple chains with two shared triangular bases, with two other polyhedra in the [010] direction (Fig. 2). This chain arrangement of Sr polyhedra is found for the first time in SrCd2Br6·8H2O. Thus it is the first double salt, belonging to the family of double-salt hydrates of Cd obtained from MX2—CdX2—H2O systems (where M is Mg, Ca, Sr and Ba, and X is Cl or Br), containing these chains of alkaline-earth polyhedra.
We now make a comparison between the structures of SrCd2Br6·8H2O and of SrCd2Cl6·8H2O, which was found to be twinned, and which exhibits a pseudo-monoclinic symmetry involving triclinic crystals defined by a twofold [001] axis and a twinned monoclinic face-centred Bravais cell described by the parameters a1, b1, c1, α1, β1 and γ1, which are deduced from the triclinic parameters by the transformations a1 = a, b1 = c and c1 = a-4 b-c. An examination of the orthorhombic structure of SrCd2Br6·8H2O by LEPAGE using PLATON (Spek, 1990) evidenced the absence of any twin phenomena. We can clearly observe inside the double chains that the Cd, Cl and O atoms show two-by-two correspondence, via a c/2 pseudotranslation in the case of the Cl− compound and a b/2 translation in the Br− compound. The twin element, that was a helicoidal binary [001] axis, becomes a real helicoidal binary axis [010] in SrCd2Br6·8H2O.
The structure of SrCd2Br6·8H2O contains four water molecules, H2O1, H2O2, H2O3 and H2O4, surrounding Sr atoms, and one water molecule, H2O5, which is not coordinated to any cations. Thus there are two categories of water molecules, which are differently coordinated to Sr, Br and O atoms.
The cohesion of the structure of SrCd2Br6·8H2O is ensured by the presence of two kinds of hydrogen bonds, O—H···Br and O—H···O. The first type connects different polyhedra through all the Br atoms, and the distances range from 3.444 (5) to 3.536 (6) Å. As seen in Table 2, the three Br atoms link differently to the O atoms. Atom Br1 establishes the most hydrogen bonds, with nearly all O atoms coordinated to Sr2+ cations (O1, O2, O3 and O4). Atom Br2 has two hydrogen bonds to atom O5 of the free water molecule H2O5, while atom Br3 establishes only one hydrogen bond to atom O5. There are O—H···O hydrogen bonds between nearly all the water molecules present in the structure. The shortest one links atom O5 to atoms O3 and O4, with distances of 2.712 (7) and 2.747 (9) Å, respectively. It is also noted that a comparison of the hydrogen bonds in SrCd2Br6·8H2O and SrCd2Cl6·8H2O shows an increase in the O—H···Br Should this be O—H···X? bond lengths and a decrease in the O—H···O bond lengths, which may be explained by the decrease in electronegativity from Cl to Br.
From these comparisons, it follows that the substitution of Cl− by Br− in SrCd2Cl6·8H2O has led to an extension of the unit cell due to the large size of the Br− ions. An important point in the present structure is the disappearance of the twin phenomenon already observed in crystals of SrCd2Cl6·8H2O. One can conclude that the substitution of one atom by another, less electronegative, one acts as a chemical pressure leading to an increase of symmetry. The same effect was observed when heating SrCd2Cl6·8H2O to 323 K. On the other hand, this substitution has conserved the environment of the different cations. However, it has induced a new arrangement of Sr(H2O)9 polyhedra in isolated chains with a reduction in the distortion of the polyhedra. A lengthening of Cd—Br and Sr—O bonds has also been observed.