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The structural characterization of the new iron–zinc heptaborate bromide with composition Fe1.59Zn1.41B7O13Br, prepared by chemical transport is reported. A rigid-body model with constrained generalized coordinates was defined in order to hold the positions of the B atoms at reasonable interatomic distances that typically would reach unacceptable values because of the weak scattering power of boron. There are three independent sites for the B atoms of which two are tetrahedrally coordinated. The bond-valence sum around the third B atom, located on a threefold rotation axis, was calculated considering two cases of coordination of boron with oxygens: trigonal-planar and tetrahedral. The contribution of the fourth O atom to the bond-valence sum was found to be only 0.06 v.u., indicating the presence of a very weak bond in the right position to have a distorted tetrahedral coordination in favour of the trigonal-planar coordination for the third B atom. X-ray fluorescence (XRF) was used to determinate the Fe/Zn ratio.
The method of preparation was based on Schmid (1965 ). For related structures, see: Mao et al. (1991 ); Dowty & Clark (1972 , 1973 ); Mendoza-Alvarez et al. (1985 ); Schindler & Hawthorne (1998 ); Knorr et al. (2007 ). For properties and potential applications of boracites, see: Campa-Molina et al. (1994 , 2002 ); Dana (1951 ); Mathews et al. (1997 ); Smart & Moore (1992 ). For bond-valence parameters for oxides, see: Brese & O’Keeffe (1991 ).
Data collection: DIFFRAC/AT (Siemens, 1993 ); cell refinement: FULLPROF (Rodríguez-Carvajal, 2006 ; Rodriguez & Rodriguez-Carvajal, 1997 , a strongly modified version of that described by Wiles & Young, 1981); data reduction: FULLPROF; method used to solve structure: coordinates were taken from an isotypic compound (Mao et al., 1991 ); program(s) used to refine structure: FULLPROF; software used to prepare material for publication: DIAMOND.
Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809044407/br2119sup1.cif
Rietveld powder data: contains datablocks I. DOI: 10.1107/S1600536809044407/br2119Isup2.rtv
Enhanced figure: interactive version of Fig. 4
The authors wish to express their thanks to J. C. Carlos Carballo-Bastida from CiCESE-Ensenada, Mexico, for his technical assistance. M. Aguilar-Franco and J. L. Ruvalcaba from Instituto de Fisica, UNAM, Mexico, are acknowledged for their valuable support in performing the XRD and XRF experiments, respectively. Thanks are also due to the Laboratorio Central de Microscopia at Instituto de Fisica, UNAM. IR acknowledges a CONACyT fellowship to support her postdoctoral programme.
Single crystals of Fe1.59Zn1.41B7O13Br were grown by a chemical vapour transport technique, commonly called the three-crucibles method, reported by Schmid (1965). Growth takes place in a closed quartz ampoule. Chemical transport reactions were carried out by heating the ampoule at about 1173 K in a resistance-heated vertical furnace, with gradients of 850 K (above) and 650 K (below), over a period of 72 h. The reactants were placed in the following order: 1.7 g of B2O3 (which was obtained by dehydrating H3BO3) was placed in the first crucible; 0.5 g of each one of both metal oxides (ZnO and FeO) in the second crucible; and 0.8 g of each one of both divalent metal halides (FeCl2 and ZnCl2) in the third crucible. Crystals of Fe1.59Zn1.41B7O13Br as large as 2 mm in size were commonly obtained. X-ray Fluorescence (XRF) spectroscopy was used to estimate the Fe/Zn ratio. A small crystallite was irradiated using the "SANDRA" system developed at Instituto de Fisica, UNAM, equipped with a 75 W Mo X-ray tube (50 kV, 1.5 mA, XTF5011 model from Oxford Instruments) and AmpTeK Si-Pin detector. The system was calibrated using reference standard materials from NIST (SRM 2711). The average percent atomic content with standard uncertainty for each element in the sample were 53 (4) % for iron, and 47 (4) % for zinc, and give a Fe:Zn ratio of 1.13. Then the stichiometric formula is Fe1.59 (12)Zn1.41 (12)B7O13Br.
The characterization of powdered Fe1.59Zn1.41B7O13Br mixed boracite by conventional X-ray powder diffraction data indicated the presence of a well crystallized phase showing reflections that matched with the isostructural phase trembathite, Mg1.56Fe1.44Mn0.02B7O13Cl (PDF 01–089-6198) reported by Schindler & Hawthorne (1998). The starting structural parameters to perform a Rietveld refinement of the Fe1.59Zn1.41B7O13Br boracite were taken from the isostructural data reported for Zn3B7O13Cl (ICSD 55444) by Mao et al. (1991). The following parameters were refined: zero point, scale factor, background parameters, unit cell dimensions, half-width, pseudo-Voigt and asymmetry parameters for the peak shape; position and thermal isotropic factors. For the case of boron, the thermal isotropic factors were fixed to 0.24 Å2, which is a reasonable value for the boron atom and for obtaining a good refinement. The occupation factors for Fe and Zn atoms sharing the same position were fixed to the values of 0.53 and 0.47 respectively, obtained by a quantitative chemical analysis from X-ray fluorescence (XRF) spectroscopy. Due to the very low scattering power of boron atoms to the X-rays, one rigid body group (RBG) containing the boron atoms was defined as ilustrated in figure 1. This RBG has its centre in O(1) atom. Then, eight atoms define the complete RGB (including the centre) and are labelled as B(1), B(2), B(3), O(1), O(2), O(3), O(4) and O(5). Each atom has their spherical internal coordinates (dm, m, θm) fixed according to the rigid character of the RBG formed by these eight atoms. The parameters χc, Θc, Φc, xo, yo, zo, that were refined in a first step are represented in fig. 1 b, c and were limited by the symmetry allowed movements for the RBG as a whole. At the end of this step, B(1)O4, B(2)O4 tetrahedra, and B(3)O3 triangle kept their interatomic angles and distances. In a second and final step of refinement the spherical internal coordinates for B(3) and O(2) were refined in such a way to allow to bring the B(3)O3 triangle closer to the O(1) atom. The RBG subroutine has been included in the program FULLPROF (Rodriguez & Rodriguez-Carvajal, 1997). The use of the RBG reduced significantly the number of positional parameters in the Rietveld refinement. The results of the refinement are shown in figure 2.
|Fe1.59Zn1.41B7O13Br||F(000) = 1546.0|
|Mr = 544.65||Dx = 4.013 Mg m−3|
|Trigonal, R3c||Cu Kα radiation, λ = 1.54175 Å|
|Hall symbol: R 3 -2"c||T = 300 K|
|a = 8.6081 (1) Å||Particle morphology: irregular|
|c = 21.0703 (3) Å||pale pink|
|V = 1352.12 (3) Å3||irregular, 20 × 20 mm|
|Z = 6||Specimen preparation: Prepared at 1173 K|
|Bruker D8 Advance diffractometer||Data collection mode: reflection|
|Radiation source: sealed X-ray tube, Cu Kα||Scan method: step|
|graphite||2θmin = 8.12°, 2θmax = 110.01°, 2θstep = 0.02°|
|Specimen mounting: packed powder sample container|
|Least-squares matrix: full with fixed elements per cycle||5240 data points|
|Rp = 0.018||Profile function: pseudo-Voigt modified by Thompson et al. (1987)|
|Rwp = 0.025||18 parameters|
|Rexp = 0.014||Weighting scheme based on measured s.u.'s|
|RBragg = 0.06||(Δ/σ)max = 0.02|
|R(F2) = 0.06||Background function: linear interpolation between a set of 72 background points with refinable heights|
|χ2 = 3.572|
|Zn||0.1488 (8)||0.3037 (3)||0.3347 (2)||0.0113 (8)||0.47000|
|Fe||0.1488 (8)||0.3037 (3)||0.3347 (2)||0.0113 (8)||0.53000|
|B1||−0.184 (1)||0.1519 (1)||−0.080 (1)||0.00304|
|B2||0.1130 (2)||−0.0902 (1)||−0.0259 (1)||0.00304|
|O1||0.00000||0.00000||−0.008 (1)||0.0025 (9)|
|O2||0.010 (1)||−0.157 (2)||0.107 (1)||0.0025 (9)|
|O3||0.282 (1)||0.274 (1)||−0.032 (1)||0.0025 (9)|
|O4||0.206 (1)||−0.006 (1)||−0.085 (1)||0.0025 (9)|
|O5||0.242 (1)||−0.059 (1)||0.024 (1)||0.0025 (9)|
|Zn—Br||2.680 (3)||B1—O4vii||1.451 (13)|
|Zn—Bri||3.412 (1)||B1—O5vi||1.49 (3)|
|Zn—O2ii||2.130 (4)||B2—O1||1.566 (3)|
|Zn—O3iii||2.081 (7)||B2—O3viii||1.452 (8)|
|Zn—O4iv||2.035 (4)||B2—O4||1.463 (18)|
|Zn—O5v||2.012 (7)||B2—O5||1.453 (17)|
|B1—O2vi||1.506 (14)||B3—O2||1.397 (14)|
|B1—O3vii||1.48 (2)||B3—O1||2.38 (3)|
|Zn—Br—Znvii||94.1 (2)||O2—B3—O2vii||119.9 (9)|
Symmetry codes: (i) −y+1/3, −x+2/3, z+1/6; (ii) −x+y+1/3, y+2/3, z+1/6; (iii) −y+2/3, x−y+1/3, z+1/3; (iv) x, x−y, z+1/2; (v) x−1/3, y+1/3, z+1/3; (vi) −y−1/3, −x+1/3, z−1/6; (vii) −y, x−y, z; (viii) −x+y, −x, z.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BR2119).