Rietveld refinement of the mixed boracite Fe1.59Zn1.41B7O13Br

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 structural characterization of the new iron-zinc heptaborate bromide with composition Fe 1.59 Zn 1.41 B 7 O 13 Br, 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 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.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BR2119).  (Dana, 1951). Unusual physical properties can be cited for a given cations located in the crystalographic sites for Me and X. Depending on this, potential applications such as an optic stopper (Smart & Moore, 1992); ferroelectric non volatile memory (Mathews, et al., 1997); and infrared (IR) detection (Campa -Molina et al., 1994, 2002 have been reported and in some sense, can be modulated by the presence of some specific types of cations.  (1991) and considering those coordination polyhedra whose bond valence calculations were based on distances and angles that were allowed to refine (this was partially true in some cases). Bond valence sum around Br is found to be 0.82 and 1.10, for BrZn 6 and BrFe 6 distorted octahedra respectively. indicating the presence of a very weak bond in the right position for have a distorted tetrahedral coordination around the planar triangle coordination for the third boron atom B(3). This fact is also a feature for the reported boracites Fe 3 B 7 O 13 Cl (ICSD 60504, Mendoza-Alvarez et al., 1985), and Zn 3 B 7 O 13 Cl (ICSD 55444, Mao et al., 1991).

Experimental
Single crystals of Fe  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 Å 2 , 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)