Crystal structure of the Fe-member of usovite

The compound with the idealized composition Ba2CaFeAl2F14 crystallizes in the usovite structure type. Two models with different treatment of the disordered Fe site are presented.


Chemical context
Fluoridoaluminates with alkaline earth cations exhibit a rich crystal chemistry (Babel & Tressaud, 1985;Weil et al., 2001). They are suitable host materials for optical applications as has been shown by luminescence exitation studies of SrAlF 5 or CaAlF 5 doped with Pr 3+ and Mn 2+ (van der Kolk et al., 2004). In order to prepare large single crystals of a related fluoridoaluminate with composition BaCaAlF 7 , a different preparation route was chosen in comparison with the reported crystal-growth procedure. Instead of using a ZnCl 2 melt (Werner & Weil, 2003), a carbon tool steel container shielded ISSN 2056-9890 Figure 1 [AlF 6 ] octahedra (yellow, with F atoms green) and [FeF 6 ] octahedra (orange) are linked into crosslinked double chains parallel to [010]. Displacement ellipsoids are drawn at the 74% probability level. with a molybdenum foil was used for solid state reactions between a mixture of the binary fluorides (Weil & Kubel, 2002). However, during one of these experiments it turned out that the container was not completely lined by the molybdenum foil which consequently led to a reaction with the container wall and an incorporation of iron into parts of the reaction products. Crystal structure analysis of selected crystals from this reaction batch revealed an Fe-containing phase that crystallizes isotypically with the mineral usovite, Ba 2 CaMgAl 2 F 14 (Litvin et al., 1980).
Compounds with the usovite-type structure are represented by the general formula Ba 2 (M II 1)(M II 2)(M III 3) 2 F 14 (M II 1 = Ca, Cd, Mn; M II 2 = Mg, Co, Mn, Cu, Cd, Fe; M III 3 = Al, V, Fe, Cr, Ga, Mn) and crystallize with four formula units in the space group C2/c. Most of the usovite-type representatives known so far were prepared and structurally determined by Babel, Tressaud and co-workers over the last three decades (Holler et al., 1984(Holler et al., , 1985Kaiser et al., 2002;Le Lirzin et al., 1990, 1991, 2008Qiang et al., 1988).

Structural commentary
The principal building units of the usovite crystal structure are distorted [BaF 12 (Fig. 2).
The unit-cell volume of the title compound [1067.9 (2) Å 3 ] is slightly larger than that of usovite Ba 2 CaMgAl 2 F 14 (1027.9 Å ; Litvin et al., 1980) due to the replacement of the Mg 2+ cations (ionic radius = 0.72 Å ; Shannon, 1976) by the larger Fe 2+ cations (ionic radius = 0.78 Å ; Shannon, 1976) at the M II 2 site. This is also reflected by the bond lengths within the individual coordination polyhedra (Table 1). Wheras the Ba-F, Ca-F and Al-F distances remain nearly unaltered between the two structures, the Mg-F and Fe-F distances show remarkable differences. The Mg-F distances in the usovite structure range from 1.939 to 2.041 Å , the corresponding Fe-F distances in the title structure from 2.015 (2) to 2.216 (2) Å , with a mean distance of 2.123 Å . The latter is in reasonable agreement with the mean Fe II -F distance of 2.106 Å in the isotypic crystal structure of Ba 2 CaFeV 2 F 14 (Kaiser et al., 2002). However, the mean bond lengths in both the title structure and Ba 2 CaFeV 2 F 14 are considerably longer than that of 2.074 Å in the structure of the binary compound FeF 2 (Jauch et al., 1993).
A similar increase of the M-F bond lengths of the [(M II 2)F 6 ] octahedra was also observed for a series of other usovite-type structures and was associated with an occupational disorder of the M II 2 site. For these models, either a mutual substitution of Ca 2+ (on the M II 1 site) with corresponding divalent transition metal ions on the M II 2 site, or partial replacement of the divalent transition metal ions by Ca 2+ at the M II 2 site was considered, resulting in stoichiometric compounds and Ca-richer compounds, respectively (Kaiser et al., 2002). In the case of the title compound, a model with mutual substitution of Ca 2+ and Fe 2+ on the M II 1 and M II 2 sites could be ruled out during refinement. However, a model with an incorporation of Ca 2+ on the Fe 2+ site resulted in a ratio of Ca:Fe = 0.155 (7) Symmetry codes: (i) Àx þ 1 2 ; y þ 1 2 ; Àz þ 1 2 ; (ii) Àx þ 1 2 ; y þ 1 2 ; Àz þ 3 2 ; (iii) x; Ày þ 1; z À 1 2 ; (iv) Àx þ 1 2 ; Ày þ 1 2 ; Àz þ 1; (v) Àx þ 1 2 ; y À 1 2 ; Àz þ 1 2 ; (vi) x; Ày þ 1; z þ 1 2 ; (vii)

Figure 2
The crystal structure of the usovite-type title compound, emphasizing the formation of the layered 2 factors and remaining electron densities as the model without an incorporation of Ca 2+ and underoccupation of the Fe 2+ site only [model (II); Table 3]. The refined formula for this model is Ba 2 Ca 2 Fe 0.90 (1) Al 2 F 14 . Bond lengths and angles of the two models are the same within the corresponding standard uncertainties (Table 1). Kaiser et al. (2002) have discussed in detail the pros and cons of the incorporation of Ca 2+ (ionic radius = 1.0 Å ; Shannon, 1976) at the M II 2 site for various usovite-type structures. Strong arguments supporting an M II 2 site with mixed Fe/Ca occupation are the resulting bond-valence sums (Brown, 2002) that deviate significantly from the expected values of 2 if only Fe 2+ ions are considered to be present at the M II 2 site (Table 2). Contrariwise, the bond-valence sums are in excellent agreement with the expected value if a mixed Fe/Ca occupancy is taken into account. The corresponding numbers are listed in Table 2 and were calculated with the weighted average occupancy ratio of Fe:Ca = 0.77:0.23 that was estimated by the program VaList (Wills, 2010). This ratio is in good agreement with the occupancy ratio from the refinement [model (I): Fe:Ca = 0.69:0.31]. The resulting global instability index (Brown, 2002) of 0.04 valence units for model (I) suggests a very tightly bonded structure with little strain. Any strain inherent in the usovite structure is obviously relieved by the substitution of Ca 2+ on the M II 2 site.
On the other hand, an M II 2 site without an incorporation of Ca 2+ would result in an underoccupation of Fe 2+ [model (II)] and consequently requires the presence of an element in a higher oxidation state (here most probably Fe 3+ ) to compensate the negative charge of À2 of the [Ba 2 CaAl 2 F 14 ] framework. Although in this case rather a decrease of M II 2-F bond lengths should be expected (contrary to the findings of the current study), it cannot competely ruled out that Fe 3+ ions are present at this site. As a matter of fact, based on diffraction data alone, there is a clear tendency towards model (

Synthesis and crystallization
The binary fluorides AlF 3 (Merck, Patinal), CaF 2 (Merck, Suprapur) and BaF 2 (Riedel de Haen, pure) were mixed in the stoichiometric ratio 1:1:1 and thoroughly ground in a ball mill, pressed into tablets and placed in a carbon tool steel container shielded with a molybdenum foil. NH 4 FÁHF (100 mg, Fluka, pÁA.) were added to the mixture to increase the HF pressure, to expel the remaining oxygen and to adjust a slightly reducing atmosphere during the reaction. The reactor was then closed and heated to 1173 K in the course of 20 h, kept at that temperature for 24 h, and then cooled slowly to 973 K at a rate of 10 K h À1 , kept at this temperature for 24 h and finally cooled to room temperature overnight. After opening the reactor it became evident that parts of the molybdenum foil were torn apart accompanied by a severe attack of the inner container wall. Single crystals of the title compound were separated from the obtained colourless to light-green bulk material. X-ray powder diffraction of the bulk revealed the formation of -BaCaAlF 7 as the main phase and the title compound as a minority phase. Some additional reflections were also present that could not be assigned to any known phases.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Coordinates of usovite (Litvin et al., 1980) were used as starting parameters for refinement. The model converged rather smoothly with R1 = 0.034 and wR2 = 0.089. However, negative residual electron density at the Fe atom pointed to an underoccupation and/or a statistical disorder of the M II 2 site with a lighter element present. In fact, free refinement of the site occupation factor for this site resulted in only 90% occupancy and significant better reliability factors (see Table 3). The same procedure applied for all other atoms resulted in full occupancy within the twofold standard uncertainty. For the final models, full occupancy was therefore considered for all atoms except Fe. Model (I) accounts for an incorporation of Ca 2+ at the Fe site under consideration of full occupancy; in model (II), the site occupation factor of the Fe site was refined freely without contribution of Ca 2+ at this site. The remaining electron densities (  (Dowty, 2006) for modelI; Atoms for Windows (Dowty, 2006) for modelII. For both compounds, software used to prepare material for publication: publCIF (Westrip, 2010).

(modelI) Dibarium calcium iron(II) dialuminium tetradecafluoride
Crystal data Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0022 (2) Special details 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 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 (Å 2 )
x y z U iso */U eq Occ. ( (2) Special details 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Geometric parameters (Å, º)