Ca5Zr3F22

Single crystals of Ca5Zr3F22, pentacalcium trizirconium docosafluoride, were obtained unexpectedly by solid-state reaction between CaF2 and ZrF4 in the presence of AgF. The structure of the title compound is isotypic with that of Sr5Zr3F22 and can be described as being composed of layers with composition [Zr3F20]8− made up from two different [ZrF8]4− square antiprisms (one with site symmetry 2) by corner-sharing. The layers extending parallel to the (001) plane are further linked by Ca2+ cations, forming a three-dimensional network. Amongst the four crystallographically different Ca2+ ions, three are located on twofold rotation axes. The Ca2+ ions exhibit coordination numbers ranging from 8 to 12, depending on the cut off, with very distorted fluorine environments. Two of the Ca2+ ions occupy interstices between the layers whereas the other two are located in void spaces of the [Zr3F20]8− layer and alternate with the two Zr atoms along [010]. The crystal under investigation was an inversion twin.

Single crystals of Ca 5 Zr 3 F 22 , pentacalcium trizirconium docosafluoride, were obtained unexpectedly by solid-state reaction between CaF 2 and ZrF 4 in the presence of AgF. The structure of the title compound is isotypic with that of Sr 5 Zr 3 F 22 and can be described as being composed of layers with composition [Zr 3 F 20 ] 8À made up from two different [ZrF 8 ] 4À square antiprisms (one with site symmetry 2) by corner-sharing. The layers extending parallel to the (001) plane are further linked by Ca 2+ cations, forming a threedimensional network. Amongst the four crystallographically different Ca 2+ ions, three are located on twofold rotation axes. The Ca 2+ ions exhibit coordination numbers ranging from 8 to 12, depending on the cut off, with very distorted fluorine environments. Two of the Ca 2+ ions occupy interstices between the layers whereas the other two are located in void spaces of the [Zr 3 F 20 ] 8À layer and alternate with the two Zr atoms along [010]. The crystal under investigation was an inversion twin.

Related literature
For the isotypic Sr analogue, see: Le Bail (1996). The crystal chemistry of fluorides has been reviewed by Babel & Tressaud (1985). For phase relationships in the CaF 2 -ZrF 4 system, see: L'Helgoualch et al. (1971); Kotsar et al. (1973); Ratnikov et al. (1977);Laval et al. (1987) Despite several reports related to investigations of the pseudo-binary system CaF 2 -ZrF 4 (L′Helgoualch et al. 1971;Kotsar et al. 1973;Ratnikov et al. 1977;Babel & Tressaud, 1985;Laval et al. 1987) the title compound has never been mentioned previously. Therefore the determination of its crystal structure was of prime importance to complete a thorough examination of the phase diagram of this system.
The orthorhombic structure of the title compound is isotypic with that of Sr 5 Zr 3 F 22 reported by Le Bail (1996). As with this latter, the structure of Ca 5 Zr 3 F 22 has also been determined from an inversion twinned crystal although the crystals were grown in very different ways. Figure

Experimental
Single crystals of Ca 5 Zr 3 F 22 were unexpectedly obtained from an equimolar mixture of AgF, CaF 2 and ZrF 4 heated at 873 K in a sealed platinum tube during the study of the phase diagram of the ternary system AgF-CaF 2 -ZrF 4 . After heating at this temperature for 24 h, the sample was cooled to room temperature at the rate of 5 K . h -1 for the first 24 h and then by switching the power off. Small platelet like crystals of Ca 5 Zr 3 F 22 could have been extracted from the batch. Both AgF and CaF 2 were commercial products whereas ZrF 4 was prepared by direct fluorination of ZrO 2 under pure fluorine gas flow at 873 K with intermediate grindings. CaF 2 was also heated at 873 K overnight under fluorine gas flow prior to use.

Refinement
The highest residual peak in the final difference Fourier map was located 0.04 Å from atom Zr2 and the deepest hole was located 1.05 Å from atom F9.

Figure 1
View of the polyhedral linkage in Ca 5 Zr 3 F 22 . Displacement ellipsoids are drawn at the 50% probability level. Symmetry   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  (7)