Twinned caesium cerium(IV) pentafluoride

Single-crystals of CsCeF5 were synthesized hydrothermally. The crystal under investigation was twinned by pseudo-merohedry with a twofold rotation around the c axis as an additional twinning operation. The crystal structure is built of layers of distorted edge- and corner-sharing CeF8 square-antiprisms. The Cs+ cations are located between the layers and exhibit coordination numbers of nine. Upon compression, CsCeF5 undergoes an irreversible phase transition at about 1 GPa.

Single-crystals of CsCeF 5 were synthesized hydrothermally. The crystal under investigation was twinned by pseudomerohedry with a twofold rotation around the c axis as an additional twinning operation. The crystal structure is built of layers of distorted edge-and corner-sharing CeF 8 squareantiprisms. The Cs + cations are located between the layers and exhibit coordination numbers of nine. Upon compression, CsCeF 5 undergoes an irreversible phase transition at about 1 GPa.

Introduction
Most of the work on structural inorganic chemistry of actinide fluorides has been primarily based on the materials that crystallize from molten alkali fluoride salts (Grzechnik et al., 2007;Grzechnik et al., 2008;Friese et al., 2011). Recently, several complex thorium fluorides have been synthesized (Underwood et al., 2012;Grzechnik et al., 2013a) and advances in the synthesis of cerium (IV) fluorides have been made (Rouse et al., 2009;Grzechnik et al., 2013b). The successful use of fluoride mineralizers in the hydrothermal crystal growth of alkali thorium fluorides suggests that additional investigations into inorganic cerium (IV) fluorides may be fruitful. The chemistry of monovalent metal cerium(IV) fluorides in hydrothermal fluids was examined by Underwood (2013) and it was found that it is considerably richer than anticipated.
We are interested in the crystal structures and stabilities of cerium (IV) fluorides as they are thought to be isostructural with the actinide fluorides (Grzechnik et al., 2013b). In the recent study by Underwood et al. (2012), three new polymorphs of CsThF 5 have been synthesized hydrothermally. The tetragonal phase (phase I, P4/nmm, Z = 2), consisting of sheets of ThF 9 tricapped trigonal prisms separated by layers of Cs atoms, is a minor product in the hydrothermal conditions. Two monoclinic polymorphs (phases II and III) are synthesized at various conditions. The phase II (P2 1 /c, Z = 4) is built of chains of corner-sharing ThF 9 polyhedra, while the phase III (P2 1 /c, Z = 8) is built of sheets of edge-and corner-sharing ThF 9 polyhedra. The chain structure of the phase II converts into the sheet structure of the phase III at 500°C. In this study, we examine cesium cerium(IV) pentafluoride, CsCeF 5 , to see whether it is indeed isostructural with CsThF 5 .

Experimental
A series of crystals mounted on glass pins was tested on a STOE IPDS 2 diffractometer and the best one was selected for the structure determination at ambient conditions and high-pressure single-crystal x-ray studies. High-pressure data were collected in the Ahsbahs-type diamond anvil cell at room temperature at 1.2, 3.1, and 5.0 GPa. A 0.250 mm hole was drilled into a stainless steel gasket preindented to a thickness of about 0.120 mm. A 4:1 mixture of methanol and ethanol was used as a pressure medium. The ruby luminescence method (Mao et al., 1986) was used for pressure calibration.
The intensities were indexed and integrated with the software X-AREA (Stoe & Cie, 2002). Due to the pseudomerohedral twinning, some reflections in the lower θ range partly overlap. During the integration the orientation matrices of both twin individuals were used simultaneously. The overlap tolerance was set to 100%, so that all overlapped reflections were included. The faces of the crystal were optimized for each individual with the program X-SHAPE (Stoe & Cie, 2002) and an absorption correction was applied with the program Jana2006 (Petříček et al., 2006). Figures 1-3 were drawn using the program DIAMOND (Brandenburg, 2000).

Synthesis and crystallization
All reagents for the synthesis of CsCeF 5 were of analytical grade and used as purchased. The compound in this study was prepared hydrothermally as follows: 0.20 g CeF 4 (Strem Chemical, 99.9%) was combined with five molar equivalents (0.703 g) of CsF (Alfa Aesar, 99.9%), placed into a Parr Instruments Teflon-lined autoclave and filled with 10.0 mL water. The autoclave was then heated at 250 °C for 24 h and slowly cooled at a rate of 10 °C/h to room temperature.
When the reaction was complete, the content of the autoclave was filtered and the product washed with deionized water to yield a colorless material. Powder X-ray diffraction was used to characterize the bulk solid and single crystal X-ray diffraction was used to structurally characterize the new species.
The structure was solved with the program SIR97 (Altomare et al., 1999) an it was refined with the program Jana2006 (Petříček et al., 2006). The reflections from both individuals and the twinning operation were taken into account during the refinement process. The refined twin volumes of the two individuals are 0.8830 (7) and 0.1170 (7).

Results and discussion
All the examined crystals of CsCeF 5 are pseudomerohedrally twinned and have their lattice parameters close to those for the high-temperature polymorph of CsThF 5 (phase III) (Underwood et al., 2012). We do not find any material whose single-crystal or powder x-ray intensities could be indexed with the lattices analogous to those in the phases I and II of CsThF 5 . It indicates that CsCeF 5 does not exhibit any temperature-induced polymorphism within the synthesis conditions.
The analysis of the structural data in Table 2 shows that, unlike the Th atoms in the phase III of CsThF 5 , the Ce atoms

Figure 1
An edge-sharing doublet of distorted square antiprisms CeF 8 .

Figure 2
Layers of CeF 8 (left) and CsF 9 (right) polyhedra viewed along the b axis.
supplementary materials

Figure 3
Crystal structure in different projections. The polyhedra around the Ce atoms are drawn.