Single-crystal structure refinement of YbF₂ with a remark about YbH₂

1 Introduction

Alkaline earth metals and some rare earth metals show similarities in their crystal chemistry due to their similar ionic radius and coordination need in the divalent state [1], e.g. Ca²⁺ and Yb²⁺ (112 pm vs. 114 pm for the coordination number CN=8) or Eu²⁺ and Sr²⁺ (125 pm vs. 126 pm for the coordination number CN=8). Therefore, it was intended to extend the chemistry of allenide [C=C=C]⁴⁻ moieties by synthesizing compounds such as Ca₂Cl₂[C₃] [2] by reacting teflon powder with metallic ytterbium or, alternatively, Yb, YbCl₃ with elemental carbon. The reaction with the chloride yielded only recrystallized powders of YbCl₃ and amorphous material, whereas the reaction of Yb with teflon produced mainly light yellow, transparent single crystalline YbF₂ and some X-ray-amorphous material.

Besides the first single-crystal structure determination of YbF₂ we also report a relatively small volume per formula unit for YbH₂ compared to those of MF₂ and MH₂ compounds (M=Ca, Yb, Eu, Sr and Ba).

2 Experimental section

All preparations were carried out under a continuously purified and monitored argon atmosphere in glove boxes (Fa. M. Braun, Garching, Germany). The reactions took place in arc-welded Nb tubes (99.99%, Plansee, Germany).

Yb (99.9%, chips, Strem, Newburyport, MA, USA), and (CF₂)_n (Sigma-Aldrich, >40 μm powder St. Louis, MO, USA) were mixed and compacted manually in a 4: 3 molar ratio (overall mass: 0.3 g). This pellet was arc-welded into a Nb ampoule and subsequently silica-jacketed under vacuum. The reaction container was placed upright into a box furnace and heated to 1100 K within six hours. After 3 days at this temperature, the furnace was shut off. The reaction products were single crystals of YbF₂ (approximately 30% of the product estimated with the naked eye) obtained next to X-ray-amorphous material as the main product. The title compound forms as irregularly shaped crystalline, moderately air and water stable material. With different molar ratios of Yb and (CF₂)_n similar products were obtained. All the crystals studied exhibit all the same composition according to single-crystal structure measurements performed on several selected specimens.

3 X-ray investigations

Samples of the material were immersed in polybutene oil (Aldrich, M_n ~320, isobutylene 90%). Suitable single crystals were selected under a polarization microscope, mounted in a drop of polybutene sustained in a plastic loop, and placed onto the goniometer. A cold stream of nitrogen [T=203(2) K] froze the polybutene oil, thus keeping the crystal stationary and protected from oxygen and moisture. Preliminary examination and subsequent data collection were performed on a Bruker X8 Apex II diffractometer equipped with a 4 K CCD detector and graphite-monochromatized MoK_α radiation (λ=71.073 pm). The intensity data was handled with the program package that came with the diffractometer [3]. The intensity data was corrected for Lorentz and polarization effects and an empirical absorption correction was applied using Sadabs [4]. Initial positions 4a for ytterbium and 8c for fluoride were refined with Shelxl-97 [5, 6] by full-matrix least-squares techniques. It was attempted to place fluoride on the highest remaining rest electrons densities, but even with an unrestrained site occupation factor, no occupation with a positive displacement factor could be achieved. This is in line with the observed residual electron density of +1.41 e⁻ Å⁻³. Selected parameters of the measurements and results of the refinements are summarized in Table 1.

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD-433046 (YbF₂).

4 Description of the crystal structures

All MF₂ compounds (M=Ca, Yb, Eu, Sr and Ba) crystallize isostructurally in the well-known cubic fluorite structure type (CaF₂, space group Fm3̅m). The fluoride anions are surrounded tetrahedrally by the metal cations which are in turn coordinated in cube shaped fashion by the F⁻ anions (Fig. 1). The hydrides of the stoichiometry MH₂ (M=Ca, Yb, Eu, Sr and Ba) crystallize also isostructurally, but in the orthorhombic PbCl₂-type structure (cotunnite, Pnma) (Fig. 2) in which the cations are coordinated in the form of a tricapped trigonal prisms which share the trigonal faces to form columns. By sharing faces, these columns build corrugated layers of [MH₉] units which are arranged perpendicular to the crystallographic c axis, but shifted by nearly exactly [0 ½ ½] with respect to each other.

Fig. 1:

Perspective view of the crystal structure of YbF₂. Displacement ellipsoids are drawn at the 95% probability level.

Fig. 2:

Perspective view of the crystal structure of YbH₂. Displacement ellipsoids are drawn at the 95% probability level.

5 Discussion

The first complete single-crystal structure determination on YbF₂ shows no indications of fluoride ions occupying positions other than the Wyckoff site 8c. This result differs from conclusions drawn before [7] from XRPD data (here a splitting of some peaks was observed on long annealed material and the lattice parameter becomes smaller, e.g. YbF₂ [light green, space group Fm3̅m, a=559.93(1) pm] compared to YbF_2.37 [nearly colorless, pseudo-cubic, a=557.05(1) pm]. The color of our product also indicates an ionic, divalent compound since trivalent fluorides are transparent, but colorless. The formula volume (Table 2) and the atomic distances of YbF₂ [d(Yb–F)=242.25(7) pm] are as expected for divalent ytterbium.

The fluoride and the hydride series attracted our attention since all compounds crystallize isotypically within the respective series either in the CaF₂ or the PbCl₂ structure type. The fluorides show the expected behavior which is a nearly linear increase of the formula volume with respect to the cation radius, but the hydrides follow a different trend. First of all, hydrides show a smaller volume per formula unit if compared to the fluoride compound with the same cation (Table 2). This can be understood because of the different crystal structure type adapted and because of the large polarizability of the soft hydride anion [1]. Nevertheless, YbH₂ and EuH₂ exhibit both deviations from a linear increase of the formula volume of the hydride series – YbH₂ in particular (Table 2). The transparency of the crystals, measurements of the magnetic susceptibilities [14] as well as Mössbauer spectroscopic studies [15] on orthorhombic ytterbium hydride YbH₂ are in accordance with the diamagnetic f¹⁴ configuration for Yb²⁺. Therefore, the presence of trivalent ytterbium in the YbH₂ gives no explanation for its relatively small volume per formula unit which is not in accordance with the cation radius of Yb²⁺.

6 Conclusion

The first complete single-crystal structure determination on YbF₂ is reported. The crystals obtained showed no signs of fluorides occupying other positions than the Wyckoff site 8c. Of the rare earth metal dihydrides, especially YbH₂ shows an as-yet unexplained deviation from a linear increase of the formula volume with regard to the increasing radius of the respective M²⁺ cation. The smaller cation radius of Yb⁺³ (compared to Yb⁺²) cannot, therefore, explain the small lattice parameter of YbH₂, since no Yb⁺³ is detected. Electronic structure calculations may produce some insight into this observation.

7 Supporting information

A picture of one frame of the X-ray data collection showing the rare incident of a streak originating from cosmic radiation is available in the online version (DOI: 10.1515/znb-2017-0147).