Redetermination of the crystal structure of tetralithium octafluoridozirconate(IV), Li4ZrF8, from single-crystal X-ray data

The structure of tetralithium octafluoridozirconate has been redetermined by high-resolution single-crystal X-ray diffraction. This result is largely consistent with a prior report, but with significant improvements in precision.


Chemical context
Zirconium fluorides are commonly examined as members of trinary and ternary-phase alkali/transition metal/actinide fluorides for molten-salt reactors. Many of these molten salts incorporate lithium, because of the favorable nuclear and thermal properties of lithium fluoride. Compounds of zirconium are a useful (if imprecise) structural surrogate for tetravalent cerium, thorium, uranium, and plutonium structures where these materials are unavailable or impractical (Thoma et al., 1965(Thoma et al., , 1968. With the increased interest in carbon-neutral energy sources, investigations of nuclear-relevant technologies such as molten-salt reactors are of increasing interest. As a result, a re-evaluation of data is necessary in some areas. High-quality structure models of Li 2 ZrF 6 and Li 3 Zr 4 F 19 from single-crystal data have previously been discussed in the literature (Brunton, 1973;Dugat et al., 1995). The structure of Li 4 ZrF 8 was reported to be isotypic to the uranium species by powder X-ray diffraction (Dugat et al., 1995), but no refined structure model from single-crystal data has been reported to date.

Structural commentary
Li 4 ZrF 8 is confirmed to be isotypic with the reported structures of Li 4 MF 8 (M = Tb, U) (El-Ghozzi et al., 1992;Brunton, 1967). The zirconium(IV) ion is surrounded by eight fluoride ions in a bicapped trigonal prism (Fig. 1), while both of the two unique lithium sites are surrounded by six fluoride ions in slightly distorted octahedra. Zr-F bond lengths range from 2.0265 (9) to 2.2550 (7) Å (Table 1), and Li-F bonds range from 1.931 (3) to 2.204 (3) Å . The octafluoridozirconate anion is isolated, separated by 4.9906 (4) Å from its crystallographic nearest neighbors. Investigation of several distinct crystals of different size and apparent crystal habit all resulted in unit-cell parameters that agreed with the published unit cell of Li 4 ZrF 8 . It is therefore likely that, despite the sub-stoichiometric ratio in the reaction (which was intended to produce other lithium zirconium fluorides), Li 4 ZrF 8 is the most stable single-crystalline zirconate formed.
The refined crystal structure model is qualitatively very similar in most respects to that reported by Dugat et al. (1995), including the connectivity and zirconium bonding environment. There are significant statistical improvements in all major metrics, including unit-cell precision, standard uncertainties of the unit cell and bond lengths, and a much finer identification of the lithium and fluoride ion sites. Despite this concordance, every zirconium-fluoride bond length reported in the literature is more than one standard uncertainty apart from the zirconium-fluoride distances determined in the structure reported here. This is not a result of systematic bias in the calculated powder-pattern bond lengths. The eight Zr-F bonds are evenly split, with four longer than reported here, and four shorter, and the obtained average bond length is very close to the one from the previous study. Among the twelve lithium-fluoride bonds, the average bond lengths for each lithium site are statistically identical to those noted in the previous model, but distinct at the standard uncertainty in the data reported here. The site designated Li1 in each structure has greater asymmetry than its neighbor, but the re-examined data do not have a difference that is nearly so marked; the literature Li1-F bond lengths range from 1.84 (2)-2.11 (2) Å , while the new result reported here has bond lengths of 1.942 (2)-2.054 (2) Å . Additionally, the axes of the unit cell are different by a margin greater than the standard uncertainty reported in the literature, as all three axes reported here are greater in size. The overall effect on the unit-cell volume is small, however, but there is an additional order of magnitude of precision obtained. For more details, direct comparisons of the bond lengths and the unit cells are given in Table 1. The crystal structure of Li 4 ZrF 8 . The large image on the left is of the crystal packing down the c axis. The inset demonstrates the displacement ellipsoids of all ions at the 95% probability level. From top-to-bottom on the right, the views of the ZrF 8 4À unit down the a, b, and c axes. Color code: green, zirconium; orange, lithium; pink, fluorine.  Dugat et al. (1995); atom labeling adapted to the current study.
The crystal examined exhibited static disorder, observable by the zirconium site (which has significantly more electron density than the other atoms). Both zirconium sites are on Wyckoff position 4c (site symmetry. m.).

Synthesis and crystallization
Lithium fluoride (43.0 mg, 1.66 mmol; 99.85% Alfa Aesar) and zirconium dioxide (61.1 mg, 0.496 mmol; 99% Aldrich) were charged into an 8 mL PTFE-lined autoclave. 1.00 mL of deionized water was then added, followed by the dropwise addition of 1.00 mL 48% hydrofluoric acid (Sigma-Aldrich). The autoclave was sealed, and heated at 473 K for twentyfour h, followed by controlled cooling to room temperature at a rate of 5 K h À1 . The title product was isolated from the supernatant by repeatedly rinsing with chilled deionized water to dilute the fluoride hazard and to remove any lithium fluoride that remained in the HF solution. Methanol was used to transfer the samples to a petri dish, followed by drying in air. Large (up to 5 mm) crystals were parallelogram columns that cleaved into parallelepipeds, while small (50 mm-scale) crystals were thin parallelogram plates.

Crystal data
Li 4  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )