J Chem Crystallogr (2012) 42:606–610 DOI 10.1007/s10870-012-0289-6
ORIGINAL PAPER
The Crystal Structures of CsTh6F25 and NaTh3F13 Christopher C. Underwood • Colin D. McMillen Joseph W. Kolis
•
Received: 19 October 2011 / Accepted: 7 February 2012 / Published online: 22 March 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Two new alkali thorium fluorides have been synthesized hydrothermally and structurally characterized. The structures of CsTh6F25 and NaTh3F13 are each described in space group P63/mmc (No. 194) with ˚ , and a = 8.2570(12) a = 8.3507(12) and c = 16.914(3) A ˚ and c = 16.891(3) A, respectively. Both structures exhibit a framework structure containing pores, unlike most of their alkali metal thorium fluoride analogs. The significant structural difference between the two compounds, however, is the arrangement of the alkali metal within the pores formed in the framework. Accordingly the pores exhibit differing degrees of distortion depending on their contents. Keywords Alkali thorium fluorides Hydrothermal synthesis Framework structures
Introduction The descriptive chemistry of solid-state inorganic thorium compounds has been somewhat neglected in the last several decades [1] but may be poised to receive new interest because of the possible use of thorium as a safe nuclear fuel in the future. Since it was determined that thorium oxide can be grown as large high-quality single crystals using fluoride mineralizers [2], we have successfully grown a C. C. Underwood C. D. McMillen J. W. Kolis (&) Department of Chemistry, Clemson University, 475 H. L. Hunter Laboratories, Clemson, SC 29634-0973, USA e-mail:
[email protected] C. C. Underwood e-mail:
[email protected] C. D. McMillen e-mail:
[email protected]
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number of alkali thorium fluorides under hydrothermal conditions [3, 4], doing so due to their potential as a fuel source for both fusion and fission reactors. The descriptive chemistry of the thorium fluorides has not been investigated in excessive detail, and major advances in this chemistry were made nearly 40 years ago. Previous work was done primarily in molten alkali fluoride salts and led to a variety of alkali metal thorium fluorides in the tetravalent state. A number of AxThyFz compounds were characterized, mostly by both powder and single crystal diffraction, suggesting that the chemistry is very rich [5–31]. The reaction of ThF4 with alkali fluorides in hydrothermal fluids leads to a wide variety of new metal fluoride compounds. Systematic exploration of phase space has uncovered a number of species that typically reflect the stoichiometry and size of the alkali ion. The cesium thorium fluorides seem particularly rich [3, 4]. In this paper we describe the chemistry and structures of two new alkali thorium fluorides grown from hydrothermal solution, and correlate their structures to other known metal fluorides and to each other. Specifically we report a new cesium thorium fluoride as well as a new addition to the limited number of sodium thorium fluorides characterized by single crystal X-ray diffraction (XRD).
Experimental Synthesis All reagents were of analytical grade and used as purchased. Compounds in this study were prepared hydrothermally as follows: For 1, 0.15 g (0.49 mmol) of ThF4 (Strem Chemical, 99.9%) was combined with 0.05 g
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(0.33 mmol) CsF (Alfa Aesar, 99.9%) in the presence of 0.4 mL 2 M BeF2 (Alfa Aesar, 99.9%); for 2, 0.15 g (0.49 mmol) of ThF4 with 0.3 mL 2 M NaF (Acros, 99%) were placed into silver ampoules and weld-sealed. The sealed ampoules were loaded into a Tuttle-seal autoclave, which was counter pressured with additional water. The autoclave was heated at 575 °C for 4 days typically generating a counter pressure of 17,000 psi resulting in a slight compression of the welded ampoules. When the reaction was complete, the contents of the ampoule were filtered and the products washed with deionized water to yield large colorless crystals. Powder XRD was used to characterize the bulk solids and single crystal XRD was used to identify and structurally characterize new species.
Table 1 Crystallographic data for structures 1 and 2
X-Ray Diffraction
Parameters
37
40
F(000)
1,640
1,592
Powder X-ray diffraction data was collected using a Rigaku Ultima IV X-ray diffractometer with Cu Ka radi˚ ). Patterns were collected from 5 to ation (k = 1.5418 A 65 ° in 2h at a scan speed of 1.0 °/min. Single crystal X-ray intensity data were collected using a Rigaku Mercury CCD detector and an AFC-8S diffractometer equipped with a graphite monochromator that emits Mo Ka radiation ˚ ). The space groups were determined from (k = 0.71073 A the observed systematic absences and confirmed using the MISSYM algorithm within the PLATON program suite [32]. Data reduction including the application of Lorentz and polarization effects (Lp) and absorption corrections were performed using the CrystalClear program [33]. The structures were solved by direct methods and refined using subsequent Fourier difference techniques, by full-matrix least squares, on F2 using SHELXTL 6.10 [34]. All atoms were refined anisotropically except where specified. Data from the single crystal structure refinements is given in Table 1. All single crystal solutions were confirmed by simulating the powder pattern from the single crystal structure determinations and comparing these to the powder patterns obtained from the bulk reaction products or those previously published.
Results and Discussion Crystal Structure of CsTh6F25 (1) Compound 1 was found to crystallize into the hexagonal space group P63/mmc (No. 194). Atoms F3 and F5 in this solution had to be refined using an ISOR restraint to prevent their principal mean square atomic displacements from being non-positive definite. This compound is isostructural to the CsU6F25 formulation previously structurally characterized by Brunton [35]. Similar phases of 1,
1
2 F13Th3Na
Chemical formula
F25Th6Cs
F.W. (g/mol)
2000.15
966.11
Space group
P63/mmc
P63/mmc
Temp./K
293 ± 2
293 ± 2
Crystal system ˚) a (A
Hexagonal
Hexagonal
8.3507(12)
8.2570(12)
˚) c (A ˚ 3) V (A
16.914(3)
16.891(3)
1021.5(3)
997.3(3)
Z
2
4
Dcal (Mg/m3)
6.503
6.434
Indices (min) Indices (max)
[-10, -10, -21] [10, 19]
[-10, -10, -21] [10, 20]
l, mm-1
45.478
44.836
2h range (°)
2.41–26.35
2.41–26.36
Collected reflections
9,073
8,785
Unique reflections
435
433
Final R (obs. data),a R1
0.0403
0.0422
wR2
0.0962
0.1110
Final R (all data), R1
0.0457
0.0454
wR2
0.1008
0.1140
Goodness of fit (S)
1.222
1.212
Extinction coefficient
0.00032(12)
0.00056(19)
Largest diff. peak
8.324
2.949
Largest diff. hole
-2.953
a
-2.842 2
R1 = [R||F0| - |Fc||]/R|F0|; wR2 = {[Rw[(F0) - (Fc)2]2}1/2
RbTh6F25 and KTh6F25, were proposed to crystallize in P63/mmc via powder diffraction data [8]. However, the only single crystal data available for this phase of alkali thorium fluoride is for a-KTh6F25, a polymorph that was found to crystallize in R-3m [16]. Although it forms a framework system that contains pores, its thorium polyhedra stack in an ABCABC order, as compared to our structure that stacks in an ABAB order, similar to CsU6F25. The powder pattern for 1 is similar to a previously-reported powder pattern for a melt of nominal composition CsTh6F25 which was not otherwise characterized or structurally analyzed [5]. The thorium atoms form slightly distorted tricapped trigonal prisms with fluorine atoms, ˚ , which having an average Th–F distance of 2.391(7) A correlates well with the expected values predicted by Shannon [36]. All the unique atoms in 1 lie on special positions. Th1, F2, and F4 have m symmetry along the b axis, F3 has a two-fold rotation along the b axis, F1 has mm2 symmetry, F5 and F6 has a three-fold rotation along the a axis and a mirror plane on the b axis, and Cs1 has 6m2 symmetry.
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The new compound consists of a framework of extended bilayers, using nine-coordinate thorium polyhedra as its principle building block (Fig. 1a). The thorium atoms adopt a slightly distorted tricapped trigonal prism. F4 comprises the apex of the prism and is also bound to ˚ ) is neighboring Cs atoms. This Th–F bond (2.566(8) A longer than the other bonds in the prism and dictates the polyhedral alignment due to is corner-sharing by the thorium polyhedra in the next layer along the c axis. The apices of the neighboring thorium polyhedra in a given layer also alternate by 180° relative to the c axis. Within a common layer, the thorium polyhedra form clusters along the ab plane by two edge-sharing pairs, F1–F5 and F2–F6. These clusters form hexagonal-shaped ellipsoidal pores that run lengthwise along [001] where a single cesium ion sits in the center of the pore (Fig. 1b). When viewed down the c axis, it can be seen that the pore is distorted; mea˚ at its lonsuring across the a axis, the pore is 5.008(1) A gest point and 4.620(1) at its shortest point. The length of ˚ (Table 2). the pore along the c axis is 10.483(31) A Crystal Structure of NaTh3F13 (2) Compound 2 was also found to crystallize into the hexagonal space group P63/mmc (No. 194). It has the same space group and unit cell parameters as 1 but contains several different structural differences as highlighted below. This compound is isostructural to the NaNp3F13 formulation made previously by Cousson [37]. However, similar phases of the formula ATh3F14 (A = Cs, Rb, K) have been identified by either single crystal [3, 14] or powder diffraction [7], but having different structures. The structure of 2 differs greatly from these isoformulaic phases. For example, CsTh3F13 exists in two modifications: as P6/mmm and Pmc21, both having channel structures. Although the hexagonal modification crystallizes in the
Fig. 1 a Compound 1 as viewed down the ab plane. b A layer of compound 1 as viewed down the c axis. Thorium polyhedra beneath the sheet are removed for clarity
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same space group, the cell parameters are clearly much different and their channels extend infinitely and are not capped to form pores. The calculated powder pattern of 2 is generally similar to a previously-reported powder pattern for CsTh6F25 [5] in that its peaks are shifted in comparison to 1 to account for the slightly smaller unit cell volume. The powder pattern for 2 also contains a few additional peaks that account for the subtle structural differences. The thorium atoms are slightly distorted tricapped trigonal ˚ , which prisms with an average Th–F distance of 2.397(6) A correlates well with the expected values predicted by Shannon [36]. As in 1, all the unique atoms in 2 also lie on special positions. Th1, F1, and F2 have a mirror plane along the b axis, F3 has a two-fold rotation along the b axis, F4 has mm2 symmetry, Na1, F5, and F6 have a three-fold rotation along the a axis and a mirror plane along the b axis, and F7 has 6m2 symmetry. F7 occupies the same crystallographic site as Cs1 in 1.
˚ ) with esd’s for compounds 1 and Table 2 Selected bond distances (A 2 Compound 1
Compound 2
˚) Bond distances (A
˚) Bond distances (A
Th1–F1 (92)
2.376(5)
Th1–F1 (92)
2.420(5)
Th1–F2 (92)
2.347(5)
Th1–F2 (92)
2.348(4)
Th1–F3 (92)
2.336(3)
Th1–F3 (92)
2.343(3)
Th1–F4
2.3304(16)
Th1–F4
2.334(2)
Th1–F5 Th1–F6
2.503(4) 2.566(8)
Th1–F5 Th1–F6
2.357(5) 2.663(7)
Cs1–F1 (96)
3.170(11)
Na1–F1 (93)
Cs1–F2 (96)
3.268(12)
i
2.473(11)
Na1–F6
2.58(2)
Na1–F7
2.511(19)
Symmetry codes: (i) –x ? 1, -y ? 1, -z
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The new compound exhibits the same framework as 1, but with some slight differences. The thorium polyhedra also form clusters along the ab plane, but also edge-share with F1–F5 and F2–F6 pairs (Fig. 2a). These clusters also form similar hexagonal-shaped pores that run along [001], but in this case two sodium ions and one fluorine atom sit in the pore (Fig. 2b). The pore is also more distorted than in 1; viewed down the c axis and measuring across the ˚ at its longest point a axis, the pore measures 5.096(1) A ˚ and 4.093(1) A at its shortest point. The contents of the pore are in a linear Na–F–Na arrangement along [001], as was observed by Cousson in the neptunium analog [37]. The Coordination of Cs? in 1 compared to Na? in 2 The pores in both 1 and 2 are ellipsoidal shaped, but have ˚ long and hexagonal sides. In 1, they measure 10.483(31) A
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˚ at its longest point and the width measures 5.008(1) A 4.620(1) at its shortest point, indicating that the very large ˚ ionic diameter as 12-coordinate cesium ion (3.76 A Cs(I) [36]) sits in the center of the ellipsoid. There it is coordinated to 12 fluorine ions within the pore (Fig. 3a), with interactions of 3.170(11) at the narrower part of the ˚ at the wider part of the pore. The pore and 3.268(12) A Cs–F polyhedra are edge-sharing with the thorium polyhedra surrounding the pore. Due to its much smaller size, it appears Na would be too weakly bound to reside in the ˚ center of this pore. In 2, the pores measure 10.185(24) A ˚ at its longest point long and the width measures 5.096(1) A ˚ at its shortest point, leaving ample space and 4.093(1) A ˚ ionic diameter for two much smaller sodium atoms (2.00 A as five-coordinate Na(I) [36]) and a fluorine atom, F7, to reside. Atom F7 is a linear bridging atom between the two sodium atoms to create two sodium polyhedra that are
Fig. 2 a Compound 2 as viewed down the ab plane. b A layer of compound 2 as viewed down the c axis. Thorium polyhedra beneath the sheet are removed for clarity
Fig. 3 a Cut away view of an ellipsoidal pore in compound 1 as viewed down the ab plane. The 12-coordinate cesium ion (shown uncoordinated) sits in the center of the pore. b Cut away view of the ellipsoidal pore in compound 2 as viewed down the ab plane. Two 5-coordinate sodium ions (Na–F bonds are bolded) are centered at the loci of the pore and are bridged by a corner-shared fluorine
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five-coordinate centered at the loci of the ellipsoid, thus yielding corner-sharing sodium polyhedra in the structure (Fig. 3b). The marginally wider pore size measured for compound 1 appears to account for its larger unit cell, particularly the lengthening of the a and b axes. However, the shorter length of the pore in 2 along the c axis does not seem to result in c axis contraction. In this case the Th1–F6 bond that is the boundary of the pore in this direction is elongated for 2 to form a shorter pore. Meanwhile the Th1–Th1 distance associated with this boundary is essentially unchanged between the two compounds to account for the similar c axis parameter. It is interesting to note that similar differences were mentioned by Cousson when comparing the aforementioned structures of NaNp3F13 to CsU6F25 in P63/mmc [35, 37].
Conclusions In our continued research into the hydrothermal phase space in the alkali thorium fluoride system we herein identify two new structures and characterize them by single crystal XRD for the first time. The two new compounds each contain a related framework of extended bilayers that form pores within a matrix of thorium fluoride polyhedra where the alkali metal(s) sit. While both crystallize in the same space group, their structures differ by the coordination environments of the alkali metal. The pores are similar size and can only accommodate one large Cs? ion whereas two much smaller Na? ions can occupy the pores along with an additional F-. This study shows that we continue to discover new species of alkali thorium fluorides and the Th–F framework is exquisitely sensitive to the size and concentration of metal ion.
Supplementary Information Further details of the crystal structure investigations may be obtained in writing from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, on quoting the depository numbers CSD-423677 and CSD-423678. Acknowledgment The authors thank the National Science Foundation Grant #DMR-0907395 for financial support.
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