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Synthesis and crystal structure of Li2[(UO2)(MoO4)2], a uranyl molybdate with chains of corner-sharing uranyl square bipyramids and MoO4 tetrahedra
Solid State Sciences 5 (2003) 481–485 www.elsevier.com/locate/ssscie
Synthesis and crystal structure of Li2 [(UO2 )(MoO4 )2], a uranyl molybdate with chains of corner-sharing uranyl square bipyramids and MoO4 tetrahedra Sergey V. Krivovichev ∗ , Peter C. Burns Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556-0767, USA Received 25 April 2002; received in revised form 21 October 2002; accepted 29 October 2002
Abstract Crystals of Li2 [(UO2 )(MoO4 )2 ] have been synthesized by solid-state reaction of Li2 CO3 , UO3 and MoO3 . The crystal structure of the compound (triclinic, P 1, a = 5.3455(4), b = 5.8297(4), c = 8.2652(6) Å, α = 108.267(2), β = 100.566(2), γ = 104.121(2)◦ , V = 227.56(3) Å3 , Z = 1) has been solved by direct methods and refined to R1 = 0.027 (wR2 = 0.047). The structure is based upon chains of composition [(UO2 )(MoO4 )2 ]2− composed of UrO4 square bipyramids that share corners with MoO4 tetrahedra. The chains are parallel to the a axis and are linked into a three-dimensional framework by Li+ cations in trigonal bipyramidal coordination. The LiO5 trigonal bipyramids form edge-sharing chains parallel to the a axis. 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Crystal structure; Uranyl molybdate; Bond valence requirements
1. Introduction Uranium minerals and inorganic compounds have received considerable attention in recent years due to their importance for geological disposal of spent nuclear fuel and remediation of radioactive waste [1]. Of known uranyl compounds, uranyl molybdates appear to show the greatest structural complexity and diversity, owing to the flexible interpolyhedral linkages and the variety of different coordination geometries about U6+ and Mo6+ [2–10]. There are at least nine inorganic uranyl molybdates that are based on uranyl dimolybdate structural units, [(UO2 )(MoO4)2 ]2− [6,11–17]. The structures of eight of these [6,11–15] are based upon sheets of UrO5 (Ur = uranyl ion, UO2+ 2 ) pentagonal bipyramids and MoO4 tetrahedra. The [(UO2 )(MoO4)2 ]2− sheets show two linkage topologies, as described in [6]. In contrast, the structure of deloryite, Cu4 [(UO2 )(MoO4)2 ](OH)6 [16, 17], contains a [(UO2)(MoO4 )2 ]2− chain of vertex-sharing UrO4 square bipyramids and MoO4 tetrahedra. * Corresponding author. Present address: Mineralogisch-Petrographisches Institut, Kiel Universitaet, Olshausenstrasse 40, 24098 Kiel, Germany. E-mail addresses: [email protected], [email protected] (S.V. Krivovichev).
All known uranyl dimolybdates with the general formula M2 [(UO2 )(MoO4 )2 ] (M = Na, K, Rb, Cs) are based upon [(UO2 )(MoO4)2 ]2− sheets, although the topology of the sheet for M = Na [6] differs from that observed in M = K [11], Rb [10] and Cs [15]. In the current study we report the synthesis and crystal structure of Li2 [(UO2)(MoO4 )2 ], which, in contrast to other alkali metal compounds with the same stoichiometry, is based upon chains of U and Mo polyhedra similar to those observed in deloryite.
2. Experimental 2.1. Synthesis Crystals used in this study were obtained by hightemperature solid-state reaction of Li2 CO3 (0.0148 g), UO3 (0.0572 g) and MoO3 (0.0576 g). The reactants were placed in a platinum crucible and heated to 650 ◦ C for 5 hours, followed by cooling to 300 ◦ C over 50 hours, after which they were cooled to 100 ◦ C over 10 hours. The product consisted of tabular yellow transparent crystals of Li2 [(UO2 )(MoO4)2 ]. Twinning and intergrowths of the crystals were common, and finding a suitable crystal for the structural study was a challenge.
1293-2558/03/$ – see front matter 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(03)00013-X
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2.2. Data collection
Table 3 Selected bond lengths (Å) for Li2 [(UO2 )(MoO4 )2 ]
The crystal selected for data collection was mounted on a Bruker three-circle diffractometer equipped with a SMART APEX CCD (charge-coupled device) detector with a crystalto-detector distance of 5 cm. The data were collected using monochromated MoKα X-radiation and frame widths of 0.3◦ in ω. The unit-cell dimensions (Table 1) were refined from 1860 reflections using least-squares techniques. More than a hemisphere of data was collected and the threedimensional data set was reduced and filtered for statistical outliers using the Bruker program SAINT. The data were corrected for Lorentz, polarization and background effects. An empirical absorption correction was done on the basis of 652 reflections by modelling the crystal as an ellipsoid, which lowered Rint from 7.8 to 2.8%. Additional information pertinent to the data collection is given in Table 1.
U(1)–O(2),a U(1)–O(5)b,c U(1)–O(3),a
1.794(3) 2× 2.250(3) 2× 2.271(3) 2×
Mo(1)–O(1) Mo(1)–O(4) Mo(1)–O(5) Mo(1)–O(3) Mo–O
1.739(3) 1.740(3) 1.772(3) 1.788(3) 1.76
Li–O(1)d Li–O(4) Li–O(2)e Li–O(4)f Li–O(1)g Li–O
1.982(8) 1.998(9) 2.027(9) 2.13(1) 2.28(1)
a = −x, −y, −z; b = x, y − 1, z; c = −x, −y + 1, −z; d = x + 1, y, z; e = x, y, z − 1; f = −x + 1, −y + 1, −z − 1; g = −x, −y, −z − 1
2.3. Structure solution and refinement
Table 1 Crystal data and structure refinement for Li2 [(UO2 )(MoO4 )2 ] Temperature Wavelength Crystal system Space group Unit cell dimensions
293(2) K 0.71073 Å Triclinic P1 a = 5.3455(4) Å, b = 5.8297(4) Å, c = 8.2652(6) Å, α = 108.267(2)◦ , β = 100.566(2)◦ , γ = 104.121(2)◦ 227.56(3) Å3 1 4.406 mg/mm3 20.491 mm−1 262 0.14 × 0.10 × 0.08 mm3 2.71 to 34.52◦ 2554 1791
Volume Z Density (calculated) Absorption coefficient F (000) Crystal size Theta range for data collection Reflections collected Independent reflections Independent observed reflections [I > 2σ (I )] Refinement method Goodness-of-fit on F 2 Final R indices
1622 Full-matrix least-squares on F 2 0.883 R1 = 0.027, wR2 = 0.047
Scattering curves for neutral atoms, together with anomalous-dispersion corrections, were taken from [18]. The Bruker SHELXTL Version 5 system of programs was used for the determination and refinement of the structure. It was solved by direct methods and refined on the basis of F 2 for all unique reflections to R1 = 2.7% in space group P 1. The final model included anisotropic displacement parameters for all atoms and a weighting scheme of the structure factors. The final atomic positional and displacement parameters, and selected interatomic distances are in Tables 2 and 3, respectively. Tables of observed and calculated structure factors are available from the authors upon request.
3. Results 3.1. Cation polyhedra There is one symmetrically unique U6+ cation in the structure and it is bonded to two O atoms, forming a linear uranyl ion, UO2+ 2 (Ur) with U–OUr bond lengths of 1.794 Å. The uranyl ion is coordinated by four O atoms arranged at the equatorial vertices of UrO4 square bipyramid. The U–Oeq (eq: equatorial) bond distance is 2.26 Å, which is
Table 2 Atomic coordinates and displacement parameters (104 × Å2 ) for Li2 [(UO2 )(MoO4 )2 ] Atom
x
y
z
U eq
U11
U22
U33
U23
U13
U Mo O(1) O(2) O(3) O(4) O(5) Li
0 0.08559(7) −0.2179(7) 0.3108(7) 0.2090(7) 0.3096(7) 0.0287(8) 0.4796(17)
0 0.34926(6) 0.1364(6) 0.0628(7) 0.2081(6) 0.4277(6) 0.6292(6) 0.2157(16)
0 −0.30111(5) −0.4512(4) 0.1587(4) −0.1536(4) −0.4188(4) −0.1743(5) −0.5747(11)
132(1) 130(1) 212(7) 220(7) 207(7) 212(7) 300(9) 219(18)
170(1) 149(2) 207(17) 208(18) 198(17) 240(18) 400(20) 190(40)
121(1) 117(2) 195(16) 291(18) 259(17) 187(15) 208(18) 240(40)
110(1) 130(2) 191(16) 156(15) 205(16) 242(17) 280(20) 240(40)
47(1) 51(1) 71(13) 110(14) 138(14) 98(13) 19(15) 100(30)
31(1) 50(1) −5(13) 20(13) 58(13) 133(14) 99(17) 30(30)
U eq is defined as one third of the trace of the orthogonalized Uij tensor.
U12 57(1) 40(2) 40(13) 62(14) 77(14) 52(13) 169(17) 110(30)
S.V. Krivovichev, P.C. Burns / Solid State Sciences 5 (2003) 481–485
483
Table 4 Bond-valence analysis (v.u.) for Li2 [(UO2 )(MoO4 )2 ]* O(1)
O(2)
O(3)
1.662→
0.702→
U Mo Li
1.57 0.25, 0.11
0.22
!
1.93
1.88
O(4)
O(5)
!
0.732→
6.18 5.96 0.99
1.38
1.57 0.24, 0.17
1.44
2.08
1.98
2.17
* Calculated on the basis of bond-valence parameters for U6+ –O bonds by Burns et al. [19] and for Mo6+ –O and Li+ –O bonds by Brese and O’Keeffe
[21].
Fig. 1. Structure of Li2 [(UO2 )(MoO4 )2 ] projected along the a (a) and b (b) axes. Legend: U polyhedra are shaded with crosses, Mo tetrahedra are shaded with lines, Li atoms are shown as circles.
consistent with the value of 2.28(5) Å obtained for uranyl square bipyramids from numerous well-refined structures [19,20]. One symmetrically independent Mo6+ cation is tetrahedrally coordinated by four O atoms. The Mo–O
distance for the MoO4 tetrahedron is 1.76 Å, in good agreement with values in uranyl molybdates containing MoO4 tetrahedra. There is one Li+ cation that is coordinated by five O atoms located at the vertices of a distorted trigonal bipyramid. The individual Li+ –O bond lengths are in the range of 1.98 to 2.28 Å. 3.2. Bond-valence analysis The bond-valence sums for the atoms in the structure was calculated using parameters given by Burns et al. [19,20] for U6+ –O bonds and by Brese and O’Keeffe [21] for Mo6+ – O and Li+ –O. The results are summarized in Table 4. The bond-valence sums for all atoms are in agreement with their expected values. 3.3. Structure description The structure is based upon chains of composition [(UO2 )(MoO4)2 ]2− that contain UrO4 square bipyramids that share vertices with MoO4 tetrahedra (Figs. 1, 2). The chains are parallel to the a axis and are linked into a threedimensional framework by Li+ cations (Fig. 1). The LiO5
trigonal bipyramids form edge-sharing chains that extend parallel to the a axis. The structural geometry of the [(UO2 )(MoO4)2 ]2− chain is shown in Fig. 2a. The O(3) and O(5) atoms are shared between UrO4 square bipyramids and MoO4 tetrahedra. The Mo–O(3)–U and Mo–O(5)–U bond angles are 131.1(2) and 174.4(2)◦. The latter value is close to linear, which is unusual for Mo–O–U linkages in uranyl molybdates. As a consequence, the anisotropic displacement ellipsoid of the O(5) atom is elongated in the direction perpendicular to the U–O and Mo–O bonds (Fig. 2a), suggesting either static or dynamic distortion of the bond angle.
4. Discussion 4.1. Comparison topologically related structures Burns et al. [22] developed a structural hierarchy of uranyl minerals and inorganic compounds that is based on polymerization of cation polyhedra of high bond-valence. Chains of corner-sharing UrO4 square bipyramids and TO4 tetrahedra (T = As, P or Mo) were found in the structures of deloryite, Cu4 [(UO2 )(MoO4)2 ](OH)6 [16,17], walpurgite, Bi4 O4 [(UO2 )(AsO4 )2 ](H2 O)2 [23], orthowalpurgite, Bi4 O4 [(UO2 )(AsO4 )2 ](H2 O)2 [24], and Cu2 [(UO2)(PO4 )2 ] [25]. Recently, Almond and Albrecht-Schmitt [26] reported
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Fig. 2. The configuration of the [(UO2 )(MoO4 )2 ]2− chain in the structure of Li2 [(UO2 )(MoO4 )2 ] in thermal displacement ellipsoids (a) (ellipsoids are drawn at the 50% probability level), its graphical (b) and polyhedral (c) representations; the [(UO2 )(MoO4 )2 ]2− chain in the structure of deloryite (d); the [(UO2 )(AsO4 )2 ]4− chain in the structure of walpurgite (e); the [(UO2 )(TeO3 )2 ]2− chain in the structure of β-Tl2 [(UO2 )(TeO3 )2 ] (f); the [(UO2 )(TeO3 )2 ]2− chain in the structure of Sr3 [(UO2 )(TeO3 )2 ](TeO3 ) (g). Table 5 Crystallographic data for minerals and inorganic compounds based upon [(UO2 )(TO4 )2 ] (T = Mo, As, P) or [(UO2 )(TeO3 )2 ] chains* Chemical formula
Mineral name
Sp. gr.
a (Å)
b (Å)
c (Å)
α (◦ )
β (◦ )
γ (◦ )
Li2 [(UO2 )(MoO4 )2 ] Cu4 [(UO2 )(MoO4 )2 ](OH)6 Bi4 O4 [(UO2 )(AsO4 )2 ](H2 O)2 Bi4 O4 [(UO2 )(AsO4 )2 ](H2 O)2 Cu2 [(UO2 )(PO4 )2 ] β-Tl2 [(UO2 )(TeO3 )2 ] Sr3 [(UO2 )(TeO3 )2 ](TeO3 )
– deloryite walpurgite orthowalpurgite – – –
P -1 C2/m P -1 Pbcm C2/m P 21 /n C2/c
5.3455 19.940 7.135 5.492 14.040 5.4766 20.546
5.8297 6.116 10.426 13.324 5.7595 8.2348 5.6571
8.2652 5.520 5.494 20.685 5.0278 20.849 13.0979
108.267 − 101.47 − − − −
100.566 104.18 110.82 − 107.24 92.329 105.8
104.121 − 88.20 − − − −
Ref. **
[16,17] [23] [24] [25] [26] [26]
* Unit-cell parameters corresponding to the chain extension are given as bold. ** This work.
syntheses and structures of β-Tl2 [(UO2 )(TeO3)2 ] and Sr3 [(UO2 )(TeO3)2 ](TeO3). Structures of these compounds are based upon the [(UO2)(TeO3 )2 ]2− chains that consist of UrO4 square bipyramids sharing corners with TeO3 groups. The Te(IV)O3 groups can be described as tetrahedral TeO3 E configurations assuming the lone electron pair E to be at the corner of a tetrahedron. Crystallographic data for the minerals and inorganic compounds based upon the [(UO2 )(TO4)2 ] (T = As, P or Mo) or [(UO2 )(TeO3 )2 ] chains are given in Table 5. Topological structure of polyhedral linkage within the [(UO2 )(TO4)2 ] or [(UO2)(TeO3 )2 ] chains can be examined by their nodal representations. Each node corresponds to a UrO4 bipyramid (black circle) or a TO4 or TeO3 E tetrahedron (white circle). Nodes are connected if the polyhedra share at least one common vertex. Using this approach, the [(UO2 )(TO4)2 ] or [(UO2)(TeO3 )2 ] chain is associated with the black-and-white graph shown in Fig. 2b. It consists of 4-connected black and 2-connected white nodes. Polyhedral representations of the [(UO2 )(TO4)2 ] and [(UO2 )(TeO3 )2 ] chains from the selected structures are shown in Figs. 2c–h. Although the [(UO2)(TO4 )2 ] chains shown in Figs. 2c, d and e have different geometrical parameters, they are topologically similar as they can be transformed one into another by simple rotation of the respective polyhedra. In contrast, the [(UO2 )(TeO3 )2 ] chains shown in Figs. 2g and h cannot be transformed into each other by rota-
tion of polyhedra. The chain shown in Fig. 2g has the E vertices of the TeO3 E tetrahedra on different sides of the chain, whereas the chain shown in Fig. 2h has the E vertices on one side of the chain only. However, the topology of polyhedral linkage of these chains is the same and correspond to the same black-and-white graph shown in Fig. 2b. According to the definitions given by Hawthorne [27], the [(UO2 )(TeO3)2 ] chains shown in Figs. 2g and h should be considered as geometrical isomers. Here geometrical isomerism is a consequence of the replacement of one of the tetrahedral vertices by the lone electron pair E which makes the terminal vertices of tetrahedra non-equivalent in a topological sense. No such isomers are possible for the [(UO2 )(TO4)2 ] chains as terminal vertices of their TO4 tetrahedra are equivalent. 4.2. Comparison to deloryite The [(UO2 )(MoO4)2 ]2− chain in the structure of deloryite, Cu4 [(UO2 )(MoO4)2 ](OH)6 [16,17], is shown in Fig. 2d. The deloryite chain is more linear than that found in the current study, which is reflected in its longer identity period (6.116 Å), in comparison with 5.830 Å found for Li2 [(UO2 )(MoO4 )2 ] (Fig. 2c). All of the Mo–O–U angles for the bridging O atoms in deloryite are 154.0◦, thus the chain is more symmetrical. Thus, the uranyl dimolybdate chain in Li2 [(UO2 )(MoO4)2 ] is strongly distorted in comparison with that observed in deloryite, which again demon-
S.V. Krivovichev, P.C. Burns / Solid State Sciences 5 (2003) 481–485
strates that uranyl molybdate units are flexible and can be distorted depending on their environment. It is noted that most uranyl molybdates are based upon structural units of UrO5 pentagonal bipyramids that share corners with MoO4 tetrahedra. The title compound is only the fourth example of a uranyl molybdate that contains corner-sharing uranyl square bipyramids and MoO4 tetrahedra. Other examples are deloryite [16,17], Cs6 [(UO2)(MoO4 )4 ] [7] and Rb6 [(UO2 )(MoO4)4 ] [10]. The two latter compounds are based upon [(UO2 )(MoO4)4 ]6− finite clusters that involve a central UrO4 square bipyramid that shares four of its equatorial vertices with MoO4 tetrahedra. The uranyl dimolybdate chain may be regarded as the result of polymerization of these finite clusters. The relative scarcity of the structures based upon units of UrO4 square bipyramids and MoO4 tetrahedra, in comparison with those containing UrO5 groups, can be understood using bond-valence theory. The Mo6+ –O bondvalence is ∼1.5 v.u. (valence units), whereas the typical residual bond-valences incident upon equatorial vertices of UrO5 and UrO4 bipyramids are 0.54 and 0.64 v.u., respectively [20]. This means that sharing of an O atom between UrO5 and MoO4 tetrahedron provides almost perfect satisfaction of its bond-valence requirements (2.00 v.u.). In contrast, sharing of an O atom between UrO4 and MoO4 groups may result in some degree of overbonding, which can be compensated by elongation of Mo–O bond within the MoO4 tetrahedron and, as a consequence, a distortion of the tetrahedron as a whole. This effect can be observed in the structure of Li2 [(UO2 )(MoO4 )2 ]: the Mo–O(3) and Mo–O(5) bonds to the O(3) and O(5) atoms that are shared between U and Mo polyhedra are 1.772 and 1.788 Å. In order to attain a bond-valence sum for the Mo site that is ∼6.0 v.u., two other Mo–O bonds (Mo–O(1) and Mo–O(4)) are shortened to 1.739 and 1.740 Å, respectively. A similar situation is observed for both Cs6 [(UO2 )(MoO4)4 ] [7] and Rb6 [(UO2 )(MoO4)4 ] [10], where the Mo–O bonds to bridging O atoms are much longer than the Mo–O bonds to terminal O atoms. The necessity of distortion of MoO4 tetrahedra makes uranyl molybdates based on UrO4 bipyramids much less abundant than those based on UrO5 groups.
Supplementary material The supplementary material has been sent to the Fachinformationzentrum Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany, as supplementary material
485
no. SUP. . . (. . .pages) and can be obtained by contacting the FIZ (quoting the article details and the corresponding SUP number). Acknowledgements This research was supported by the Environmental Management Sciences Program of the United States Department of Energy (grant DE-FG07-97ER 14820) and by the Russian Foundation for Basic Research for S.V.K. (grant 0105-64883).
References [1] P.C. Burns, R.J. Finch (Eds.), Uranium: Mineralogy, Geochemistry and the Environment, Reviews in Mineralogy, Vol. 38, Mineralogical Society of America, Washington DC, 1999. [2] S.V. Krivovichev, P.C. Burns, Can. Mineral. 38 (2000) 717. [3] S.V. Krivovichev, P.C. Burns, Can. Mineral. 38 (2000) 847. [4] S.V. Krivovichev, P.C. Burns, Can. Mineral. 39 (2001) 197. [5] S.V. Krivovichev, P.C. Burns, Can. Mineral. 39 (2001) 215. [6] S.V. Krivovichev, R.J. Finch, P.C. Burns, Can. Mineral. 40 (2002) 193. [7] S.V. Krivovichev, P.C. Burns, Can. Mineral. 40 (2002) 201. [8] S.V. Krivovichev, C.L. Cahill, P.C. Burns, Inorg. Chem. 41 (2002) 34. [9] S.V. Krivovichev, P.C. Burns, Can. Mineral. 40 (2002) 1571. [10] S.V. Krivovichev, P.C. Burns, J. Solid State Chem. 168 (2002) 245. [11] G.G. Sadikov, T.I. Krasovskaya, Y.A. Polyakov, V.P. Nikolaev, Izv. Akad. Nauk SSSR, Neorg. Mater. 24 (1988) 109. [12] R.K. Rastsvetaeva, A.V. Barinova, A.M. Fedoseev, N.A. Budantseva, Yu.V. Nekrasov, Dokl. Akad. Nauk 365 (1999) 68. [13] V.N. Khrustalev, G.B. Andreev, M.Yu. Antipin, A.M. Fedoseev, N.A. Budantseva, I.B. Shirokova, Zh. Neorg. Khim. 45 (2000) 1996. [14] G.B. Andreev, M.Yu. Antipin, A.M. Fedoseev, N.A. Budantseva, Koord. Khim. 27 (2001) 227–230. [15] S.V. Krivovichev, P.C. Burns, in preparation. [16] R. Tali, V.V. Tabachenko, L.M. Kovba, L.N. Dem’yanets, Zh. Neorg. Khim. 39 (1994) 1752. [17] D.Yu. Pushcharovsky, R.K. Rastsvetaeva, H. Sarp, J. Alloys Compd. 239 (1996) 23. [18] J.A. Ibers, W.C. Hamilton (Eds.), International Tables for X-ray Crystallography, Vol. IV, The Kynoch Press, Birmingham, UK, 1974. [19] P.C. Burns, R.C. Ewing, F.C. Hawthorne, Can. Mineral. 35 (1997) 1551. [20] P.C. Burns, Rev. Mineral. 38 (1999) 23. [21] N.E. Brese, M. O’Keeffe, Acta Crystallogr. B 47 (1991) 192. [22] P.C. Burns, M.L. Miller, R.C. Ewing, Can. Mineral. 34 (1996) 845. [23] K. Mereiter, Tschch. Miner. Petrogr. Mitt. 30 (1982) 277. [24] W. Krause, H. Effenberger, F. Brandstätter, Eur. J. Mineral. 7 (1995) 1313. [25] A. Guesdon, J. Chardon, J. Provost, B. Raveau, J. Solid State Chem. 165 (2002) 89–93. [26] P.M. Almond, T.E. Albrecht-Schmitt, Inorg. Chem. 41 (2002) 5495. [27] F.C. Hawthorne, Acta Crystallogr. A 39 (1983) 724–736.
Synthesis and crystal structure of Li2 [(UO2 )(MoO4 )2], a uranyl molybdate with chains of corner-sharing uranyl square bipyramids and MoO4 tetrahedra Sergey V. Krivovichev ∗ , Peter C. Burns Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556-0767, USA Received 25 April 2002; received in revised form 21 October 2002; accepted 29 October 2002
Abstract Crystals of Li2 [(UO2 )(MoO4 )2 ] have been synthesized by solid-state reaction of Li2 CO3 , UO3 and MoO3 . The crystal structure of the compound (triclinic, P 1, a = 5.3455(4), b = 5.8297(4), c = 8.2652(6) Å, α = 108.267(2), β = 100.566(2), γ = 104.121(2)◦ , V = 227.56(3) Å3 , Z = 1) has been solved by direct methods and refined to R1 = 0.027 (wR2 = 0.047). The structure is based upon chains of composition [(UO2 )(MoO4 )2 ]2− composed of UrO4 square bipyramids that share corners with MoO4 tetrahedra. The chains are parallel to the a axis and are linked into a three-dimensional framework by Li+ cations in trigonal bipyramidal coordination. The LiO5 trigonal bipyramids form edge-sharing chains parallel to the a axis. 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Crystal structure; Uranyl molybdate; Bond valence requirements
1. Introduction Uranium minerals and inorganic compounds have received considerable attention in recent years due to their importance for geological disposal of spent nuclear fuel and remediation of radioactive waste [1]. Of known uranyl compounds, uranyl molybdates appear to show the greatest structural complexity and diversity, owing to the flexible interpolyhedral linkages and the variety of different coordination geometries about U6+ and Mo6+ [2–10]. There are at least nine inorganic uranyl molybdates that are based on uranyl dimolybdate structural units, [(UO2 )(MoO4)2 ]2− [6,11–17]. The structures of eight of these [6,11–15] are based upon sheets of UrO5 (Ur = uranyl ion, UO2+ 2 ) pentagonal bipyramids and MoO4 tetrahedra. The [(UO2 )(MoO4)2 ]2− sheets show two linkage topologies, as described in [6]. In contrast, the structure of deloryite, Cu4 [(UO2 )(MoO4)2 ](OH)6 [16, 17], contains a [(UO2)(MoO4 )2 ]2− chain of vertex-sharing UrO4 square bipyramids and MoO4 tetrahedra. * Corresponding author. Present address: Mineralogisch-Petrographisches Institut, Kiel Universitaet, Olshausenstrasse 40, 24098 Kiel, Germany. E-mail addresses: [email protected], [email protected] (S.V. Krivovichev).
All known uranyl dimolybdates with the general formula M2 [(UO2 )(MoO4 )2 ] (M = Na, K, Rb, Cs) are based upon [(UO2 )(MoO4)2 ]2− sheets, although the topology of the sheet for M = Na [6] differs from that observed in M = K [11], Rb [10] and Cs [15]. In the current study we report the synthesis and crystal structure of Li2 [(UO2)(MoO4 )2 ], which, in contrast to other alkali metal compounds with the same stoichiometry, is based upon chains of U and Mo polyhedra similar to those observed in deloryite.
2. Experimental 2.1. Synthesis Crystals used in this study were obtained by hightemperature solid-state reaction of Li2 CO3 (0.0148 g), UO3 (0.0572 g) and MoO3 (0.0576 g). The reactants were placed in a platinum crucible and heated to 650 ◦ C for 5 hours, followed by cooling to 300 ◦ C over 50 hours, after which they were cooled to 100 ◦ C over 10 hours. The product consisted of tabular yellow transparent crystals of Li2 [(UO2 )(MoO4)2 ]. Twinning and intergrowths of the crystals were common, and finding a suitable crystal for the structural study was a challenge.
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2.2. Data collection
Table 3 Selected bond lengths (Å) for Li2 [(UO2 )(MoO4 )2 ]
The crystal selected for data collection was mounted on a Bruker three-circle diffractometer equipped with a SMART APEX CCD (charge-coupled device) detector with a crystalto-detector distance of 5 cm. The data were collected using monochromated MoKα X-radiation and frame widths of 0.3◦ in ω. The unit-cell dimensions (Table 1) were refined from 1860 reflections using least-squares techniques. More than a hemisphere of data was collected and the threedimensional data set was reduced and filtered for statistical outliers using the Bruker program SAINT. The data were corrected for Lorentz, polarization and background effects. An empirical absorption correction was done on the basis of 652 reflections by modelling the crystal as an ellipsoid, which lowered Rint from 7.8 to 2.8%. Additional information pertinent to the data collection is given in Table 1.
U(1)–O(2),a U(1)–O(5)b,c U(1)–O(3),a
1.794(3) 2× 2.250(3) 2× 2.271(3) 2×
Mo(1)–O(1) Mo(1)–O(4) Mo(1)–O(5) Mo(1)–O(3) Mo–O
1.739(3) 1.740(3) 1.772(3) 1.788(3) 1.76
Li–O(1)d Li–O(4) Li–O(2)e Li–O(4)f Li–O(1)g Li–O
1.982(8) 1.998(9) 2.027(9) 2.13(1) 2.28(1)
a = −x, −y, −z; b = x, y − 1, z; c = −x, −y + 1, −z; d = x + 1, y, z; e = x, y, z − 1; f = −x + 1, −y + 1, −z − 1; g = −x, −y, −z − 1
2.3. Structure solution and refinement
Table 1 Crystal data and structure refinement for Li2 [(UO2 )(MoO4 )2 ] Temperature Wavelength Crystal system Space group Unit cell dimensions
293(2) K 0.71073 Å Triclinic P1 a = 5.3455(4) Å, b = 5.8297(4) Å, c = 8.2652(6) Å, α = 108.267(2)◦ , β = 100.566(2)◦ , γ = 104.121(2)◦ 227.56(3) Å3 1 4.406 mg/mm3 20.491 mm−1 262 0.14 × 0.10 × 0.08 mm3 2.71 to 34.52◦ 2554 1791
Volume Z Density (calculated) Absorption coefficient F (000) Crystal size Theta range for data collection Reflections collected Independent reflections Independent observed reflections [I > 2σ (I )] Refinement method Goodness-of-fit on F 2 Final R indices
1622 Full-matrix least-squares on F 2 0.883 R1 = 0.027, wR2 = 0.047
Scattering curves for neutral atoms, together with anomalous-dispersion corrections, were taken from [18]. The Bruker SHELXTL Version 5 system of programs was used for the determination and refinement of the structure. It was solved by direct methods and refined on the basis of F 2 for all unique reflections to R1 = 2.7% in space group P 1. The final model included anisotropic displacement parameters for all atoms and a weighting scheme of the structure factors. The final atomic positional and displacement parameters, and selected interatomic distances are in Tables 2 and 3, respectively. Tables of observed and calculated structure factors are available from the authors upon request.
3. Results 3.1. Cation polyhedra There is one symmetrically unique U6+ cation in the structure and it is bonded to two O atoms, forming a linear uranyl ion, UO2+ 2 (Ur) with U–OUr bond lengths of 1.794 Å. The uranyl ion is coordinated by four O atoms arranged at the equatorial vertices of UrO4 square bipyramid. The U–Oeq (eq: equatorial) bond distance is 2.26 Å, which is
Table 2 Atomic coordinates and displacement parameters (104 × Å2 ) for Li2 [(UO2 )(MoO4 )2 ] Atom
x
y
z
U eq
U11
U22
U33
U23
U13
U Mo O(1) O(2) O(3) O(4) O(5) Li
0 0.08559(7) −0.2179(7) 0.3108(7) 0.2090(7) 0.3096(7) 0.0287(8) 0.4796(17)
0 0.34926(6) 0.1364(6) 0.0628(7) 0.2081(6) 0.4277(6) 0.6292(6) 0.2157(16)
0 −0.30111(5) −0.4512(4) 0.1587(4) −0.1536(4) −0.4188(4) −0.1743(5) −0.5747(11)
132(1) 130(1) 212(7) 220(7) 207(7) 212(7) 300(9) 219(18)
170(1) 149(2) 207(17) 208(18) 198(17) 240(18) 400(20) 190(40)
121(1) 117(2) 195(16) 291(18) 259(17) 187(15) 208(18) 240(40)
110(1) 130(2) 191(16) 156(15) 205(16) 242(17) 280(20) 240(40)
47(1) 51(1) 71(13) 110(14) 138(14) 98(13) 19(15) 100(30)
31(1) 50(1) −5(13) 20(13) 58(13) 133(14) 99(17) 30(30)
U eq is defined as one third of the trace of the orthogonalized Uij tensor.
U12 57(1) 40(2) 40(13) 62(14) 77(14) 52(13) 169(17) 110(30)
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483
Table 4 Bond-valence analysis (v.u.) for Li2 [(UO2 )(MoO4 )2 ]* O(1)
O(2)
O(3)
1.662→
0.702→
U Mo Li
1.57 0.25, 0.11
0.22
!
1.93
1.88
O(4)
O(5)
!
0.732→
6.18 5.96 0.99
1.38
1.57 0.24, 0.17
1.44
2.08
1.98
2.17
* Calculated on the basis of bond-valence parameters for U6+ –O bonds by Burns et al. [19] and for Mo6+ –O and Li+ –O bonds by Brese and O’Keeffe
[21].
Fig. 1. Structure of Li2 [(UO2 )(MoO4 )2 ] projected along the a (a) and b (b) axes. Legend: U polyhedra are shaded with crosses, Mo tetrahedra are shaded with lines, Li atoms are shown as circles.
consistent with the value of 2.28(5) Å obtained for uranyl square bipyramids from numerous well-refined structures [19,20]. One symmetrically independent Mo6+ cation is tetrahedrally coordinated by four O atoms. The Mo–O
distance for the MoO4 tetrahedron is 1.76 Å, in good agreement with values in uranyl molybdates containing MoO4 tetrahedra. There is one Li+ cation that is coordinated by five O atoms located at the vertices of a distorted trigonal bipyramid. The individual Li+ –O bond lengths are in the range of 1.98 to 2.28 Å. 3.2. Bond-valence analysis The bond-valence sums for the atoms in the structure was calculated using parameters given by Burns et al. [19,20] for U6+ –O bonds and by Brese and O’Keeffe [21] for Mo6+ – O and Li+ –O. The results are summarized in Table 4. The bond-valence sums for all atoms are in agreement with their expected values. 3.3. Structure description The structure is based upon chains of composition [(UO2 )(MoO4)2 ]2− that contain UrO4 square bipyramids that share vertices with MoO4 tetrahedra (Figs. 1, 2). The chains are parallel to the a axis and are linked into a threedimensional framework by Li+ cations (Fig. 1). The LiO5
trigonal bipyramids form edge-sharing chains that extend parallel to the a axis. The structural geometry of the [(UO2 )(MoO4)2 ]2− chain is shown in Fig. 2a. The O(3) and O(5) atoms are shared between UrO4 square bipyramids and MoO4 tetrahedra. The Mo–O(3)–U and Mo–O(5)–U bond angles are 131.1(2) and 174.4(2)◦. The latter value is close to linear, which is unusual for Mo–O–U linkages in uranyl molybdates. As a consequence, the anisotropic displacement ellipsoid of the O(5) atom is elongated in the direction perpendicular to the U–O and Mo–O bonds (Fig. 2a), suggesting either static or dynamic distortion of the bond angle.
4. Discussion 4.1. Comparison topologically related structures Burns et al. [22] developed a structural hierarchy of uranyl minerals and inorganic compounds that is based on polymerization of cation polyhedra of high bond-valence. Chains of corner-sharing UrO4 square bipyramids and TO4 tetrahedra (T = As, P or Mo) were found in the structures of deloryite, Cu4 [(UO2 )(MoO4)2 ](OH)6 [16,17], walpurgite, Bi4 O4 [(UO2 )(AsO4 )2 ](H2 O)2 [23], orthowalpurgite, Bi4 O4 [(UO2 )(AsO4 )2 ](H2 O)2 [24], and Cu2 [(UO2)(PO4 )2 ] [25]. Recently, Almond and Albrecht-Schmitt [26] reported
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Fig. 2. The configuration of the [(UO2 )(MoO4 )2 ]2− chain in the structure of Li2 [(UO2 )(MoO4 )2 ] in thermal displacement ellipsoids (a) (ellipsoids are drawn at the 50% probability level), its graphical (b) and polyhedral (c) representations; the [(UO2 )(MoO4 )2 ]2− chain in the structure of deloryite (d); the [(UO2 )(AsO4 )2 ]4− chain in the structure of walpurgite (e); the [(UO2 )(TeO3 )2 ]2− chain in the structure of β-Tl2 [(UO2 )(TeO3 )2 ] (f); the [(UO2 )(TeO3 )2 ]2− chain in the structure of Sr3 [(UO2 )(TeO3 )2 ](TeO3 ) (g). Table 5 Crystallographic data for minerals and inorganic compounds based upon [(UO2 )(TO4 )2 ] (T = Mo, As, P) or [(UO2 )(TeO3 )2 ] chains* Chemical formula
Mineral name
Sp. gr.
a (Å)
b (Å)
c (Å)
α (◦ )
β (◦ )
γ (◦ )
Li2 [(UO2 )(MoO4 )2 ] Cu4 [(UO2 )(MoO4 )2 ](OH)6 Bi4 O4 [(UO2 )(AsO4 )2 ](H2 O)2 Bi4 O4 [(UO2 )(AsO4 )2 ](H2 O)2 Cu2 [(UO2 )(PO4 )2 ] β-Tl2 [(UO2 )(TeO3 )2 ] Sr3 [(UO2 )(TeO3 )2 ](TeO3 )
– deloryite walpurgite orthowalpurgite – – –
P -1 C2/m P -1 Pbcm C2/m P 21 /n C2/c
5.3455 19.940 7.135 5.492 14.040 5.4766 20.546
5.8297 6.116 10.426 13.324 5.7595 8.2348 5.6571
8.2652 5.520 5.494 20.685 5.0278 20.849 13.0979
108.267 − 101.47 − − − −
100.566 104.18 110.82 − 107.24 92.329 105.8
104.121 − 88.20 − − − −
Ref. **
[16,17] [23] [24] [25] [26] [26]
* Unit-cell parameters corresponding to the chain extension are given as bold. ** This work.
syntheses and structures of β-Tl2 [(UO2 )(TeO3)2 ] and Sr3 [(UO2 )(TeO3)2 ](TeO3). Structures of these compounds are based upon the [(UO2)(TeO3 )2 ]2− chains that consist of UrO4 square bipyramids sharing corners with TeO3 groups. The Te(IV)O3 groups can be described as tetrahedral TeO3 E configurations assuming the lone electron pair E to be at the corner of a tetrahedron. Crystallographic data for the minerals and inorganic compounds based upon the [(UO2 )(TO4)2 ] (T = As, P or Mo) or [(UO2 )(TeO3 )2 ] chains are given in Table 5. Topological structure of polyhedral linkage within the [(UO2 )(TO4)2 ] or [(UO2)(TeO3 )2 ] chains can be examined by their nodal representations. Each node corresponds to a UrO4 bipyramid (black circle) or a TO4 or TeO3 E tetrahedron (white circle). Nodes are connected if the polyhedra share at least one common vertex. Using this approach, the [(UO2 )(TO4)2 ] or [(UO2)(TeO3 )2 ] chain is associated with the black-and-white graph shown in Fig. 2b. It consists of 4-connected black and 2-connected white nodes. Polyhedral representations of the [(UO2 )(TO4)2 ] and [(UO2 )(TeO3 )2 ] chains from the selected structures are shown in Figs. 2c–h. Although the [(UO2)(TO4 )2 ] chains shown in Figs. 2c, d and e have different geometrical parameters, they are topologically similar as they can be transformed one into another by simple rotation of the respective polyhedra. In contrast, the [(UO2 )(TeO3 )2 ] chains shown in Figs. 2g and h cannot be transformed into each other by rota-
tion of polyhedra. The chain shown in Fig. 2g has the E vertices of the TeO3 E tetrahedra on different sides of the chain, whereas the chain shown in Fig. 2h has the E vertices on one side of the chain only. However, the topology of polyhedral linkage of these chains is the same and correspond to the same black-and-white graph shown in Fig. 2b. According to the definitions given by Hawthorne [27], the [(UO2 )(TeO3)2 ] chains shown in Figs. 2g and h should be considered as geometrical isomers. Here geometrical isomerism is a consequence of the replacement of one of the tetrahedral vertices by the lone electron pair E which makes the terminal vertices of tetrahedra non-equivalent in a topological sense. No such isomers are possible for the [(UO2 )(TO4)2 ] chains as terminal vertices of their TO4 tetrahedra are equivalent. 4.2. Comparison to deloryite The [(UO2 )(MoO4)2 ]2− chain in the structure of deloryite, Cu4 [(UO2 )(MoO4)2 ](OH)6 [16,17], is shown in Fig. 2d. The deloryite chain is more linear than that found in the current study, which is reflected in its longer identity period (6.116 Å), in comparison with 5.830 Å found for Li2 [(UO2 )(MoO4 )2 ] (Fig. 2c). All of the Mo–O–U angles for the bridging O atoms in deloryite are 154.0◦, thus the chain is more symmetrical. Thus, the uranyl dimolybdate chain in Li2 [(UO2 )(MoO4)2 ] is strongly distorted in comparison with that observed in deloryite, which again demon-
S.V. Krivovichev, P.C. Burns / Solid State Sciences 5 (2003) 481–485
strates that uranyl molybdate units are flexible and can be distorted depending on their environment. It is noted that most uranyl molybdates are based upon structural units of UrO5 pentagonal bipyramids that share corners with MoO4 tetrahedra. The title compound is only the fourth example of a uranyl molybdate that contains corner-sharing uranyl square bipyramids and MoO4 tetrahedra. Other examples are deloryite [16,17], Cs6 [(UO2)(MoO4 )4 ] [7] and Rb6 [(UO2 )(MoO4)4 ] [10]. The two latter compounds are based upon [(UO2 )(MoO4)4 ]6− finite clusters that involve a central UrO4 square bipyramid that shares four of its equatorial vertices with MoO4 tetrahedra. The uranyl dimolybdate chain may be regarded as the result of polymerization of these finite clusters. The relative scarcity of the structures based upon units of UrO4 square bipyramids and MoO4 tetrahedra, in comparison with those containing UrO5 groups, can be understood using bond-valence theory. The Mo6+ –O bondvalence is ∼1.5 v.u. (valence units), whereas the typical residual bond-valences incident upon equatorial vertices of UrO5 and UrO4 bipyramids are 0.54 and 0.64 v.u., respectively [20]. This means that sharing of an O atom between UrO5 and MoO4 tetrahedron provides almost perfect satisfaction of its bond-valence requirements (2.00 v.u.). In contrast, sharing of an O atom between UrO4 and MoO4 groups may result in some degree of overbonding, which can be compensated by elongation of Mo–O bond within the MoO4 tetrahedron and, as a consequence, a distortion of the tetrahedron as a whole. This effect can be observed in the structure of Li2 [(UO2 )(MoO4 )2 ]: the Mo–O(3) and Mo–O(5) bonds to the O(3) and O(5) atoms that are shared between U and Mo polyhedra are 1.772 and 1.788 Å. In order to attain a bond-valence sum for the Mo site that is ∼6.0 v.u., two other Mo–O bonds (Mo–O(1) and Mo–O(4)) are shortened to 1.739 and 1.740 Å, respectively. A similar situation is observed for both Cs6 [(UO2 )(MoO4)4 ] [7] and Rb6 [(UO2 )(MoO4)4 ] [10], where the Mo–O bonds to bridging O atoms are much longer than the Mo–O bonds to terminal O atoms. The necessity of distortion of MoO4 tetrahedra makes uranyl molybdates based on UrO4 bipyramids much less abundant than those based on UrO5 groups.
Supplementary material The supplementary material has been sent to the Fachinformationzentrum Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany, as supplementary material
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no. SUP. . . (. . .pages) and can be obtained by contacting the FIZ (quoting the article details and the corresponding SUP number). Acknowledgements This research was supported by the Environmental Management Sciences Program of the United States Department of Energy (grant DE-FG07-97ER 14820) and by the Russian Foundation for Basic Research for S.V.K. (grant 0105-64883).
References [1] P.C. Burns, R.J. Finch (Eds.), Uranium: Mineralogy, Geochemistry and the Environment, Reviews in Mineralogy, Vol. 38, Mineralogical Society of America, Washington DC, 1999. [2] S.V. Krivovichev, P.C. Burns, Can. Mineral. 38 (2000) 717. [3] S.V. Krivovichev, P.C. Burns, Can. Mineral. 38 (2000) 847. [4] S.V. Krivovichev, P.C. Burns, Can. Mineral. 39 (2001) 197. [5] S.V. Krivovichev, P.C. Burns, Can. Mineral. 39 (2001) 215. [6] S.V. Krivovichev, R.J. Finch, P.C. Burns, Can. Mineral. 40 (2002) 193. [7] S.V. Krivovichev, P.C. Burns, Can. Mineral. 40 (2002) 201. [8] S.V. Krivovichev, C.L. Cahill, P.C. Burns, Inorg. Chem. 41 (2002) 34. [9] S.V. Krivovichev, P.C. Burns, Can. Mineral. 40 (2002) 1571. [10] S.V. Krivovichev, P.C. Burns, J. Solid State Chem. 168 (2002) 245. [11] G.G. Sadikov, T.I. Krasovskaya, Y.A. Polyakov, V.P. Nikolaev, Izv. Akad. Nauk SSSR, Neorg. Mater. 24 (1988) 109. [12] R.K. Rastsvetaeva, A.V. Barinova, A.M. Fedoseev, N.A. Budantseva, Yu.V. Nekrasov, Dokl. Akad. Nauk 365 (1999) 68. [13] V.N. Khrustalev, G.B. Andreev, M.Yu. Antipin, A.M. Fedoseev, N.A. Budantseva, I.B. Shirokova, Zh. Neorg. Khim. 45 (2000) 1996. [14] G.B. Andreev, M.Yu. Antipin, A.M. Fedoseev, N.A. Budantseva, Koord. Khim. 27 (2001) 227–230. [15] S.V. Krivovichev, P.C. Burns, in preparation. [16] R. Tali, V.V. Tabachenko, L.M. Kovba, L.N. Dem’yanets, Zh. Neorg. Khim. 39 (1994) 1752. [17] D.Yu. Pushcharovsky, R.K. Rastsvetaeva, H. Sarp, J. Alloys Compd. 239 (1996) 23. [18] J.A. Ibers, W.C. Hamilton (Eds.), International Tables for X-ray Crystallography, Vol. IV, The Kynoch Press, Birmingham, UK, 1974. [19] P.C. Burns, R.C. Ewing, F.C. Hawthorne, Can. Mineral. 35 (1997) 1551. [20] P.C. Burns, Rev. Mineral. 38 (1999) 23. [21] N.E. Brese, M. O’Keeffe, Acta Crystallogr. B 47 (1991) 192. [22] P.C. Burns, M.L. Miller, R.C. Ewing, Can. Mineral. 34 (1996) 845. [23] K. Mereiter, Tschch. Miner. Petrogr. Mitt. 30 (1982) 277. [24] W. Krause, H. Effenberger, F. Brandstätter, Eur. J. Mineral. 7 (1995) 1313. [25] A. Guesdon, J. Chardon, J. Provost, B. Raveau, J. Solid State Chem. 165 (2002) 89–93. [26] P.M. Almond, T.E. Albrecht-Schmitt, Inorg. Chem. 41 (2002) 5495. [27] F.C. Hawthorne, Acta Crystallogr. A 39 (1983) 724–736.