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Article

Sc2[Se2O5]3: The First Rare-Earth Metal Oxoselenate(IV) with Exclusively [Se2O5]2− Anions

by
Stefan Greiner
and
Thomas Schleid
*
Institute for Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
*
Author to whom correspondence should be addressed.
Crystals 2018, 8(5), 187; https://doi.org/10.3390/cryst8050187
Submission received: 23 March 2018 / Revised: 20 April 2018 / Accepted: 20 April 2018 / Published: 26 April 2018
(This article belongs to the Special Issue Crystal Structures of Compounds Containing Ions Selenite)
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Abstract

:
The scandium oxodiselenate(IV) Sc2[Se2O5]3 was synthesized via solid-state reactions between scandium sesquioxide (Sc2O3) and selenium dioxide (SeO2) with thallium(I) chloride (TlCl) as fluxing agent in molar ratios of 1:4:2. Evacuated fused silica ampoules were used as reactions vessels for annealing the mixtures for five days at 800 °C. The new scandium compound crystallizes in the triclinic space group P 1 ¯ with the lattice parameters a = 663.71(5) pm, b = 1024.32(7) pm, c = 1057.49(8) pm, α = 81.034(2)°, β = 87.468(2)°, γ = 89.237(2)° and Z = 2. There are two distinct Sc3+ positions, which show six-fold coordination by oxygen atoms as [ScO6]9− octahedra (d(Sc–O) = 205–212 pm). Three different [Se2O5]2− anions provide these oxygen atoms with their terminal ligands (Ot). Each of the six selenium(IV) central atoms exhibit a stereochemically active lone pair of electrons, so that all [Se2O5]2− anions consist of two ψ1-tetrahedral [SeO3]2− subunits (d(Se–Ot) = 164–167 pm, d(Se–Ob) = 176–185 pm, ∢(O–Se–O) = 93–104°) sharing one bridging oxygen atom (Ob) with ∢(Se–Ob–Se) = 121–128°. The vibrational modes of the complex anionic [Se2O5]2− entities were characterized via single-crystal Raman spectroscopy.

1. Introduction

Owing to their potential as materials with an inorganic antenna effect [1] within energy transfer processes for lighting applications, many rare-earth metal(III) oxoselenates(IV) were published with different structural characteristics in the past decades. So even the pure rare-earth metal(III) oxoselenates(IV) with the simple formula RE2[SeO3]3 (≡ RE2Se3O9; RE = Sc, Y, La–Lu) [2,3,4,5,6,7,8,9,10,11,12] show a cornucopia of structure and space-group types. For the smallest rare-earth metal, scandium Sc2[SeO3]3 crystallizes in a hexagonal crystal structure in space group P63/m [3]. In contrast, the RE2[SeO3]3-type phases with lanthanum and cerium show an orthorhombic structure in space-group Pnma [2,4]. The next two lanthanides, praseodymium and neodymium, exhibit for their RE2[SeO3]3 compounds the monoclinic crystal system with the space group P21/n [2,8]. All further rare-earth metals(III) (RE = Y, Sm–Lu) [6,7,10] adopt the same triclinic crystal structure for their RE2[SeO3]3-type representatives with space-group P 1 ¯ . Besides this formula type, representatives containing oxygen atoms not bonded to the Se4+ cations are known with the formula RE2O[SeO3]2 (≡ RE2Se2O7; RE = Y, Sm–Tm) [13,14,15,16]. Most of the compounds crystallize in the tetragonal Tb2O[SeO3]2 structure-type [16], but the newest experimental research shows that monoclinic Sm3O2Sm[SeO3]4 (≡ Sm2Se2O7) is also accessible [15]. All these compounds exhibit the same basic module, which is responsible for the crystal structure, and these basic modules are oxygen-centered [ORE4]10+ tetrahedra, which are interconnected in different ways. Furthermore, in the literature is a rare-earth metal(III) oxoselenate(IV) known with two ‘free’ oxygen atoms per formula unit and the composition Sc2O2[SeO3] [17,18]. This formula type RE2O2[SeO3] (≡ RE2SeO5) was long postulated from Oppermann et al. [19,20,21,22,23] in various RE2O3/SeO2 systems. Nevertheless, no single crystals were available to confirm this crystal structure, so it was only assumed via powder diffraction methods [19,20,21,22,23]. Only one representative, including rare-earth metals, is known so far in the literature, which exhibits complex anionic [Se2O5]2− units. Sm2[SeO3][Se2O5]2 (≡ Sm2Se5O13) [24] contains only two of these anions, while with the title compound the first rare-earth metal(III) oxoselenate(IV) was accessible with exclusively these sort of anions. The aim of the synthesis was a TlSc[SeO3]2, however, which should be structurally similar to the alkali-metal scandium oxoselenates(IV) ASc[SeO3]2 (A = Na–Cs) [25] known in the literature. This paper describes the crystal structure of the new title compound Sc2[Se2O5]3 as well as the vibrational modes of the [Se2O5]2− groups via single-crystal Raman measurements.

2. Materials and Methods

For the synthesis of the scandium oxodiselenate(IV) Sc2[Se2O5]3 a mixture of Sc2O3 (ChemPur: 99.9%), SeO2 (ChemPur: 99.9%) and TlCl (Alfa Aesar: ‘pure’) in a molar ratio of 1:4:2 was used. The target product was thus TlSc[SeO3]2 according to the known alkali-metal scandium oxoselenates(IV) ASc[SeO3]2 (A = Na–Cs) [25]. The chemicals were stored and handled in a glove box (GS Glove-Box Systems) under an argon atmosphere. The reactants were filled into glassy fused silica ampoules, which were evacuated to 10−3 mbar and torch sealed. Afterwards, the reaction vessels were heated up over 8 h to 800 °C and tempered for five days at this temperature. The furnace was cooled down to 500 °C within 99 h, and held at this temperature for another 2 h, before it was completely cooled down within 4 h to room temperature. The crude product was checked for its water- and air-stability. Afterwards it was washed with demineralized water to remove most of the fluxing agents (SeO2 and TlCl). Some colorless, plate-like crystals were selected and characterized via X-ray structure analysis. For this purpose, the crystals were measured at room temperature on a κ-CCD X-ray diffractometer (Bruker Nonius, Karlsruhe, Germany) with graphite-monochromatized Mo-Kα radiation (λ = 71.07 pm). A numerical absorption correction was carried out with the program HABITUS [26], but the structure solution and refinement was performed with the program SHELX-97 [27,28]. In Table 1 the crystallographic data of Sc2[Se2O5]3 are summarized, while Table 2 contains the atomic positions and their equivalent isotropic displacement coefficients. The interatomic distances and bond angles are shown in Table 3.
The reflections in the X-ray powder pattern (Figure 1) exhibit a low intensity due to the poor crystallinity of the product, therefore a good evaluation and characterization was not possible (Figure 1). However, scandium oxoselenate(IV) Sc2[SeO3]3 and some residual thallium(I) chloride TlCl could be identified as by-products. In addition, the powder pattern shows reflections of another side phase, which could not be determined so far. Hence, a successful synthesis of the phase pure title compound Sc2[Se2O5]3 was not successful yet.
The single-crystal Raman measurements were operated on a XploRa device (Horiba, Bensheim, Germany) from 200–1000 cm−1 to detect the vibrational modes of the complex anionic [Se2O5]2− anions. Further details of the crystal structure investigations can be obtained at the Fachinfomrationszentrum Karlsruhe, 76334 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: [email protected], http://www.fiz-karlsruhe.de/request for deposited data.html) for the depository number CSD-433355 of Sc2[Se2O5]3.

3. Results and Discussion

The new scandium oxodiselenate(IV) Sc2[Se2O5]3 (≡ Sc2Se6O15) crystallizes in the triclinic space group P 1 ¯ (no. 2) with the lattice parameters a = 663.71(5) pm, b = 1024.32(7) pm, c = 1057.49(8) pm, α = 81.034(2)°, β = 87.468(2)°, γ = 89.237(2)°, and Z = 2 (Table 1). In the crystal structure, two distinct Sc3+ cations exist (Table 2). Both of them are six-fold coordinated by oxygen atoms as [ScO6]9− octahedra, which belong to six terminally attached [Se2O5]2− units (Figure 2). The Sc3+–O2− distances range in the area of 204 to 212 pm (Table 3). These correlate well with them known from the scandium oxoselenate(IV) Sc2[SeO3]3 (d = 201–220 pm) [3].
Moreover, three different complex anionic [Se2O5]2− groups occur in the crystal structure (Figure 3), showing different dihedral angles between the two (O,O,O)-faces ((O1,O2,O3)–(O3,O4,O5) = 121.2°; (O6,O7,O8)–(O8,O9,O10) = 146.0°; (O11,O12,O13)–(O13,O14,O15) = 152.2°). The Se4+–O2− distances are located in an interval from 164 to 167 pm (Table 3), this is slightly smaller than those in downeyite-type SeO2 (d = 171–173 pm) [29]. In contrast, the Se4+–(Ob)2− distances are significantly longer with 177 to 185 pm. Similar distances were found in the first known lanthanoid(III) oxoselenate(IV) with [Se2O5]2− units, Sm2[SeO3][Se2O5]2 (≡ Sm2Se5O13, d = 164–184 pm) [24] namely. The [ScO6]9− octahedra are interconnected via the terminally grafting [Se2O5]2− units resulting in the three-dimensional network of Sc2[Se2O5]3 (Figure 4). The displacements of the Se4+ cations from their triangular (O,O,O)-plane are for all three complex anionic [Se2O5]2− anions very much alike. These displacements range from 77.3 pm (Se1), 78.3 pm (Se3), 79.4 pm (Se6), 79.5 pm (Se5), 79.7 pm (Se2), to 82.0 pm (Se4). Also the Se–Ob–Se angles are located in a small area with (Se1)–(O3)b–(Se2) = 121.3°, (Se3)–(O8)b–(Se4) = 126.3°, and (Se5)–(O13)b–(Se6) = 127.5°.
Raman-spectroscopic measurements of Sc2[Se2O5]3 single crystals show typically modes, known from free [SeO3]2− units with ψ1-tetrahedral shape and ideal C3v symmetry. These modes belong to two symmetric vibrations with A1 at 810 and 425 cm−1 and E at 740 and 375 cm−1 according to Siebert [30]. By the influence of the crystal field these bands of the [SeO3]2− anions split up into wide areas. The symmetric stretching modes are located in the range of 890–790 cm−1 (νs) and the anti-symmetric stretching modes occur between 760 and 660 cm−1 (νas). According to these, also the bending modes have an expanded area with the symmetric bending mode between 510 and 420 cm−1 (δs) and anti-symmetric bending mode from 410 up to 330 cm−1 [30]. The complex anionic oxodiselenate(IV) units generate one more stretching mode resulted from the slightly longer Se4+–(Ob)2− bond by about 10 to 15 pm expanded. This mode is located between 650 and 550 cm−1 [12] and seems to be specific for [Se2O5]2− anions (νSeOB). In the Raman spectrum three signals can be detected in this area with ν = 578, 606 and 632 cm−1. For the symmetric stretching mode (νs) two signals appear at 901 and 934 cm−1 and for the anti-symmetric stretching mode (νas) three at 739, 800, and 811 cm−1. In the area of the symmetric bending mode ν = 420–550 cm−1 (δs) four signals can be found at 435, 469, 523, and 539 cm−1. At smaller wavenumbers between 208 and 387 cm−1, the anti-symmetric bending modes as well as lattices vibrations are located. Seven signals at 208, 235, 274, 292, 322, 347, and 387 cm−1 become obvious in these areas (Figure 5). For the characterization of these modes modern articles were used as references [11,18,31,32,33].

4. Conclusions

Unlike yttrium, its heavier congener in group III of the periodic table of the elements, scandium hardly forms compounds suitable as hosts for doping with appropriate lanthanoid(III) cations (e.g., Eu3+ or Tb3+) for luminescence materials. Its small ionic radius (r:(Sc3+) = 74.5 pm for C.N. = 6 [34]) and its preference for octahedral coordination figures explain this finding plausibly, whereas yttrium (r:(Y3+) = 101.9 pm for C.N. = 8 [34]) ranges within the heavy lanthanoid compartment (Dy3+, Ho3+, or Er3+) under these circumstances, without being a 4f element. So even with nice [SeO3]2− pyramids as building blocks with a lone-pair of electron at the Se4+ cations, scandium(III) oxoselenate(IV) derivatives are unable to host Ln3+ cations, as long as they contain only six-fold coordinated Sc3+ cations, e.g., Sc2[SeO3]3 [3], Sc2O2[SeO3] [17,18] and our new example Sc2[Se2O5]3. However this changes, whenever the Sc3+ cations reside in coordination polyhedra with higher coordination numbers. In the case of ScF[SeO3] [17,18,35] and ScCl[SeO3] [17,36] with seven-fold coordinated Sc3+ cations in pentagonal bipyramids, Eu3+ cations can replace these partially (1–5%), which leads to red luminescent materials in the case of ScF[SeO3]:Eu3+ [17]. The tight [ScO5F2]9− polyhedra provoke a slight red-shift of the Eu3+-centered luminescence, however, as compared to the YF[SeO3]:Eu3+ example [10] with Eu3+ in the eight-fold coordination of [YO6F2]11− polyhedra.

Supporting Materials

The CIF of the title compound can be downloaded directly with the publication, containing all crystallographic data.

Author Contributions

Stefan Greiner and Thomas Schleid conceived and designed the experiments; Stefan Greiner performed the experiments; Stefan Greiner and Thomas Schleid analyzed the data; Thomas Schleid contributed reagents/materials/analysis tools; Stefan Greiner wrote the paper and Thomas Schleid polished it.

Acknowledgments

We thank Sabine Strobel for the single-crystal X-ray diffraction measurement. Additionally, we are grateful to Adrian H. Geyer for the single-crystal Raman analysis. Finally, we acknowledge the considerable financial support by the State of Baden-Württemberg (Stuttgart).

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 08 00187 g001 550
Figure 1. Measured powder pattern of the title compound (black) with the reference of Sc2[Se2O5]3 (red), Sc2[SeO3]3 (green), and TlCl (blue) from single crystal data.
Figure 1. Measured powder pattern of the title compound (black) with the reference of Sc2[Se2O5]3 (red), Sc2[SeO3]3 (green), and TlCl (blue) from single crystal data.
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Crystals 08 00187 g002 550
Figure 2. Coordination sphere of the two crystallographically distinct Sc3+, which are six-fold coordinated by oxygen atoms as [ScO6]9− octahedra ((top left) and (bottom left)). The oxygen atoms themselves belong to six terminally attached [Se2O5]2− anions. Symmetry related atoms are shown with an a(x, y, z) or b(−x, −y, −z) and a thermal ellipsoid representation with probability factor of 95% (left).
Figure 2. Coordination sphere of the two crystallographically distinct Sc3+, which are six-fold coordinated by oxygen atoms as [ScO6]9− octahedra ((top left) and (bottom left)). The oxygen atoms themselves belong to six terminally attached [Se2O5]2− anions. Symmetry related atoms are shown with an a(x, y, z) or b(−x, −y, −z) and a thermal ellipsoid representation with probability factor of 95% (left).
Crystals 08 00187 g002
Crystals 08 00187 g003 550
Figure 3. Configuration of the three different [Se2O5]2− groups in the crystal structure of triclinic Sc2[Se2O5]3. These consist of two via vertex interconnected ψ1-tetrahedral [SeO3]2− subunits with different tilting angles between their two (O,O,O)-faces. In addition, the cationic coordination sphere of the [Se2O5]2− anions is shown (thermal ellipsoid representation with probability factor of 95%). Note that none of the bridging oxygen atoms (O3, O8, and O13) has a coordinative contacts to the Sc3+ cations.
Figure 3. Configuration of the three different [Se2O5]2− groups in the crystal structure of triclinic Sc2[Se2O5]3. These consist of two via vertex interconnected ψ1-tetrahedral [SeO3]2− subunits with different tilting angles between their two (O,O,O)-faces. In addition, the cationic coordination sphere of the [Se2O5]2− anions is shown (thermal ellipsoid representation with probability factor of 95%). Note that none of the bridging oxygen atoms (O3, O8, and O13) has a coordinative contacts to the Sc3+ cations.
Crystals 08 00187 g003
Crystals 08 00187 g004 550
Figure 4. All [ScO6]9− octahedra are interconnected by terminally attached [Se2O5]2− anions to form the three-dimensional network of triclinic Sc2[Se2O5]3.
Figure 4. All [ScO6]9− octahedra are interconnected by terminally attached [Se2O5]2− anions to form the three-dimensional network of triclinic Sc2[Se2O5]3.
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Crystals 08 00187 g005 550
Figure 5. Single-crystal Raman spectrum of Sc2[Se2O5]3 in the area from 200 to 1000 cm−1. The different stretching and bending modes of the complex anionic [Se2O5]2− units are shown.
Figure 5. Single-crystal Raman spectrum of Sc2[Se2O5]3 in the area from 200 to 1000 cm−1. The different stretching and bending modes of the complex anionic [Se2O5]2− units are shown.
Crystals 08 00187 g005
Table
Table 1. Crystallographic data of Sc2[Se2O5]3 and their determination.
Table 1. Crystallographic data of Sc2[Se2O5]3 and their determination.
Empirical FormulaSc2[Se2O5]3 (≡ Sc2Se6O15)
Crystal systemtriclinic
Space groupP 1 ¯ (no. 2)
Lattice parameters
a/pm663.71(5)
b/pm1024.32(7)
c/pm1057.49(8)
α/deg81.034(2)
β/deg87.468(2)
γ/deg89.237(2)
Number of formula units (Z)2
Calculated density (Dx in g/cm3)3.762
Molar volume (Vm in cm3/mol)188.58(9)
Diffractometerκ-CCD (Bruker Nonius)
Wavelength (λ in pm)71.07 (Mo Kα)
Index range (±hmax, ±kmax, ±lmax)8/13/13
Number of e− per unit cell (F(000))732
Absorption coefficient (μ in mm−1)16.426
Number of collected vs. unique reflections31217/3256
Data-set residuals (Rint/Rσ)0.084/0.036
Structure residuals (R1/wR2)0.034/0.074
Goodness of fit (GooF)1.058
Extinction coefficient (g)0.0101(5)
Residual electron density (max./min. in e− 10−6 pm−3)1.318/−1.267
CSD-number433,355
Table
Table 2. Atomic coordinates and equivalent isotropic displacement coefficients (Ueq/pm2) for Sc2[Se2O5]3 (all atoms occupy the general 2i site)
Table 2. Atomic coordinates and equivalent isotropic displacement coefficients (Ueq/pm2) for Sc2[Se2O5]3 (all atoms occupy the general 2i site)
Atomx/ay/bz/cUeq1
Sc10.16448(9)0.58715(7)0.26677(7)145(2)
Sc20.31102(9)0.06071(7)0.28974(7)143(2)
Se10.22321(5)0.77750(4)0.51356(4)173(1)
Se20.31368(5)0.30185(4)0.49159(4)156(1)
Se30.14680(5)0.33785(4)0.08075(4)197(1)
Se40.34062(5)0.46713(4)0.80898(4)189(1)
Se50.20134(5)0.01376(4)0.77299(4)171(1)
Se60.32216(5)0.85034(4)0.04488(4)173(1)
O10.3565(4)0.8834(3)0.4083(3)257(6)
O20.1123(4)0.6849(3)0.4203(3)241(6)
O30.4252(4)0.6720(3)0.5744(3)229(6)
O40.2120(4)0.4093(3)0.3801(3)245(6)
O50.2599(4)0.1567(3)0.4489(3)210(6)
O60.3013(4)0.2435(3)0.1727(3)288(7)
O70.1988(4)0.4927(3)0.1010(3)289(7)
O80.2873(4)0.3302(3)0.9362(3)354(8)
O90.1421(4)0.4542(3)0.7240(3)308(7)
O100.4805(4)0.6157(3)0.2594(3)241(6)
O110.0055(4)0.0288(3)0.2913(3)289(7)
O120.3733(4)0.9148(3)0.7186(3)227(6)
O130.1454(4)0.9259(3)0.9300(3)346(8)
O140.3719(4)0.9735(3)0.1228(3)288(7)
O150.1445(4)0.7649(3)0.1370(3)229(6)
1 defined as temperature factor according to: exp[−2π2(U11h2a*2 + U22k2b*2 + U33l2c*2 + 2U13hla*c* + 2U12hka*b* + 2U23hlb*c*)].
Table
Table 3. Selected interatomic distances (d/pm) and angles (∢/°) in the crystal structure of Sc2[Se2O5]3.
Table 3. Selected interatomic distances (d/pm) and angles (∢/°) in the crystal structure of Sc2[Se2O5]3.
Sc1–O4204.7(3)Sc2–O11205.7(3)
Sc1–O2205.1(3)Sc1–O1206.7(3)
Sc1–O9207.9(3)Sc1–O6207.9(3)
Sc1–O15210.9(3)Sc1–O5209.1(3)
Sc1–O10211.8(3)Sc1–O12211.0(3)
Sc1–O7213.3(3)Sc1–O14212.1(3)
Se1–O1166.1(3)Se2–O4164.4(3)
Se1–O2167.0(3)Se1–O5166.7(3)
Se1–O3178.9(3)Se1–O3184.6(3)
Se3–O6164.3(3)Se4–O9164.5(3)
Se1–O7167.6(3)Se1–O10165.6(3)
Se1–O8176.7(3)Se1–O8181.3(3)
Se5–O11165.2(3)Se6–O14165.5(3)
Se1–O12166.0(3)Se1–O15166.3(6)
Se1–O13178.7(3)Se1–O13180.6(3)
O1–Se1–O3098.52(13)O3–Se2–O4095.05(13)
O2–Se1–O3101.80(14)O3–Se2–O5101.58(14)
O1–Se1–O2102.67(14)O4–Se2–O5103.23(14)
O6–Se3–O8095.50(15)O8–Se4–O10093.04(15)
O7–Se3–O8099.83(15)O8–Se4–O9098.24(16)
O6–Se3–O7105.18(15)O9–Se4–O10104.03(15)
O11–Se5–O13096.13(15)O13–Se6–O15093.44(13)
O12–Se5–O13100.96(15)O13–Se6–O14102.13(15)
O11–Se5–O12102.11(14)O14–Se6–O15103.96(15)
Se1–O3–Se2121.32(14)Se3–O8–Se4126.31(18)
Se5–O13–Se6127.48(17)

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Greiner, S.; Schleid, T. Sc2[Se2O5]3: The First Rare-Earth Metal Oxoselenate(IV) with Exclusively [Se2O5]2− Anions. Crystals 2018, 8, 187. https://doi.org/10.3390/cryst8050187

AMA Style

Greiner S, Schleid T. Sc2[Se2O5]3: The First Rare-Earth Metal Oxoselenate(IV) with Exclusively [Se2O5]2− Anions. Crystals. 2018; 8(5):187. https://doi.org/10.3390/cryst8050187

Chicago/Turabian Style

Greiner, Stefan, and Thomas Schleid. 2018. "Sc2[Se2O5]3: The First Rare-Earth Metal Oxoselenate(IV) with Exclusively [Se2O5]2− Anions" Crystals 8, no. 5: 187. https://doi.org/10.3390/cryst8050187

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