 | Acta Crystallographica Section E Acta Crystallographica Section E CRYSTALLOGRAPHIC COMMUNICATIONS |
journal menu
inorganic compounds
| CRYSTALLOGRAPHIC COMMUNICATIONS |
accessThortveitite-type Tm2Si2O7
aUniversity of Innsbruck, Institute of Mineralogy & Petrography, Innrain 52, A-6020 Innsbruck, Austria
*Correspondence e-mail: volker.kahlenberg@uibk.ac.at
Single crystals of dithulium disilicate, Tm2Si2O7, were obtained in synthesis experiments in the system SiO2–Tm2O3–LiF at ambient pressure. The compound belongs to the group of sorosilicates, i.e. it is based on [Si2O7]-units and crystallizes in the thortveitite (Sc2Si2O7) structure type. The Tm3+ cation (site symmetry .2.) occupies a distorted octahedral site, with Tm—O bond lengths in the range 2.217 (4)–2.289 (4) Å. Each of the octahedra shares three of its edges with adjacent [TmO6] groups, resulting in the formation of layers parallel to (001). The individual [SiO4] tetrahedra are more regular, i.e. the differences between the bond lengths between Si and the bridging and non-bridging O atoms are not very pronounced. The layers containing the octahedra and the sheets containing the [Si2O7] groups (point group symmetry 2/m) form an alternating sequence. Linkage is provided by sharing common oxygen vertices.
CCDC reference: 1006971
Related literature
For applications of oxosilicates containing trivalent rare earth elements (REE), see: Kitai (2008![[Kitai, A. (2008). Luminescent Materials and Applications, p. 298. London: John Wiley & Sons.]](../../../../../../logos/arrows/e_arr.gif "Kitai, A. (2008). Luminescent Materials and Applications, p. 298. London: John Wiley & Sons.")
); Piccinelli et al. (2009); Qiao et al. (2014); Luo et al. (2012); Streit et al. (2013); Han et al. (2006); Sun et al. (2012). For structures isotypic with that of the title compound, see: Zachariasen (1930); Smolin et al. (1973); Christensen (1994); Redhammer & Roth (2003). For polymorphic forms of Tm2Si2O7 and other structure types adopted by (REE)2Si2O7 compounds, see: Bocquillon et al. (1977); Hartenbach et al. (2003); Felsche (1973); Fleet & Liu (2005); Shannon & Prewitt (1970). For discussions of the [Si2O7]-unit with a linear bridging angle, see: Baur (1980); Bianchi et al. (1988); Cruickshank et al. (1962); Kimata et al. (1998); Liebau (1961). For general aspects on the crystal chemistry of silicates, see: Liebau (1985). For definition of distortion parameters, see: Robinson et al. (1971). For bond-valence analysis, see: Brown (2002). For definition and calculation of similarity descriptors, see: Tasci et al. (2012); Bergerhoff et al. (1999). For ionic radii, see: Shannon (1976). For the Inorganic Database, see: ICSD (2014).
Experimental
Crystal data
|
Data collection: CrysAlis PRO (Agilent, 2014); cell CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SIR2002 (Burla et al., 2003); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2011); software used to prepare material for publication: publCIF (Westrip, 2010) and WinGX (Farrugia, 2012).
Supporting information
CCDC reference: 1006971
contains datablocks global, I, New_Global_Publ_Block. DOI: https://doi.org/10.1107/S1600536814013142/wm5029sup1.cif
Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536814013142/wm5029Isup2.hkl
Starting materials for the growth experiments were dried reagent grade Tm2O3, SiO2 and LiF. Due to the pronounced hygroscopicity of the alkali fluoride, sample preparation was performed in a glove bag under nitrogen atmosphere. 0.1 g of the nutrient consisting of a mixture of Tm2O3:SiO2 in the molar ratio 1:4 was homogenized in an agate mortar with 0.1 g LiF. Subsequently, the educts were loaded into a platinum tube with an outer diameter of 3 mm and with 20 mm length. After sealing, the tube and its content were heated in a resistance furnace from 373 K to 1373 K with a rate of 50 K/h and isothermed for 2 h at the target temperature. The sample was cooled down to 1073 K with a rate of 5 K/h and, finally, the temperature was reduced to 373 K with a rate of 100 K/h. Removal of the with water left a residue of transparent, colorless, optically biaxial and highly birefringent crystals up to 500 µm in size. One of the optically biaxial crystals showing sharp extinction when observed between crossed polarizers was selected for further structural studies and mounted on the tip of a glass fiber using finger nail hardener as glue.
Similar sets of lattice parameters could be found in the recent WEB-based version of the Inorganic Database (ICSD, 2014) for the chemically closely related thortveitite-type materials with composition (REE)2Si2O7 pointing to an isostructural relationship, which was confirmed by the subsequent structure analysis by For a data set corresponding to a hemisphere of was collected.
Oxosilicates that contain trivalent rare earth elements have been studied frequently because of their potential usage in the field of luminescense including applications in devices and circuits for electronic, optoelectronic as well as communication industries (Kitai, 2008; Piccinelli et al., 2009; Qiao et al., 2014; Luo et al., 2012; Streit et al., 2013; Han et al., 2006; Sun et al., 2012).
In the course of an ongoing project on the synthesis of alkali-REE-silicates (REE is a rare earth element), single-crystals of Tm2Si2O7 have been obtained and structurally characterized. Synthetic Tm2Si2O7 is isotypic with thortveitite (Sc2Si2O7), a rare scandium silicate mineral (Zachariasen, 1930; Smolin et al., 1973). The compound is a sorosilicate and contains isolated [Si2O7]-groups. The bridging oxygen atom of the dimer resides on a centre of symmetry, resulting in a linear Si—O—Si angle. The conformation of the group is staggered with a dihedral angle (or azimuth) of 60° (Fig. 1). In the past, the question whether or not Si—O—Si angles can exhibit a value of 180° has been discussed controversially and, actually, the thortveitite structure-type played an important role in this debate (Liebau, 1961; Cruickshank, et al., 1962). However, a critical analysis of published data performed by Baur (1980) revealed that linear Si—O—Si bridging angles do exist and cannot be attributed to incorrect assignments. To date, it is generally accepted that a description of the thortveitite structure-type in the centrosymmetric C2/m (implying a linear Si—O—Si angle) is correct (Bianchi et al., 1988; Kimata et al., 1998). The present of Tm2Si2O7 also confirms this model. The spread in the Si—O and O—Si—O angles is not very pronounced and the values are in the expected limits for silicates (Liebau, 1985). Numerically, the degree of distortion can be expressed by the quadratic elongation QE and the angle variance AV (Robinson et al., 1971). The values of these distortion parameters for a single [SiO4]-tetrahedron are very small: 1.001 (for QE) and 4.95 (for AV), respectively. The Tm3+ cations are octahedrally coordinated by O atoms (Fig. 2), with Tm—O bond lengths in the range 2.217 (4) – 2.289 (4) Å and an average of 2.247 Å. The mean value compares well with those observed for the thortveitite representatives of the directly neighbouring REE Yb (<Yb—O>=2.240 Å) and Er (<Er—O>=2.253 Å) (Christensen, 1994). The differences can be attributed to the increasing ionic radii of the trivalent cations in the series Yb3+ - Tm3+ - Er3+ (Shannon, 1976). The octahedra show a distortion with moderate QE-values (1.061) and very high values for the angle variance. The high AV value of 219.7 seems to be a characteristic feature of the thortveitite structure-type and has been also observed for other members of this family (Redhammer & Roth, 2003). Each of the octahedra shares three of its edges with adjacent [TmO6]-groups resulting in the formation of layers parallel to (001). These pseudo-hexagonal sheets (Fig. 3) are similar to the layers in dioctahedral micas. The above-mentioned pronounced angular distortions can be rationalized by a combination of (i) a shortening of the common edges of adjacent octahedra (in order to reduce the repulsive interactions between adjoining Tm3+ cations) and (ii) a widening of the corresponding opposite unshared O—O edges. Successive layers containing octahedra are linked by the [Si2O7]-groups in such a way that each of both tetrahedra shares two of its corners with two different octahedra from the same and one corner with an octahedron from the other surrounding layer.
Bond valence sum calculations using the parameter sets for the Tm—O and Si—O bonds given by Brown (2002) resulted in the following values (in v.u.) for the cation-anion interactions up to 3.4 Å: Tm: 3.09, Si: 4.01, O1: 2.12, O2: 1.99 and O3: 1.99.
As mentioned above, the present structure is isotypic with that of thortveitite. For the calculation of several quantitative descriptors for the characterization of the degree of similarity between the crystal structures of Tm2Si2O7 and Sc2Si2O7, the program COMPSTRU (Tasci et al., 2012) was employed. For the given two structures, the degree of (S), i.e. the spontaneous strain obtained from the eigenvalues of the finite Lagrangian strain tensor calculated in a Cartesian reference system, has a value of (S) = 0.0222. After application of an origin shift of p = (0, 0, 1/2) the structure of Tm2Si2O7 was transformed to the most similar configuration of Sc2Si2O7. The calculations revealed the following atomic displacements (in Å) between the corresponding atoms in Sc2Si2O7 (first entry) and Tm2Si2O7 (second entry): Sc—Tm: 0.025; Si—Si: 0.043; O1—O2: 0.000; O2—O1: 0.083; O3—O3: 0.070 i.e. the maximum displacement is lower than 0.10 Å. The measure of similarity (Δ) as defined by Bergerhoff et al. (1999) has a value of 0.059.
Since the beginning of the 1970ies a large number of different structure types have been described for rare earth element silicates with composition (REE)2Si2O7 (Felsche, 1973). To date, at least twelve different forms (A—I, K, L and X) have to be distinguished (Fleet & Liu, 2005). Tm2Si2O7, for example, exhibits a high degree of where five different modifications can be realised. The synthesis of polycrystalline Tm2Si2O7 adopting the thortveitite- or C-type has been described by Bocquillon et al. (1977) in the temperature range between 1473 and 1673 K. However, the stability field of C-type Tm2Si2O7 extends to higher pressures as well: synthesis runs performed at 65 kbar/1773 K (Shannon & Prewitt, 1970) as well as 10 kbar/973 K and 18 kbar/973 K (Bocquillon et al., 1977) also resulted in the formation of the C-phase. Other high-pressure modifications of Tm2Si2O7 crystallize in the B–, D–, X– and L-types (Fleet & Liu, 2005; Shannon & Prewitt, 1970). The B-type, however, has been also prepared at ambient pressure and 1173 K (Hartenbach et al., 2003). In summary, one can say that more than forty years after the first systematic investigations to chart the p,T-behaviour of Tm2Si2O7, there are still open questions. The new synthesis route using lithium fluoride as a mineralizer offers the possibility to grow large single-crystals suited for in situ X-ray diffraction or Raman spectroscopic high-pressure studies in diamond anvil cells.
For applications of oxosilicates containing trivalent rare earth elements (REE), see: Kitai (2008); Piccinelli et al. (2009); Qiao et al. (2014); Luo et al. (2012); Streit et al. (2013); Han et al. (2006); Sun et al. (2012). For structures isotypic with that of the title compound, see: Zachariasen (1930); Smolin et al. (1973); Christensen (1994); Redhammer & Roth (2003). For polymorphic forms of Tm2Si2O7 and other structure types adopted by (REE)2Si2O7 compounds, see: Bocquillon et al. (1977); Hartenbach et al. (2003); Felsche (1973); Fleet & Liu (2005); Shannon & Prewitt (1970). For discussions of the [Si2O7]-unit with a linear bridging angle, see: Baur (1980); Bianchi et al. (1988); Cruickshank et al. (1962); Kimata et al. (1998); Liebau (1961). For general aspects on the crystal chemistry of silicates, see: Liebau (1985). For definition of distortion parameters, see: Robinson et al. (1971). For bond-valence analysis, see: Brown (2002). For definition and calculation of similarity descriptors, see: Tasci et al. (2012); Bergerhoff et al. (1999). For ionic radii, see: Shannon (1976). For the Inorganic Database, see: ICSD (2014).
Data collection: CrysAlis PRO (Agilent, 2014); cell CrysAlis PRO (Agilent, 2014); data reduction: CrysAlis PRO (Agilent, 2014); program(s) used to solve structure: SIR2002 (Burla et al., 2003); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2011); software used to prepare material for publication: publCIF (Westrip, 2010) and WinGX (Farrugia, 2012).
![[Figure 1]](./wm5029fig1thm.gif) | Fig. 1. Representation of a single [Si2O7]-unit. Ellipsoids are drawn at the 60% probability level. [Symmetry codes: (i) x, y, -1 + z (ii) -x, y, -z (iii) -x, y, 1 - z (iv) x, -y, -1 + z]. |
![[Figure 2]](./wm5029fig2thm.gif) | Fig. 2. Representation of the coordination around the trivalent Tm ion. Ellipsoids are drawn at the 60% probability level. [Symmetry codes: (i) 1 - x, y, 1 - z (ii) 1/2 + x, 1/2 - y, z (iii) 1/2 - x, 1/2 - y, 1 - z]. |
![[Figure 3]](./wm5029fig3thm.gif) | Fig. 3. Single layer of edge-sharing octahedra and one of the two adjacent sheets containing [Si2O7]-units in a projection parallel to [001]. Red, grey and blue spheres represent oxygen, silicon and thulium ions. |
| Tm2Si2O7 | F(000) = 444 |
| Mr = 506.04 | Dx = 6.021 Mg m−3 |
| Monoclinic, C2/m | Mo Kα radiation, λ = 0.71073 Å |
| Hall symbol: -C 2y | Cell parameters from 762 reflections |
| a = 6.8205 (14) Å | θ = 3.8–29.3° |
| b = 8.9062 (18) Å | µ = 31.99 mm−1 |
| c = 4.6937 (11) Å | T = 293 K |
| β = 101.78 (2)° | Platy fragment, colourless |
| V = 279.11 (10) Å3 | 0.05 × 0.03 × 0.01 mm |
| Z = 2 |
| Agilent Xcalibur (Ruby, Gemini ultra) diffractometer | 340 independent reflections |
| Radiation source: Enhance (Mo) X-ray Source | 330 reflections with I > 2σ(I) |
| Graphite monochromator | Rint = 0.020 |
| Detector resolution: 10.3575 pixels mm-1 | θmax = 27.6°, θmin = 3.8° |
| ω scans | h = −8→8 |
| Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2014) | k = −11→11 |
| Tmin = 0.231, Tmax = 1 | l = −4→6 |
| 894 measured reflections |
| Refinement on F2 | Primary atom site location: structure-invariant direct methods |
| Least-squares matrix: full | Secondary atom site location: difference Fourier map |
| R[F2 > 2σ(F2)] = 0.018 | w = 1/[σ2(Fo2) + (0.0265P)2] where P = (Fo2 + 2Fc2)/3 |
| wR(F2) = 0.045 | (Δ/σ)max < 0.001 |
| S = 1.15 | Δρmax = 1.62 e Å−3 |
| 340 reflections | Δρmin = −1.42 e Å−3 |
| 32 parameters | Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
| 0 restraints | Extinction coefficient: 0.0072 (6) |
| Tm2Si2O7 | V = 279.11 (10) Å3 |
| Mr = 506.04 | Z = 2 |
| Monoclinic, C2/m | Mo Kα radiation |
| a = 6.8205 (14) Å | µ = 31.99 mm−1 |
| b = 8.9062 (18) Å | T = 293 K |
| c = 4.6937 (11) Å | 0.05 × 0.03 × 0.01 mm |
| β = 101.78 (2)° |
| Agilent Xcalibur (Ruby, Gemini ultra) diffractometer | 340 independent reflections |
| Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2014) | 330 reflections with I > 2σ(I) |
| Tmin = 0.231, Tmax = 1 | Rint = 0.020 |
| 894 measured reflections |
| R[F2 > 2σ(F2)] = 0.018 | 32 parameters |
| wR(F2) = 0.045 | 0 restraints |
| S = 1.15 | Δρmax = 1.62 e Å−3 |
| 340 reflections | Δρmin = −1.42 e Å−3 |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in lengths, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. |
| x | y | z | Uiso*/Ueq | ||
| Tm | 0.5 | 0.19345 (4) | 0.5 | 0.0045 (2) | |
| Si | 0.2186 (3) | 0 | 0.9130 (5) | 0.0044 (5) | |
| O1 | 0.3804 (9) | 0 | 0.2130 (12) | 0.0069 (12) | |
| O2 | 0 | 0 | 0 | 0.0129 (19) | |
| O3 | 0.2357 (6) | 0.1505 (5) | 0.7213 (9) | 0.0073 (8) |
| U11 | U22 | U33 | U12 | U13 | U23 | |
| Tm | 0.0032 (3) | 0.0043 (3) | 0.0058 (3) | 0 | 0.00056 (15) | 0 |
| Si | 0.0049 (11) | 0.0043 (11) | 0.0042 (11) | 0 | 0.0013 (9) | 0 |
| O1 | 0.006 (3) | 0.007 (3) | 0.005 (3) | 0 | −0.004 (2) | 0 |
| O2 | 0.008 (4) | 0.019 (5) | 0.011 (5) | 0 | 0.001 (4) | 0 |
| O3 | 0.006 (2) | 0.007 (2) | 0.009 (2) | 0.0042 (17) | 0.0026 (17) | 0.0045 (17) |
| Tm—O3i | 2.217 (4) | Si—O3vi | 1.632 (4) |
| Tm—O3ii | 2.217 (4) | Si—O3 | 1.632 (4) |
| Tm—O1 | 2.236 (4) | O1—Sivii | 1.602 (6) |
| Tm—O1iii | 2.236 (4) | O1—Tmiii | 2.236 (4) |
| Tm—O3 | 2.289 (4) | O2—Siviii | 1.624 (2) |
| Tm—O3iv | 2.289 (4) | O2—Sivii | 1.624 (2) |
| Si—O1v | 1.602 (6) | O3—Tmii | 2.217 (4) |
| Si—O2v | 1.624 (2) | ||
| O3i—Tm—O3ii | 102.3 (2) | O3—Tm—O3iv | 160.7 (2) |
| O3i—Tm—O1 | 154.9 (2) | O1v—Si—O2v | 106.4 (2) |
| O3ii—Tm—O1 | 93.47 (17) | O1v—Si—O3vi | 111.8 (2) |
| O3i—Tm—O1iii | 93.47 (17) | O2v—Si—O3vi | 108.14 (18) |
| O3ii—Tm—O1iii | 154.9 (2) | O1v—Si—O3 | 111.8 (2) |
| O1—Tm—O1iii | 79.2 (2) | O2v—Si—O3 | 108.14 (18) |
| O3i—Tm—O3 | 117.09 (17) | O3vi—Si—O3 | 110.4 (3) |
| O3ii—Tm—O3 | 75.78 (17) | Sivii—O1—Tm | 129.10 (12) |
| O1—Tm—O3 | 85.4 (2) | Sivii—O1—Tmiii | 129.10 (12) |
| O1iii—Tm—O3 | 79.74 (18) | Tm—O1—Tmiii | 100.8 (2) |
| O3i—Tm—O3iv | 75.78 (17) | Siviii—O2—Sivii | 180.00 (14) |
| O3ii—Tm—O3iv | 117.09 (17) | Si—O3—Tmii | 130.3 (3) |
| O1—Tm—O3iv | 79.74 (18) | Si—O3—Tm | 122.6 (2) |
| O1iii—Tm—O3iv | 85.43 (19) | Tmii—O3—Tm | 104.22 (17) |
| Symmetry codes: (i) x+1/2, −y+1/2, z; (ii) −x+1/2, −y+1/2, −z+1; (iii) −x+1, −y, −z+1; (iv) −x+1, y, −z+1; (v) x, y, z+1; (vi) x, −y, z; (vii) x, y, z−1; (viii) −x, −y, −z+1. |
Experimental details
| Crystal data | |
| Chemical formula | Tm2Si2O7 |
| Mr | 506.04 |
| Crystal system, space group | Monoclinic, C2/m |
| Temperature (K) | 293 |
| a, b, c (Å) | 6.8205 (14), 8.9062 (18), 4.6937 (11) |
| β (°) | 101.78 (2) |
| V (Å3) | 279.11 (10) |
| Z | 2 |
| Radiation type | Mo Kα |
| µ (mm−1) | 31.99 |
| Crystal size (mm) | 0.05 × 0.03 × 0.01 |
| Data collection | |
| Diffractometer | Agilent Xcalibur (Ruby, Gemini ultra) |
| Absorption correction | Multi-scan (CrysAlis PRO; Agilent, 2014) |
| Tmin, Tmax | 0.231, 1 |
| No. of measured, independent and observed [I > 2σ(I)] reflections | 894, 340, 330 |
| Rint | 0.020 |
| (sin θ/λ)max (Å−1) | 0.653 |
| Refinement | |
| R[F2 > 2σ(F2)], wR(F2), S | 0.018, 0.045, 1.15 |
| No. of reflections | 340 |
| No. of parameters | 32 |
| Δρmax, Δρmin (e Å−3) | 1.62, −1.42 |
Computer programs: CrysAlis PRO (Agilent, 2014), SIR2002 (Burla et al., 2003), SHELXL97 (Sheldrick, 2008), ATOMS for Windows (Dowty, 2011), publCIF (Westrip, 2010) and WinGX (Farrugia, 2012).
References
Agilent (2014). CrysAlis PRO. Agilent Technologies, Yarnton, England. Google Scholar
Baur, W. H. (1980). Acta Cryst. B36, 2198–2202. CrossRef CAS IUCr Journals Web of Science Google Scholar
Bergerhoff, G., Berndt, M., Brandenburg, K. & Degen, T. (1999). Acta Cryst. B55, 147–156. Web of Science CrossRef CAS IUCr Journals Google Scholar
Bianchi, R., Pilati, T., Diella, V., Gramaccioli, C. M. & Mannucci, G. (1988). Am. Mineral. 73, 601–607. CAS Google Scholar
Bocquillon, C., Chateau, C., Loriers, C. & Loriers, L. (1977). J. Solid State Chem. 20, 135–141. CrossRef CAS Web of Science Google Scholar
Brown, I. D. (2002). The Chemical Bond in Inorganic Chemistry: The Bond Valence Model, p. 292. Oxford University Press. Google Scholar
Burla, M. C., Camalli, M., Carrozzini, B., Cascarano, G. L., Giacovazzo, C., Polidori, G. & Spagna, R. (2003). J. Appl. Cryst. 36, 1103. CrossRef IUCr Journals Google Scholar
Christensen, A. N. (1994). Z. Kristallogr. 209, 7–13. CrossRef CAS Web of Science Google Scholar
Cruickshank, D. W. J., Lynton, H. & Barclay, G. A. (1962). Acta Cryst. 15, 491–498. CrossRef CAS IUCr Journals Web of Science Google Scholar
Dowty, E. (2011). ATOMS for Windows. Shape Software, Kingsport, USA. Google Scholar
Farrugia, L. J. (2012). J. Appl. Cryst. 45, 849–854. Web of Science CrossRef CAS IUCr Journals Google Scholar
Felsche, J. (1973). Struct. Bond. 13, 99–197. CrossRef CAS Google Scholar
Fleet, M. E. & Liu, X. (2005). J. Solid State Chem. 178, 3275–3283. Web of Science CrossRef CAS Google Scholar
Han, X., Lin, J., Li, Z., Qi, X., Li, M. & Wang, X. (2006). J. Rare Earth, 24, 108–110. Google Scholar
Hartenbach, I., Lissner, F. & Schleid, T. (2003). Z. Naturforsch. Teil B, 58, 925–927. CAS Google Scholar
ICSD (2014). Inorganic Database. FIZ-Karlsruhe, Germany, and the National Institute of Standards and Technology (NIST), USA. http://www.fiz-karlsruhe.de/icsd_home.html Google Scholar
Kimata, M., Saito, S., Matsui, T., Shimizu, M. & Nishida, N. (1998). Neues Jarhb. Mineral. Monatsh. 1998, 361–372. Google Scholar
Kitai, A. (2008). Luminescent Materials and Applications, p. 298. London: John Wiley & Sons. Google Scholar
Liebau, F. (1961). Acta Cryst. 14, 1103–1109. CrossRef CAS IUCr Journals Web of Science Google Scholar
Liebau, F. (1985). Structural Chemistry of Silicates, p. 347. Berlin, Heidelberg, New York, Tokyo: Springer. Google Scholar
Luo, Y. Y., Jo, D. S., Senthil, L., Tezuka, S., Kakihana, M., Toda, K., Masaki, T. & Yoon, D. H. (2012). J. Solid State Chem. 189, 68–74. Web of Science CrossRef CAS Google Scholar
Piccinelli, F., Speghini, A., Mariotto, G., Bovo, L. & Bettinelli, M. (2009). J. Rare Earth, 27, 555–559. Web of Science CrossRef Google Scholar
Qiao, J., Zhang, J., Zhang, X., Hao, Z., Liu, Y. & Luo, Y. (2014). Dalton Trans. 43, 4146–4150. Web of Science CrossRef CAS PubMed Google Scholar
Redhammer, G. J. & Roth, G. (2003). Acta Cryst. C59, i103–i106. Web of Science CrossRef CAS IUCr Journals Google Scholar
Robinson, K., Gibbs, G. V. & Ribbe, P. H. (1971). Science, 172, 567–570. CrossRef PubMed CAS Web of Science Google Scholar
Shannon, R. D. (1976). Acta Cryst. A32, 751–767. CrossRef CAS IUCr Journals Web of Science Google Scholar
Shannon, R. D. & Prewitt, C. T. (1970). J. Solid State Chem. 2, 199–202. CrossRef CAS Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Smolin, Yu. I., Shepelev, Yu. F. & Titov, A. P. (1973). Sov. Phys. Crystallogr. 17, 749–750. Google Scholar
Streit, H. C., Kramer, J., Suta, M. & Wickleder, C. (2013). Materials, 6, 3079-3093. Web of Science CrossRef CAS Google Scholar
Sun, Z., Wang, M., Song, X. & Jiang, Z. (2012). J. Rare Earth, 31, 957–961. CrossRef Google Scholar
Tasci, E. S., de la Flor, G., Orobengoa, D., Capillas, C., Perez-Mato, J. M. & Aroyo, M. I. (2012). EPJ Web of Conferences, 22, 00009. CrossRef Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar
Zachariasen, W. H. (1930). Z. Kristallogr. 73, 1–6. CrossRef CAS Google Scholar
This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.
| CRYSTALLOGRAPHIC COMMUNICATIONS |
Oxosilicates that contain trivalent rare earth elements have been studied frequently because of their potential usage in the field of luminescense including applications in devices and circuits for electronic, optoelectronic as well as communication industries (Kitai, 2008; Piccinelli et al., 2009; Qiao et al., 2014; Luo et al., 2012; Streit et al., 2013; Han et al., 2006; Sun et al., 2012).
In the course of an ongoing project on the synthesis of alkali-REE-silicates (REE is a rare earth element), single-crystals of Tm2Si2O7 have been obtained and structurally characterized. Synthetic Tm2Si2O7 is isotypic with thortveitite (Sc2Si2O7), a rare scandium silicate mineral (Zachariasen, 1930; Smolin et al., 1973). The compound is a sorosilicate and contains isolated [Si2O7]-groups. The bridging oxygen atom of the dimer resides on a centre of symmetry, resulting in a linear Si—O—Si angle. The conformation of the group is staggered with a dihedral angle (or azimuth) of 60° (Fig. 1). In the past, the question whether or not Si—O—Si angles can exhibit a value of 180° has been discussed controversially and, actually, the thortveitite structure-type played an important role in this debate (Liebau, 1961; Cruickshank, et al., 1962). However, a critical analysis of published data performed by Baur (1980) revealed that linear Si—O—Si bridging angles do exist and cannot be attributed to incorrect space group assignments. To date, it is generally accepted that a description of the thortveitite structure-type in the centrosymmetric space group C2/m (implying a linear Si—O—Si angle) is correct (Bianchi et al., 1988; Kimata et al., 1998). The present structure determination of Tm2Si2O7 also confirms this model. The spread in the Si—O and O—Si—O angles is not very pronounced and the values are in the expected limits for silicates (Liebau, 1985). Numerically, the degree of distortion can be expressed by the quadratic elongation QE and the angle variance AV (Robinson et al., 1971). The values of these distortion parameters for a single [SiO4]-tetrahedron are very small: 1.001 (for QE) and 4.95 (for AV), respectively. The Tm3+ cations are octahedrally coordinated by O atoms (Fig. 2), with Tm—O bond lengths in the range 2.217 (4) – 2.289 (4) Å and an average of 2.247 Å. The mean value compares well with those observed for the thortveitite representatives of the directly neighbouring REE Yb (<Yb—O>=2.240 Å) and Er (<Er—O>=2.253 Å) (Christensen, 1994). The differences can be attributed to the increasing ionic radii of the trivalent cations in the series Yb3+ - Tm3+ - Er3+ (Shannon, 1976). The octahedra show a distortion with moderate QE-values (1.061) and very high values for the angle variance. The high AV value of 219.7 seems to be a characteristic feature of the thortveitite structure-type and has been also observed for other members of this family (Redhammer & Roth, 2003). Each of the octahedra shares three of its edges with adjacent [TmO6]-groups resulting in the formation of layers parallel to (001). These pseudo-hexagonal sheets (Fig. 3) are similar to the layers in dioctahedral micas. The above-mentioned pronounced angular distortions can be rationalized by a combination of (i) a shortening of the common edges of adjacent octahedra (in order to reduce the repulsive interactions between adjoining Tm3+ cations) and (ii) a widening of the corresponding opposite unshared O—O edges. Successive layers containing octahedra are linked by the [Si2O7]-groups in such a way that each of both tetrahedra shares two of its corners with two different octahedra from the same and one corner with an octahedron from the other surrounding layer.
Bond valence sum calculations using the parameter sets for the Tm—O and Si—O bonds given by Brown (2002) resulted in the following values (in v.u.) for the cation-anion interactions up to 3.4 Å: Tm: 3.09, Si: 4.01, O1: 2.12, O2: 1.99 and O3: 1.99.
As mentioned above, the present structure is isotypic with that of thortveitite. For the calculation of several quantitative descriptors for the characterization of the degree of similarity between the crystal structures of Tm2Si2O7 and Sc2Si2O7, the program COMPSTRU (Tasci et al., 2012) was employed. For the given two structures, the degree of lattice distortion (S), i.e. the spontaneous strain obtained from the eigenvalues of the finite Lagrangian strain tensor calculated in a Cartesian reference system, has a value of (S) = 0.0222. After application of an origin shift of p = (0, 0, 1/2) the structure of Tm2Si2O7 was transformed to the most similar configuration of Sc2Si2O7. The calculations revealed the following atomic displacements (in Å) between the corresponding atoms in Sc2Si2O7 (first entry) and Tm2Si2O7 (second entry): Sc—Tm: 0.025; Si—Si: 0.043; O1—O2: 0.000; O2—O1: 0.083; O3—O3: 0.070 i.e. the maximum displacement is lower than 0.10 Å. The measure of similarity (Δ) as defined by Bergerhoff et al. (1999) has a value of 0.059.
Since the beginning of the 1970ies a large number of different structure types have been described for rare earth element silicates with composition (REE)2Si2O7 (Felsche, 1973). To date, at least twelve different forms (A—I, K, L and X) have to be distinguished (Fleet & Liu, 2005). Tm2Si2O7, for example, exhibits a high degree of polymorphism where five different modifications can be realised. The synthesis of polycrystalline Tm2Si2O7 adopting the thortveitite- or C-type has been described by Bocquillon et al. (1977) in the temperature range between 1473 and 1673 K. However, the stability field of C-type Tm2Si2O7 extends to higher pressures as well: synthesis runs performed at 65 kbar/1773 K (Shannon & Prewitt, 1970) as well as 10 kbar/973 K and 18 kbar/973 K (Bocquillon et al., 1977) also resulted in the formation of the C-phase. Other high-pressure modifications of Tm2Si2O7 crystallize in the B–, D–, X– and L-types (Fleet & Liu, 2005; Shannon & Prewitt, 1970). The B-type, however, has been also prepared at ambient pressure and 1173 K (Hartenbach et al., 2003). In summary, one can say that more than forty years after the first systematic investigations to chart the p,T-behaviour of Tm2Si2O7, there are still open questions. The new flux synthesis route using lithium fluoride as a mineralizer offers the possibility to grow large single-crystals suited for in situ X-ray diffraction or Raman spectroscopic high-pressure studies in diamond anvil cells.