Thortveitite-type Tm2Si2O7

Single crystals of dithulium disilicate, Tm2Si2O7, were obtained in flux 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.

Single crystals of dithulium disilicate, Tm 2 Si 2 O 7 , were obtained in flux synthesis experiments in the system SiO 2 -Tm 2 O 3 -LiF at ambient pressure. The compound belongs to the group of sorosilicates, i.e. it is based on [Si 2 O 7 ]-units and crystallizes in the thortveitite (Sc 2 Si 2 O 7 ) structure type. The Tm 3+ 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 [TmO 6 ] groups, resulting in the formation of layers parallel to (001). The individual [SiO 4 ] 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 [Si 2 O 7 ] groups (point group symmetry 2/m) form an alternating sequence. Linkage is provided by sharing common oxygen vertices.
Supporting information for this paper is available from the IUCr electronic archives (Reference: WM5029).

Comment
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 Tm 2 Si 2 O 7 have been obtained and structurally characterized. Synthetic Tm 2 Si 2 O 7 is isotypic with thortveitite (Sc 2 Si 2 O 7 ), a rare scandium silicate mineral (Zachariasen, 1930;Smolin et al., 1973). The compound is a sorosilicate and contains isolated [Si 2 O 7 ]-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 Tm 2 Si 2 O 7 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 [SiO 4 ]-tetrahedron are very small: 1.001 (for QE) and 4.95 (for AV), respectively. The Tm 3+ 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 Yb 3+ -Tm 3+ -Er 3+ (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 [TmO 6 ]-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 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 Tm 2 Si 2 O 7 and Sc 2 Si 2 O 7 , 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 Tm 2 Si 2 O 7 was transformed to the most similar configuration of Sc 2 Si 2 O 7. The calculations revealed the following atomic displacements (in Å) between the corresponding atoms in Sc 2 Si 2 O 7 (first entry) and Tm 2 Si 2 O 7 (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) 2 Si 2 O 7 (Felsche, 1973). To date, at least twelve different forms (A-I, K, L and X) have to be distinguished (Fleet & Liu, 2005). Tm 2 Si 2 O 7 , for example, exhibits a high degree of polymorphism where five different modifications can be realised. The synthesis of polycrystalline Tm 2 Si 2 O 7 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 Tm 2 Si 2 O 7 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 Tm 2 Si 2 O 7 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 Tm 2 Si 2 O 7 , 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.

Experimental
Starting materials for the flux growth experiments were dried reagent grade Tm 2 O 3 , SiO 2 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 Tm 2 O 3 :SiO 2 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 flux 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.

Refinement
Similar sets of lattice parameters could be found in the recent WEB-based version of the Inorganic Crystal Structure Database (ICSD, 2014) for the chemically closely related thortveitite-type materials with composition (REE) 2 Si 2 O 7 pointing to an isostructural relationship, which was confirmed by the subsequent structure analysis by direct methods. For supplementary materials sup-3 Acta Cryst. (2014). E70, i34-i35 structure determination a data set corresponding to a hemisphere of reciprocal space was collected.

Figure 1
Representation of a single [Si 2 O 7 ]-unit. Ellipsoids are drawn at the 60% probability level. [Symmetry codes: (i) x, y, -1 + 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].  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.62 e Å −3 Δρ min = −1.42 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0072 (6) Special details 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq