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 Tm(3+) cation (site symmetry .2.) occupies a distorted octa-hedral site, with Tm-O bond lengths in the range 2.217 (4)-2.289 (4) Å. Each of the octa-hedra shares three of its edges with adjacent [TmO6] groups, resulting in the formation of layers parallel to (001). The individual [SiO4] tetra-hedra 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 octa-hedra and the sheets containing the [Si2O7] groups (point group symmetry 2/m) form an alternating sequence. Linkage is provided by sharing common oxygen vertices.

Related literature

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 Crystal Structure Database, see: ICSD (2014 ▶).

Experimental

Crystal data

Tm2Si2O7

M = 506.04

Monoclinic,

a = 6.8205 (14) Å

b = 8.9062 (18) Å

c = 4.6937 (11) Å

β = 101.78 (2)°

V = 279.11 (10) Å3

Z = 2

Mo Kα radiation

μ = 31.99 mm−1

T = 293 K

0.05 × 0.03 × 0.01 mm

Data collection

Agilent Xcalibur (Ruby, Gemini ultra) diffractometer

Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2014 ▶) T min = 0.231, T max = 1

894 measured reflections

340 independent reflections

330 reflections with I > 2σ(I)

R int = 0.020

Refinement

R[F 2 > 2σ(F 2)] = 0.018

wR(F 2) = 0.045

S = 1.15

340 reflections

32 parameters

Δρmax = 1.62 e Å−3

Δρmin = −1.42 e Å−3

Data collection: CrysAlis PRO (Agilent, 2014 ▶); cell refinement: 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 ▶).

Crystal structure: contains datablock(s) global, I, New_Global_Publ_Block. DOI: 10.1107/S1600536814013142/wm5029sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536814013142/wm5029Isup2.hkl

CCDC reference: 1006971

Additional supporting information: crystallographic information; 3D view; checkCIF report

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)

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)