inorganic compounds
Rb2Lu[Si4O10]F, a tubular chain silicate
aUniversity of Innsbruck, Institute of Mineralogy & Petrography, Innrain 52, A-6020 Innsbruck, Austria
*Correspondence e-mail: volker.kahlenberg@uibk.ac.at
Single crystals of Rb2Lu[Si4O10]F (dirubidium lutetium tetrasilicate fluoride) were obtained in flux-synthesis experiments in the system SiO2–Lu2O3–RbF. The compound belongs to the group of tubular chain silicates, i.e. it is based on multiple chains of condensed [SiO4] tetrahedra forming closed columns. The periodicity of the unbranched multiple chains is four and corresponds to the length of the b axis. Adjacent columns are connected by Lu3+ cations, which are coordinated by four oxide and two fluoride anions in the form of slightly distorted octahedra. By sharing common fluoride corners, the single octahedra are linked into chains running parallel to the silicate tubes. Electroneutrality is achieved by the incorporation of additional Rb+ cations. All four symmetrically independent rubidium ions, four out of twelve oxide as well as the two fluoride anions are located on mirror planes. The remaining atoms reside on general positions.
CCDC reference: 986192
Related literature
Oxosilicates that contain monovalent alkali cations, trivalent rare earth elements and additional fluorine anions have potential application in the field of luminescense (Jacobsen & Meyer, 1994; Tang et al., 2008; Schäfer & Schleid, 2007, 2011; Kahlenberg & Manninger, 2014). For structures isotypic to that of the title compound, see: Chigarov et al. (1983); Hung et al. (2003). For general aspects of the crystal chemistry of silicates, see: Liebau (1985). For the definition of distortion parameters, see: Robinson et al. (1971). For bond-valence analysis, see: Brown (2002). For the definition and calculation of similarity descriptors, see: Tasci et al. (2012); Bergerhoff et al. (1999). For the Inorganic Structure Database, see: ICSD (2014).
Experimental
Crystal data
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Data collection
Data collection: CrysAlis PRO (Oxford Diffraction, 2006); 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: 986192
10.1107/S1600536814003043/wm5002sup1.cif
contains datablocks global, I, New_Global_Publ_Block. DOI:Structure factors: contains datablock I. DOI: 10.1107/S1600536814003043/wm5002Isup2.hkl
Single-crystals of Rb2Lu[Si4O10]F were obtained in the course of a series of
syntheses experiments aiming on the preparation of new Rb-REE-fluoride silicates. 0.1 g of the nutrient consisting of a mixture of Lu2O3:SiO2 in the molar ratio 1:4 was homogenized in an agate mortar with 0.1 g RbF, transferred into a platinum tube and welded shut. The container was heated in a laboratory chamber furnace from 373 K to 1373 K with a ramp of 50 K/h and isothermed for 2 h at the target temperature. Subsequently, the sample was cooled down to 1073 K with a rate of 5 K and, finally, the temperature was reduced to 373 K with a rate of 100 K/h. After the removal of the platinum tube the solidified melt cake was immediately crashed in an agate mortar and transferred to a glass slide under a polarizing binocular. A first optical inspection revealed the presence of two phases: a polycrystalline matrix of RbF in which transparent birefringent single-crystals up to 400µm in size were embedded. One of the optically biaxial crystals showing sharp extinction when observed between crossed polarizers was selected for further structural studies and was mounted on the tip of a glass fiber using fingernail 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 compounds K2Lu[Si4O10]F (Chigarov et al., 1983) and K2In[Si4O10](OH) (Hung et al., 2003) pointing to isostructural relationships, which were confirmed by subsequent structure analysis. A data set corresponding to a hemisphere of was collected.Data collection: CrysAlis PRO (Oxford Diffraction, 2006); cell
CrysAlis PRO (Oxford Diffraction, 2006); data reduction: CrysAlis PRO (Oxford Diffraction, 2006); 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).Rb2Lu[Si4O10]F | F(000) = 1160 |
Mr = 637.27 | Dx = 3.877 Mg m−3 |
Monoclinic, P21/m | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2yb | Cell parameters from 8764 reflections |
a = 11.6695 (3) Å | θ = 3.0–29.4° |
b = 8.52379 (18) Å | µ = 18.4 mm−1 |
c = 11.8165 (3) Å | T = 298 K |
β = 111.753 (3)° | Prism, colourless |
V = 1091.67 (5) Å3 | 0.32 × 0.08 × 0.08 mm |
Z = 4 |
Oxford Diffraction Xcalibur (Ruby, Gemini ultra) diffractometer | 2388 independent reflections |
Graphite monochromator | 2276 reflections with I > 2σ(I) |
Detector resolution: 10.3575 pixels mm-1 | Rint = 0.028 |
ω scans | θmax = 26.4°, θmin = 3.0° |
Absorption correction: analytical [CrysAlis PRO (Oxford Diffraction, 2006), based on expressions derived by Clark & Reid (1995)] | h = −14→14 |
Tmin = 0.106, Tmax = 0.562 | k = −10→10 |
15185 measured reflections | l = −14→14 |
Refinement on F2 | 0 constraints |
Least-squares matrix: full | Primary atom site location: structure-invariant direct methods |
R[F2 > 2σ(F2)] = 0.017 | Secondary atom site location: difference Fourier map |
wR(F2) = 0.037 | w = 1/[σ2(Fo2) + (0.008P)2 + 3.5409P] where P = (Fo2 + 2Fc2)/3 |
S = 1.2 | (Δ/σ)max = 0.001 |
2388 reflections | Δρmax = 0.67 e Å−3 |
178 parameters | Δρmin = −0.69 e Å−3 |
0 restraints |
Rb2Lu[Si4O10]F | V = 1091.67 (5) Å3 |
Mr = 637.27 | Z = 4 |
Monoclinic, P21/m | Mo Kα radiation |
a = 11.6695 (3) Å | µ = 18.4 mm−1 |
b = 8.52379 (18) Å | T = 298 K |
c = 11.8165 (3) Å | 0.32 × 0.08 × 0.08 mm |
β = 111.753 (3)° |
Oxford Diffraction Xcalibur (Ruby, Gemini ultra) diffractometer | 2388 independent reflections |
Absorption correction: analytical [CrysAlis PRO (Oxford Diffraction, 2006), based on expressions derived by Clark & Reid (1995)] | 2276 reflections with I > 2σ(I) |
Tmin = 0.106, Tmax = 0.562 | Rint = 0.028 |
15185 measured reflections |
R[F2 > 2σ(F2)] = 0.017 | 178 parameters |
wR(F2) = 0.037 | 0 restraints |
S = 1.2 | Δρmax = 0.67 e Å−3 |
2388 reflections | Δρmin = −0.69 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 distances, 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 | ||
Lu | 0.293348 (13) | 0.003290 (17) | 0.709429 (13) | 0.00494 (5) | |
Rb1 | 0.44695 (5) | 0.75 | 0.54955 (5) | 0.01305 (11) | |
Rb2 | 0.08212 (5) | 0.25 | 0.07908 (5) | 0.01314 (11) | |
Rb3 | −0.00057 (5) | 0.75 | 0.52863 (5) | 0.01827 (13) | |
Rb4 | 0.46419 (5) | 0.25 | 0.03113 (5) | 0.01750 (12) | |
Si1 | 0.38671 (9) | −0.06260 (11) | 0.22969 (8) | 0.00464 (19) | |
Si2 | 0.01773 (9) | 0.06858 (11) | 0.75550 (9) | 0.00508 (19) | |
Si3 | 0.24334 (9) | 0.43354 (11) | 0.38343 (8) | 0.00493 (19) | |
Si4 | 0.22493 (9) | 0.93232 (11) | 0.96450 (8) | 0.00543 (19) | |
O1 | 0.4021 (3) | 0.75 | 0.2204 (3) | 0.0099 (8) | |
O2 | 0.7259 (3) | 0.75 | 0.6326 (4) | 0.0129 (8) | |
O3 | 0.2743 (2) | 0.9946 (3) | 0.1053 (2) | 0.0084 (5) | |
O4 | 0.3423 (2) | 0.9684 (3) | 0.3435 (2) | 0.0100 (5) | |
O5 | −0.0274 (3) | 0.25 | 0.7485 (3) | 0.0112 (8) | |
O6 | −0.1052 (2) | 0.9599 (3) | 0.7182 (2) | 0.0090 (5) | |
O7 | 0.0945 (2) | 0.0289 (3) | 0.9002 (2) | 0.0084 (5) | |
O8 | 0.1833 (3) | 0.75 | 0.9630 (3) | 0.0100 (8) | |
O9 | 0.5102 (2) | 0.0289 (3) | 0.2481 (2) | 0.0110 (5) | |
O10 | 0.0957 (2) | 0.0320 (3) | 0.6743 (2) | 0.0096 (5) | |
O11 | 0.3210 (2) | 0.9616 (3) | 0.9020 (2) | 0.0095 (5) | |
O12 | 0.2534 (3) | 0.0222 (3) | 0.5157 (2) | 0.0156 (6) | |
F1 | 0.2695 (3) | 0.75 | 0.6782 (3) | 0.0142 (7) | |
F2 | 0.3190 (3) | 0.25 | 0.7478 (3) | 0.0172 (7) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Lu | 0.00544 (8) | 0.00452 (8) | 0.00516 (8) | 0.00016 (5) | 0.00231 (6) | 0.00015 (5) |
Rb1 | 0.0146 (3) | 0.0120 (2) | 0.0119 (2) | 0 | 0.0043 (2) | 0 |
Rb2 | 0.0170 (3) | 0.0114 (2) | 0.0131 (2) | 0 | 0.0080 (2) | 0 |
Rb3 | 0.0248 (3) | 0.0133 (3) | 0.0122 (3) | 0 | 0.0016 (2) | 0 |
Rb4 | 0.0143 (3) | 0.0133 (3) | 0.0212 (3) | 0 | 0.0024 (2) | 0 |
Si1 | 0.0050 (5) | 0.0044 (4) | 0.0044 (4) | 0.0007 (4) | 0.0015 (4) | 0.0002 (4) |
Si2 | 0.0043 (5) | 0.0047 (5) | 0.0053 (4) | 0.0002 (4) | 0.0007 (4) | 0.0003 (4) |
Si3 | 0.0049 (5) | 0.0052 (5) | 0.0044 (4) | −0.0007 (4) | 0.0013 (4) | −0.0001 (4) |
Si4 | 0.0055 (5) | 0.0057 (5) | 0.0042 (4) | 0.0006 (4) | 0.0009 (4) | 0.0007 (4) |
O1 | 0.0112 (19) | 0.0054 (17) | 0.0149 (19) | 0 | 0.0071 (16) | 0 |
O2 | 0.0094 (19) | 0.0051 (17) | 0.021 (2) | 0 | 0.0015 (16) | 0 |
O3 | 0.0070 (12) | 0.0113 (13) | 0.0050 (12) | 0.0022 (10) | 0.0001 (10) | −0.0006 (10) |
O4 | 0.0100 (13) | 0.0113 (13) | 0.0101 (12) | 0.0043 (10) | 0.0053 (11) | −0.0006 (10) |
O5 | 0.0093 (19) | 0.0069 (18) | 0.0156 (19) | 0 | 0.0027 (16) | 0 |
O6 | 0.0048 (12) | 0.0099 (12) | 0.0097 (12) | −0.0021 (10) | −0.0002 (10) | −0.0001 (10) |
O7 | 0.0074 (13) | 0.0118 (13) | 0.0063 (12) | 0.0036 (10) | 0.0029 (10) | −0.0007 (10) |
O8 | 0.0134 (19) | 0.0035 (17) | 0.0127 (18) | 0 | 0.0045 (16) | 0 |
O9 | 0.0082 (13) | 0.0091 (13) | 0.0145 (13) | −0.0019 (10) | 0.0028 (11) | −0.0004 (11) |
O10 | 0.0084 (13) | 0.0127 (13) | 0.0076 (12) | 0.0016 (10) | 0.0030 (10) | 0.0011 (10) |
O11 | 0.0089 (13) | 0.0137 (13) | 0.0069 (12) | 0.0010 (10) | 0.0042 (10) | 0.0023 (10) |
O12 | 0.0179 (15) | 0.0221 (15) | 0.0075 (12) | 0.0039 (11) | 0.0054 (11) | 0.0023 (11) |
F1 | 0.0164 (17) | 0.0044 (14) | 0.0194 (17) | 0 | 0.0039 (14) | 0 |
F2 | 0.0220 (18) | 0.0056 (15) | 0.0250 (18) | 0 | 0.0100 (15) | 0 |
Si1—O9 | 1.581 (3) | Rb1—O4xi | 3.332 (3) |
Si1—O1i | 1.6157 (11) | Rb2—O7xii | 2.875 (2) |
Si1—O4i | 1.631 (3) | Rb2—O7xiii | 2.875 (2) |
Si1—O3i | 1.640 (2) | Rb2—O6iii | 2.921 (3) |
Si2—O10 | 1.579 (3) | Rb2—O6xiv | 2.921 (3) |
Si2—O6i | 1.625 (3) | Rb2—O8xiv | 2.949 (4) |
Si2—O5 | 1.6259 (14) | Rb2—O3iv | 3.058 (2) |
Si2—O7 | 1.645 (3) | Rb2—O3i | 3.058 (2) |
Si3—O12ii | 1.569 (3) | Rb2—O7xv | 3.214 (3) |
Si3—O6iii | 1.630 (3) | Rb2—O7xvi | 3.214 (3) |
Si3—O4iv | 1.631 (3) | Rb2—O2v | 3.312 (4) |
Si3—O2v | 1.6318 (15) | Rb3—O10xvi | 2.909 (3) |
Si4—O11 | 1.574 (3) | Rb3—O10xiv | 2.909 (2) |
Si4—O8 | 1.6264 (14) | Rb3—O10vii | 2.927 (3) |
Si4—O3vi | 1.634 (3) | Rb3—O10ii | 2.927 (3) |
Si4—O7vii | 1.648 (3) | Rb3—F1 | 2.988 (3) |
Lu—F2 | 2.1487 (7) | Rb3—O12xvi | 3.407 (3) |
Lu—O12 | 2.167 (3) | Rb3—O12xiv | 3.407 (3) |
Lu—O9viii | 2.175 (3) | Rb3—O5xiv | 3.409 (4) |
Lu—F1i | 2.1906 (6) | Rb3—O6 | 3.426 (3) |
Lu—O10 | 2.200 (2) | Rb3—O6iv | 3.426 (3) |
Lu—O11i | 2.205 (2) | Rb4—O11xi | 2.949 (3) |
Rb1—O9v | 2.938 (3) | Rb4—O11v | 2.949 (3) |
Rb1—O9ix | 2.938 (3) | Rb4—O11xvii | 3.045 (3) |
Rb1—O4iv | 2.947 (3) | Rb4—O11xviii | 3.045 (3) |
Rb1—O4 | 2.947 (3) | Rb4—O9ii | 3.065 (3) |
Rb1—F1 | 2.989 (3) | Rb4—O9 | 3.065 (3) |
Rb1—O2 | 3.031 (4) | Rb4—F2xii | 3.142 (3) |
Rb1—O12ii | 3.158 (3) | Rb4—O3iv | 3.444 (3) |
Rb1—O12vii | 3.158 (3) | Rb4—O3i | 3.444 (3) |
Rb1—O4x | 3.332 (3) | ||
F2—Lu—O12 | 96.25 (12) | O6i—Si2—O7 | 104.51 (14) |
F2—Lu—O9viii | 91.29 (11) | O5—Si2—O7 | 106.84 (17) |
O12—Lu—O9viii | 92.79 (10) | O12ii—Si3—O6iii | 112.89 (15) |
F2—Lu—F1i | 177.69 (12) | O12ii—Si3—O4iv | 111.44 (15) |
O12—Lu—F1i | 86.05 (11) | O6iii—Si3—O4iv | 109.22 (14) |
O9viii—Lu—F1i | 88.38 (11) | O12ii—Si3—O2v | 114.08 (18) |
F2—Lu—O10 | 89.16 (11) | O6iii—Si3—O2v | 104.29 (16) |
O12—Lu—O10 | 89.57 (10) | O4iv—Si3—O2v | 104.34 (17) |
O9viii—Lu—O10 | 177.53 (9) | O11—Si4—O8 | 114.10 (17) |
F1i—Lu—O10 | 91.08 (11) | O11—Si4—O3vi | 112.49 (14) |
F2—Lu—O11i | 89.10 (11) | O8—Si4—O3vi | 108.21 (17) |
O12—Lu—O11i | 173.81 (10) | O11—Si4—O7vii | 113.66 (14) |
O9viii—Lu—O11i | 90.18 (9) | O8—Si4—O7vii | 104.55 (16) |
F1i—Lu—O11i | 88.61 (11) | O3vi—Si4—O7vii | 102.93 (13) |
O10—Lu—O11i | 87.40 (9) | Si1vii—O1—Si1ii | 162.7 (3) |
O9—Si1—O1i | 112.24 (17) | Si3ix—O2—Si3v | 146.9 (3) |
O9—Si1—O4i | 110.99 (14) | Si4xii—O3—Si1vii | 132.77 (16) |
O1i—Si1—O4i | 107.06 (17) | Si3iv—O4—Si1vii | 142.83 (17) |
O9—Si1—O3i | 111.24 (14) | Si2ii—O5—Si2 | 144.0 (2) |
O1i—Si1—O3i | 107.75 (16) | Si2vii—O6—Si3xvi | 144.19 (17) |
O4i—Si1—O3i | 107.32 (13) | Si2—O7—Si4i | 129.48 (16) |
O10—Si2—O6i | 111.85 (14) | Si4—O8—Si4iv | 145.7 (3) |
O10—Si2—O5 | 113.95 (17) | Luvii—F1—Luii | 160.51 (16) |
O6i—Si2—O5 | 106.95 (16) | Lu—F2—Luii | 156.30 (18) |
O10—Si2—O7 | 112.13 (14) |
Symmetry codes: (i) x, y−1, z; (ii) x, −y+1/2, z; (iii) −x, y−1/2, −z+1; (iv) x, −y+3/2, z; (v) −x+1, −y+1, −z+1; (vi) x, y, z+1; (vii) x, y+1, z; (viii) −x+1, −y, −z+1; (ix) −x+1, y+1/2, −z+1; (x) −x+1, −y+2, −z+1; (xi) −x+1, y−1/2, −z+1; (xii) x, y, z−1; (xiii) x, −y+1/2, z−1; (xiv) −x, −y+1, −z+1; (xv) −x, −y, −z+1; (xvi) −x, y+1/2, −z+1; (xvii) x, y−1, z−1; (xviii) x, −y+3/2, z−1. |
Experimental details
Crystal data | |
Chemical formula | Rb2Lu[Si4O10]F |
Mr | 637.27 |
Crystal system, space group | Monoclinic, P21/m |
Temperature (K) | 298 |
a, b, c (Å) | 11.6695 (3), 8.52379 (18), 11.8165 (3) |
β (°) | 111.753 (3) |
V (Å3) | 1091.67 (5) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 18.4 |
Crystal size (mm) | 0.32 × 0.08 × 0.08 |
Data collection | |
Diffractometer | Oxford Diffraction Xcalibur (Ruby, Gemini ultra) diffractometer |
Absorption correction | Analytical [CrysAlis PRO (Oxford Diffraction, 2006), based on expressions derived by Clark & Reid (1995)] |
Tmin, Tmax | 0.106, 0.562 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 15185, 2388, 2276 |
Rint | 0.028 |
(sin θ/λ)max (Å−1) | 0.625 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.017, 0.037, 1.2 |
No. of reflections | 2388 |
No. of parameters | 178 |
Δρmax, Δρmin (e Å−3) | 0.67, −0.69 |
Computer programs: CrysAlis PRO (Oxford Diffraction, 2006), SIR2002 (Burla et al., 2003), SHELXL97 (Sheldrick, 2008), ATOMS for Windows (Dowty, 2011), publCIF (Westrip, 2010) and WinGX (Farrugia, 2012).
References
Bergerhoff, G., Berndt, M., Brandenburg, K. & Degen, T. (1999). Acta Cryst. B55, 147–156. Web of Science CrossRef CAS IUCr Journals 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
Chigarov, M. I., Mamedov, Kh. S. & Kulieva, T. Z. (1983). Sov. Phys. Crystallogr. 28, 708–709. Google Scholar
Clark, R. C. & Reid, J. S. (1995). Acta Cryst. A51, 887–897. CrossRef CAS Web of Science IUCr Journals Google Scholar
Dowty, E. (2011). ATOMS for Windows. Shape Software, Kingsport, Tennessee, USA. Google Scholar
Farrugia, L. J. (2012). J. Appl. Cryst. 45, 849–854. Web of Science CrossRef CAS IUCr Journals Google Scholar
Hung, L.-I., Wang, S.-L., Kao, H.-M. & Lii, K.-H. (2003). Inorg. Chem. 42, 4057–4061. Web of Science CrossRef PubMed 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/ecid/Internet/en/DB/icsd/ . Google Scholar
Jacobsen, H. & Meyer, G. (1994). Z. Kristallogr. 209, 348–350. CrossRef CAS Web of Science Google Scholar
Kahlenberg, V. & Manninger, T. (2014). Acta Cryst. E70, i11. CrossRef IUCr Journals Google Scholar
Liebau, F. (1985). Structural Chemistry of Silicates, p. 347. Berlin, Heidelberg, New York, Tokyo: Springer. Google Scholar
Oxford Diffraction (2006). CrysAlis PRO. Oxford Diffraction Ltd, Abingdon, England. Google Scholar
Robinson, K., Gibbs, G. V. & Ribbe, P. H. (1971). Science, 172, 567–570. CrossRef PubMed CAS Web of Science Google Scholar
Schäfer, M. C. & Schleid, Th. (2007). Z. Anorg. Allg. Chem. 633, 1018–1023. Google Scholar
Schäfer, M. C. & Schleid, Th. (2011). Z. Anorg. Allg. Chem. 637, 1152–1157. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Tang, M.-F., Chiang, P.-Y., Su, Y.-H., Jung, Y.-C., Hou, G.-Y., Chang, B.-C. & Lii, K.-H. (2008). Inorg. Chem. 47, 8985–8989. Web of Science CrossRef PubMed CAS 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
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Up to now several alkali-REE-fluoride silicates (REE is a rare earth metal) including compounds such as KEu2[Si4O10]F (Jacobsen & Meyer, 1994), K9(REE)3[Si12O32]F2 (Tang et al., 2008; Kahlenberg & Manninger, 2014), Cs2Y[Si4O10]F2 (Schäfer & Schleid, 2007) or Rb3Sc2[Si4O10]F5 (Schäfer & Schleid, 2011) have been reported. In the course of an ongoing project on the synthesis of new representatives of this class, single-crystals of the previously unknown compound Rb2Lu[Si4O10]F have been structurally characterized. Following Liebau's classification of oxosilicates (Liebau, 1985), the crystal structure of this phase belongs to the group of tubular chain silicates and is based on unbranched multiple silicate chains running along [010] (Fig. 1). The periodicity of the chains is four. Alternatively, the tubular chains can also be thought of as being built from the condensation of an infinite number of fundamental rings with mean planes perpendicular to the chain direction (Liebau, 1985). In Rb2Lu[Si4O10]F these rings are eight-membered (Fig. 2) and exhibit a twisted chair conformation. Within the tubes, cages can be identified that are formed by additional four-, six- and eight-membered rings, the mean planes of which are running parallel to the chains. These six- and eight-membered rings are in boat configurations.
According to the Si:O ratio of 1:2.5 the structure is exclusively based on tertiary (Q3) [SiO4] tetrahedra. This structural feature is also reflected in the spread of the Si—O bond lengths. Each of the four crystallographically independent tetrahedra has one short (1.569 (3)–1.581 (3) Å) Si—O bond involving the non-bridging O atoms. The distances between Si and the bridging O atoms are considerably longer. The O–Si–O angles show a significant scatter throughout all present [SiO4] tetrahedra. Nevertheless, the values are in the expected limits for silicates (Liebau, 1985). Numerically, the degree of distortion can be expressed by the quadratic elongation λ and the angle variance σ2 (Robinson et al., 1971). For the four tetrahedra these two parameters vary between 1.001 and 1.005 (for λ) and 5.29 and 23.24 (for σ2) indicating that the deviation from regularity is not very pronounced. The Lu3+ cations are octahedrally coordinated by four oxygen and two additional fluoride anions (Fig. 3). The latter two are in trans-configuration. By sharing common fluorine corners, the octahedra in turn form chains running parallel to the directions of the silicate tubes (Fig. 4). However, these chains are not straight. The polyhedra are tilted with tilt angles (Lu—F1—Lu) and (Lu—F2—Lu) of 160.5 (3) and 156.3 (3)°, respectively. Charge compensation is achieved by the incorporation of additional Rb+ ions. The coordination numbers of these cations are as follows: Rb1, Rb3: 10-coordinate, including one F atom each; Rb2: 10-coordinate; Rb4: 9-coordinate, including one F atom (Fig. 5–8). The Rb2 cations, which are exclusively coordinated by O atoms, are located within the abovementioned cages of the tubes. The remaining rubidium cations reside in tunnel-like cavities formed by [SiO4]-tetrahedra and [LuO4F2]-octahedra. A side view of the crystal structure is given in Fig. 9.
Bond valence sum calculations using the parameter sets for the Rb–O, Rb–F, Lu–O, Lu–F 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: Rb(1): 1.152, Rb(2): 1.324, Rb(3): 1.008, Rb(4): 0.936, Lu: 3.140, Si(1): 4.062, Si(2): 4.044, Si(3): 4.082 and Si(4): 4.028.
The present compound is isostructural with K2Lu[Si4O10](OH) (Chigarov et al., 1983) and K2In[Si4O10](OH) (Hung et al., 2003). For the calculation of several quantitative descriptors for the characterization of the degree of similarity between the crystal structures of Rb2Lu[Si4O10]F and K2Lu[Si4O10](OH) containing the same REE, the program COMPSTRU (Tasci et al., 2012) was employed. For the 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) = 0.0053. For further investigations on an atomic level, the proton positions of the hydroxyl groups in K2Lu[Si4O10](OH) have been neglected. After a transformation to the standard setting according to a' = b, b' = c and c' = a and the application of an origin shift of p = (0, 1/2, 1/2) the structure of K2Lu[Si4O10](OH) was mapped on the most similar configuration of Rb2Lu[Si4O10]F. The calculations revealed the following atomic displacements (in Å) between the corresponding atoms in Rb2Lu[Si4O10]F (first entry) and K2Lu[Si4O10](OH) (second entry): Rb1—K1: 0.091; Rb2—K3: 0.063; Rb3—K2: 0.275; Rb4—K4: 0.064; Lu—Lu: 0.098; Si1—Si1: 0.055; Si2—Si2: 0.114; Si3—Si4: 0.089; Si4—Si3: 0.095; O1—O4: 0.083; O2—O13: 0.134; O3—O9: 0.067; O4—O5: 0.207; O5—O3: 0.168; O6—O11: 0.149; O7—O10: 0.101; O8—O2: 0.116; O9—O7: 0.082; O10—O14: 0.173; O11—O6: 0.247; O12—O8: 0.134; F1—O1: 0.246; F2—O12: 0.125, i.e. the maximum displacement is 0.275 Å. The measure of similarity (Δ) as defined by Bergerhoff et al. (1999) has a value of 0.022.