PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of actaeInternational Union of Crystallographysearchopen accessarticle submissionjournal home pagethis article
 
Acta Crystallogr Sect E Struct Rep Online. 2009 November 1; 65(Pt 11): i74.
Published online 2009 October 7. doi:  10.1107/S1600536809040008
PMCID: PMC2971154

Synchrotron study of the garnet-type oxide Li6CaSm2Ta2O12

Abstract

Hexalithium calcium disamarium(III) ditantalum(V) dodeca­oxide, Li6CaSm2Ta2O12, crystallizes in a cubic garnet-type structure. In the crystal structure, disordered Li atoms occupy two crystallographic sites. One Li has a tetra­hedral coordination environment in the oxide lattice, whereas the other Li atom occupies a significantly distorted octa­hedral site, with site occupancies for the two Li atoms of 0.26 (7) and 0.44 (2), respectively. Ca and Sm atoms are statistically distributed over the same crystallographic position with a occupancy of 2/3 for Sm and of 1/3 for Ca, and are eightfold coordinated by O atoms. The TaO6 octa­hedron is joined to six others via corner-sharing LiO4 tetra­hedra. One Li and the O atoms lie on general positions, while the other atoms are situated on special positions. The Sm/Ca position has 222, Ta has An external file that holds a picture, illustration, etc.
Object name is e-65-00i74-efi1.jpg, and the tetra­hedrally coordinated Li atom has An external file that holds a picture, illustration, etc.
Object name is e-65-00i74-efi2.jpg site symmetry.

Related literature

For a general description of structures and physical properties of garnets, see: Geller (1967 [triangle]). Recently, high Li-ion conductivity was discovered in garnet-related compounds such as Li5La3 M 2O12 (M = Nb, Ta), see: Thangadurai et al. (2003 [triangle]); Cussen (2006 [triangle]). For studies focused on the substitution of La3+ by divalent alkaline earth ions (Ca, Sr, Ba), see: Murugan et al. (2007 [triangle]); Thangadurai & Weppner (2005a [triangle],b [triangle]); O’Callaghan & Cussen (2007 [triangle]); Percival & Slater (2007 [triangle]). For indexing the powder diffraction pattern, see: Boultif & Louër (2004 [triangle]).

Experimental

Crystal data

  • Li6CaSm2Ta2O12
  • M r = 936.45
  • Cubic, An external file that holds a picture, illustration, etc.
Object name is e-65-00i74-efi3.jpg
  • a = 12.55128 (7) Å
  • V = 1977.26 (2) Å3
  • Z = 8
  • Synchrotron radiation
  • λ = 1.54900 Å
  • T = 298 K
  • Specimen shape: flat sheet
  • 20 × 20 × 0.5 mm
  • Specimen prepared at 103 kPa
  • Specimen prepared at 1223 K
  • Particle morphology: particle, yellowish-white

Data collection

  • Pohang Light Source 8C2 HRPD Beamline diffractometer
  • Specimen mounting: ’packed powder pellet’
  • Specimen mounted in reflection mode
  • Scan method: step
  • min = 10.0, 2θmax = 131.0°
  • Increment in 2θ = 0.01°

Refinement

  • R p = 15.0
  • R wp = 22.0
  • R exp = 13.1
  • R B = 6.62
  • S = 1.67
  • Excluded region(s): None
  • Profile function: pseudo Voigt
  • 151 reflections
  • 20 parameters
  • Preferred orientation correction: none

Data collection: local software at 8C2 HRPD beamline; cell refinement: FULLPROF (Rodriguez-Carvajal, 2001 [triangle]); data reduction: FULLPROF; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 [triangle]); program(s) used to refine structure: FULLPROF; molecular graphics: DIAMOND (Brandenburg, 1999 [triangle]); software used to prepare material for publication: FULLPROF.

Table 1
Selected bond lengths (Å)

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809040008/wm2261sup1.cif

Rietveld powder data: contains datablocks I. DOI: 10.1107/S1600536809040008/wm2261Isup2.rtv

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007- 412-J04001). The authors thank Dr Nam-Soo Shin for his help in performing the synchrotron XRD experiment at the Pohang light source.

supplementary crystallographic information

Comment

Conventional garnet-type oxides with general formula A3B3C2O12 contain tetrahedral, cubic and octahedral coordination environments filled with A, B, and C atoms, respectively. The garnet structure has attracted renewed interest since a Li+ ionic conductivity was observed in the compound Li5La3Ta2O12, which contains an excess of Li beyond the usual garnet composition. The ionic conductivity was enhanced through the increase of the Li content via partial substitution of trivalent La3+ by divalent alkaline earth ions (Murugan et al., 2007). The structure of the title compound is closely related to that of Li6SrLa2Ta2O12 (Percival & Slater, 2007). Li1 atoms are located at site 24d (tetrahedral), Li2 atoms at site 96h (distorted octahedral), Sm/Ca atoms are at site 24c (cubic), Ta atoms at site 16a (octahedral), and O atoms at general site 96h (Fig. 1). As shown in Fig. 2, the partially occupied Li2 site exhibits a significantly distorted [4 + 2] coordination polyhedron with Li—O bond lengths between 1.63 (6) - 2.69 (6) Å. The Li1 atoms at the tetrahedral sites and adjacent Li2 atoms at the octahedral sites are connected by common oxygen atoms via face-sharing. Considering the site occupation factors (SOF) for Li1 and Li2 sites, Li6CaSm2Ta2O12 can be described as Li2(3+x)[Li1(3-x)(Ca1/3Sm2/3)3Ta2O12] with x = 2.23.

For a general description of structures and physical properties of garnets, see: Geller (1967). High Li-ion conductivity was discovered in garnet-related compounds such as Li5La3M2O12, where M = Nb, Ta (Thangadurai et al., 2003; Cussen, 2006). For studies focused on the substitution of La3+ by divalent alkaline earth ions (Ca, Sr, Ba), see: Thangadurai & Weppner (2005a,b), O'Callaghan & Cussen (2007).

Experimental

The polycrystalline sample of Li6Sm2CaTa2O12 was prepared by solid-state reaction of stoichiometric amounts of Sm2O3, Ta2O5, CaCO3 and 10% excess of Li2CO3. Sm2O3 was preheated at 1173 K for 24 h to remove absorbed water before using. The finely ground samples were heated at 1123 and 1173 K for 12 h and then 1223 K for 24 h, with intermediate regrindings. Synchrotron X-ray diffraction (sXRD) measurement was performed on beamline 8 C2-HRPD at Pohang Accelerator Laboratory, Pohang, Korea. The incident X-rays were vertically collimated by a mirror, and monochromated to the wavelength of 1.5490 Å by double-crystal Si (111) monochromator. The datasets were collected in the range of 10° ≤ 2θ≤ 130° with a step size of 0.01° (2θ range).

Refinement

All reflections could be indexed with a body centered cubic cell. Any additional peaks due to symmetry lowering or impurity phase were not detected. The unit-cell parameter was determined with the DICVOL program (Boultif & Louër, 2004). The figures of merit were M(20) = 175.8, F(20) = 134.8(0.0013, 114). The reflection conditions for (hkl): h + k + l = even, (0kl): k,l = even, (hhl): 2 h + l = 4n and l = even, (00 l): l = 4n suggested that the Li containing-garnet structure belonged to Ia3d space group. The atomic positions of Ta, Sm, Ca and O atoms were determined employing direct methods using the synchrotron XRD data. The total amplitude factors (148, 'Fobs') were converted into structure factors and used as an input for the SHELXS97 program (Sheldrick, 2008). The positions of Li1 and Li2 were then determined by difference Fourier analysis in SHELXL97 program (Sheldrick, 2008). Structure refinements of atomic positions, occupancy and isotropic displacement parameters were carried out by the Rietveld method using the FULLPROF program with pseudo-Voigt peak shapes and manually selected backgrounds (Rodriguez-Carvajal, 2001). In the refinement, the SOFs of two Li sites were constrained in such a way that the total amount of Li atoms was constant to maintain the chemical composition. The isotropic displacement parameters for two Li atoms were set to the same refined value because there was a strong correlation between the isotropic displacement parameters and the SOFs for both Li1 and Li2 sites. The Rietveld refinement plot based on the synchrotron XRD data is shown in Fig. 3.

Figures

Fig. 1.
Schematic representation of the crystal structure of Li6CaSm2Ta2O12; the crossed spheres and the hollow spheres represent Li and Sm/Ca atoms, respectively.
Fig. 2.
(a) Local environment of Li2 site: The crossed spheres and the hollow spheres represent lithium and oxygen atoms, respectively. The relatively long Li—O bonds are dotted. (b) The Li1O4 tetrahedron shares each face with a Li2O6 octahedron.
Fig. 3.
Rietveld refinement plot of Li6CaSm2Ta2O12 based on synchrotron X-ray powder diffraction data.

Crystal data

Li6CaSm2Ta2O12Dx = 6.292 Mg m3
Mr = 936.45Synchrotron radiation, λ = 1.5490 Å
Cubic, Ia3dT = 298 K
Hall symbol: -I 4bd 2c 3Particle morphology: particle
a = 12.55128 (7) Åyellowish-white
V = 1977.26 (2) Å3flat sheet, 20 × 20 mm
Z = 8Specimen preparation: Prepared at 1223 K and 103 kPa, cooled at 5 K/min K min1
F(000) = 3200

Data collection

Pohang Light Source 8C2 HRPD Beamline diffractometerData collection mode: reflection
Radiation source: SynchrotronScan method: step
Si 111min = 10.00°, 2θmax = 131.00°, 2θstep = 0.01°
Specimen mounting: 'packed powder pellet'

Refinement

Rp = 15.0Profile function: pseudo Voigt
Rwp = 22.020 parameters
Rexp = 13.10 restraints
RBragg = 6.62(Δ/σ)max < 0.001
χ2 = 2.789Background function: manual background
12100 data pointsPreferred orientation correction: 'None'
Excluded region(s): None

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)

xyzUiso*/UeqOcc. (<1)
Sm10.000000.250000.625000.0089 (2)*0.6666
Ca10.000000.250000.625000.0089 (2)*0.3333
Ta10.000000.000000.500000.0071 (2)*
Li10.125000.000000.750000.0278 (11)*0.26 (7)
Li20.101 (5)0.192 (5)0.412 (5)0.0278 (11)*0.44 (2)
O10.0323 (5)0.0521 (5)0.6488 (6)0.0079 (13)*

Geometric parameters (Å, °)

(Sm,Ca)—O1i2.441 (18)Li1—O1xiv1.843 (7)
(Sm,Ca)—O1ii2.441 (18)Li2—O1xv1.63 (6)
(Sm,Ca)—O1iii2.441 (18)Li2—O1xi2.14 (6)
(Sm,Ca)—O1iv2.441 (18)Li2—O1i2.12 (6)
(Sm,Ca)—O1v2.561 (17)Li2—O1iii2.20 (6)
(Sm,Ca)—O12.561 (17)Li2—O1xvi2.55 (6)
(Sm,Ca)—O1vi2.561 (17)Li2—O1vii2.69 (6)
(Sm,Ca)—O1vii2.561 (17)Li1—Li2viii1.53 (6)
Ta1—O1viii2.014 (6)Li1—Li2xvii1.53 (6)
Ta1—O12.014 (6)Li1—Li2xviii1.53 (6)
Ta1—O1ix2.014 (6)Li1—Li2xix1.53 (6)
Ta1—O1x2.014 (6)Li1—Li2xx2.33 (6)
Ta1—O1xi2.014 (6)Li1—Li2ix2.33 (6)
Ta1—O1i2.014 (6)Li1—Li2vii2.33 (6)
Li1—O1xii1.843 (7)Li1—Li2xxi2.33 (6)
Li1—O11.843 (7)Li2—Li2xxii2.27 (9)
Li1—O1xiii1.843 (7)Li2—Li2xxiii2.27 (9)
O1i—(Sm,Ca)—O1ii158.8 (8)O1—Ta1—O1x180.000 (1)
O1i—(Sm,Ca)—O1iii72.8 (2)O1ix—Ta1—O1x87.2 (3)
O1ii—(Sm,Ca)—O1iii111.2 (2)O1viii—Ta1—O1xi180.0 (4)
O1i—(Sm,Ca)—O1iv111.2 (2)O1—Ta1—O1xi92.8 (3)
O1ii—(Sm,Ca)—O1iv72.8 (2)O1ix—Ta1—O1xi87.2 (3)
O1iii—(Sm,Ca)—O1iv158.8 (8)O1x—Ta1—O1xi87.2 (3)
O1i—(Sm,Ca)—O1v74.0 (2)O1viii—Ta1—O1i87.2 (3)
O1ii—(Sm,Ca)—O1v124.5 (4)O1—Ta1—O1i87.2 (3)
O1iii—(Sm,Ca)—O1v95.4 (2)O1ix—Ta1—O1i180.000 (2)
O1iv—(Sm,Ca)—O1v67.2 (2)O1x—Ta1—O1i92.8 (3)
O1i—(Sm,Ca)—O167.9 (8)O1xi—Ta1—O1i92.8 (3)
O1ii—(Sm,Ca)—O195.4 (2)O1xii—Li1—O1113.7 (6)
O1iii—(Sm,Ca)—O1124.5 (4)O1xii—Li1—O1xiii101.7 (3)
O1iv—(Sm,Ca)—O174.0 (2)O1—Li1—O1xiii113.7 (6)
O1v—(Sm,Ca)—O1108.13 (16)O1xii—Li1—O1xiv113.7 (6)
O1i—(Sm,Ca)—O1vi124.5 (4)O1—Li1—O1xiv101.7 (3)
O1ii—(Sm,Ca)—O1vi74.0 (2)O1xiii—Li1—O1xiv113.7 (6)
O1iii—(Sm,Ca)—O1vi67.2 (2)O1xv—Li2—O1i110 (3)
O1iv—(Sm,Ca)—O1vi95.4 (2)O1xi—Li2—O1i87 (2)
O1v—(Sm,Ca)—O1vi73.41 (16)O1xv—Li2—O1iii106 (3)
O1—(Sm,Ca)—O1vi167.0 (2)O1xi—Li2—O1iii150 (10)
O1i—(Sm,Ca)—O1vii95.4 (2)O1i—Li2—O1iii83 (2)
O1ii—(Sm,Ca)—O1vii67.2 (2)O1xv—Li2—O1xvi87 (10)
O1iii—(Sm,Ca)—O1vii74.0 (2)O1xi—Li2—O1xvi82 (2)
O1iv—(Sm,Ca)—O1vii124.5 (4)O1i—Li2—O1xvi165 (10)
O1v—(Sm,Ca)—O1vii167.0 (2)O1iii—Li2—O1xvi101 (2)
O1—(Sm,Ca)—O1vii73.41 (16)O1xv—Li2—O1vii148 (10)
O1vi—(Sm,Ca)—O1vii108.13 (16)O1xi—Li2—O1vii78.4 (19)
O1viii—Ta1—O187.2 (3)O1i—Li2—O1vii98 (10)
O1viii—Ta1—O1ix92.8 (3)O1iii—Li2—O1vii74.8 (18)
O1—Ta1—O1ix92.8 (3)O1xvi—Li2—O1vii67 (6)
O1viii—Ta1—O1x92.8 (3)

Symmetry codes: (i) −y, z−1/2, −x+1/2; (ii) −z+3/4, y+1/4, x+3/4; (iii) y, −z+1, −x+1/2; (iv) z−3/4, −y+1/4, x+3/4; (v) y−1/4, x+1/4, −z+5/4; (vi) −x, −y+1/2, z; (vii) −y+1/4, −x+1/4, −z+5/4; (viii) −z+1/2, −x, y+1/2; (ix) y, −z+1/2, x+1/2; (x) −x, −y, −z+1; (xi) z−1/2, x, −y+1/2; (xii) −x+1/4, −z+3/4, y+3/4; (xiii) −x+1/4, z−3/4, −y+3/4; (xiv) x, −y, −z+3/2; (xv) −z+3/4, −y+1/4, x+1/4; (xvi) y+1/4, −x+1/4, z−1/4; (xvii) z−1/4, y−1/4, x+3/4; (xviii) z−1/4, −y+1/4, −x+3/4; (xix) −z+1/2, x, −y+1; (xx) −y+1/4, x−1/4, z+1/4; (xxi) y, z−1/2, −x+1; (xxii) y−1/4, −x+1/4, −z+3/4; (xxiii) −y+1/4, x+1/4, −z+3/4.

Footnotes

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2261).

References

  • Boultif, A. & Louër, D. (2004). J. Appl. Cryst.37, 724–731.
  • Brandenburg, K. (1999). DIAMOND. Crystal Impact GbR, Bonn, Germany.
  • Cussen, E. J. (2006). Chem. Commun. pp. 412–413. [PubMed]
  • Geller, S. (1967). Z. Kristallogr.125, 1–47.
  • Murugan, R., Thangadurai, V. & Weppner, W. (2007). Ionics, 13, 195–203.
  • O’Callaghan, M. P. & Cussen, E. J. (2007). Chem. Commun. pp. 2048–2050. [PubMed]
  • Percival, J. & Slater, P. R. (2007). Solid State Commun.142, 355–357.
  • Rodriguez-Carvajal, J. (2001). FULLPROF Commission on Powder Diffraction (IUCr), Newsletter 26, pp. 12–19.
  • Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. [PubMed]
  • Thangadurai, V., Kaack, H. & Weppner, W. (2003). J. Am. Ceram. Soc.86, 437–440.
  • Thangadurai, V. & Weppner, W. (2005a). Adv. Funct. Mater.15, 107–112.
  • Thangadurai, V. & Weppner, W. (2005b). J. Am. Ceram. Soc.88, 411–418.

Articles from Acta Crystallographica Section E: Structure Reports Online are provided here courtesy of International Union of Crystallography