supplementary materials
Synchrotron study of the garnet-type oxide Li6CaSm2Ta2O12
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 
, and the tetrahedrally coordinated Li atom has 
site symmetry.
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).
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.
Data collection: local software at 8C2 HRPD beamline; cell refinement: FULLPROF (Rodriguez-Carvajal, 2001); data reduction: FULLPROF (Rodriguez-Carvajal, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: FULLPROF (Rodriguez-Carvajal, 2001); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: FULLPROF (Rodriguez-Carvajal, 2001).
Hexalithium calcium disamarium(III) ditantalum(V) dodecaoxide
top
Crystal data top
| Li6CaSm2Ta2O12 | Dx = 6.292 Mg m−3 |
| Mr = 936.45 | Synchrotron radiation, λ = 1.5490 Å |
| Cubic, Ia3d | T = 298 K |
| Hall symbol: -I 4bd 2c 3 | Particle morphology: particle |
| a = 12.55128 (7) Å | yellowish-white |
| V = 1977.26 (2) Å3 | flat sheet, 20 × 20 mm |
| Z = 8 | Specimen preparation: Prepared at 1223 K and 103 kPa, cooled at 5 K/min K min−1 |
| F(000) = 3200 | |
Data collection top
Pohang Light Source 8C2 HRPD Beamline
diffractometer | Data collection mode: reflection |
| Radiation source: Synchrotron | Scan method: step |
| Si 111 | 2θmin = 10.000°, 2θmax = 131.000°, 2θstep = 0.010° |
| Specimen mounting: 'packed powder pellet' | |
Refinement top
| Rp = 15.0 | Profile function: pseudo Voigt |
| Rwp = 22.0 | 20 parameters |
| Rexp = 13.1 | 0 restraints |
| RBragg = 6.62 | (Δ/σ)max < 0.001 |
| χ2 = 2.789 | Background function: manual background |
| 12100 data points | Preferred orientation correction: 'None' |
| Excluded region(s): None | |
Crystal data top
| Li6CaSm2Ta2O12 | Z = 8 |
| Mr = 936.45 | Synchrotron radiation, λ = 1.5490 Å |
| Cubic, Ia3d | T = 298 K |
| a = 12.55128 (7) Å | flat sheet, 20 × 20 mm |
| V = 1977.26 (2) Å3 | |
Data collection top
Pohang Light Source 8C2 HRPD Beamline
diffractometer | Scan method: step |
| Specimen mounting: 'packed powder pellet' | 2θmin = 10.000°, 2θmax = 131.000°, 2θstep = 0.010° |
| Data collection mode: reflection | |
Refinement top
| Rp = 15.0 | R(F2) = ? |
| Rwp = 22.0 | χ2 = 2.789 |
| Rexp = 13.1 | 12100 data points |
| RBragg = 6.62 | 20 parameters |
| R(F) = ? | 0 restraints |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top| | x | y | z | Uiso*/Ueq | Occ. (<1) |
| Sm1 | 0.00000 | 0.25000 | 0.62500 | 0.0089 (2)* | 0.6666 |
| Ca1 | 0.00000 | 0.25000 | 0.62500 | 0.0089 (2)* | 0.3333 |
| Ta1 | 0.00000 | 0.00000 | 0.50000 | 0.0071 (2)* | |
| Li1 | 0.12500 | 0.00000 | 0.75000 | 0.0278 (11)* | 0.26 (7) |
| Li2 | 0.101 (5) | 0.192 (5) | 0.412 (5) | 0.0278 (11)* | 0.44 (2) |
| O1 | 0.0323 (5) | 0.0521 (5) | 0.6488 (6) | 0.0079 (13)* | |
Geometric parameters (Å, °) top
| (Sm,Ca)—O1i | 2.441 (18) | Li1—O1xiv | 1.843 (7) |
| (Sm,Ca)—O1ii | 2.441 (18) | Li2—O1xv | 1.63 (6) |
| (Sm,Ca)—O1iii | 2.441 (18) | Li2—O1xi | 2.14 (6) |
| (Sm,Ca)—O1iv | 2.441 (18) | Li2—O1i | 2.12 (6) |
| (Sm,Ca)—O1v | 2.561 (17) | Li2—O1iii | 2.20 (6) |
| (Sm,Ca)—O1 | 2.561 (17) | Li2—O1xvi | 2.55 (6) |
| (Sm,Ca)—O1vi | 2.561 (17) | Li2—O1vii | 2.69 (6) |
| (Sm,Ca)—O1vii | 2.561 (17) | Li1—Li2viii | 1.53 (6) |
| Ta1—O1viii | 2.014 (6) | Li1—Li2xvii | 1.53 (6) |
| Ta1—O1 | 2.014 (6) | Li1—Li2xviii | 1.53 (6) |
| Ta1—O1ix | 2.014 (6) | Li1—Li2xix | 1.53 (6) |
| Ta1—O1x | 2.014 (6) | Li1—Li2xx | 2.33 (6) |
| Ta1—O1xi | 2.014 (6) | Li1—Li2ix | 2.33 (6) |
| Ta1—O1i | 2.014 (6) | Li1—Li2vii | 2.33 (6) |
| Li1—O1xii | 1.843 (7) | Li1—Li2xxi | 2.33 (6) |
| Li1—O1 | 1.843 (7) | Li2—Li2xxii | 2.27 (9) |
| Li1—O1xiii | 1.843 (7) | Li2—Li2xxiii | 2.27 (9) |
| | | |
| O1i—(Sm,Ca)—O1ii | 158.8 (8) | O1—Ta1—O1x | 180.000 (1) |
| O1i—(Sm,Ca)—O1iii | 72.8 (2) | O1ix—Ta1—O1x | 87.2 (3) |
| O1ii—(Sm,Ca)—O1iii | 111.2 (2) | O1viii—Ta1—O1xi | 180.0 (4) |
| O1i—(Sm,Ca)—O1iv | 111.2 (2) | O1—Ta1—O1xi | 92.8 (3) |
| O1ii—(Sm,Ca)—O1iv | 72.8 (2) | O1ix—Ta1—O1xi | 87.2 (3) |
| O1iii—(Sm,Ca)—O1iv | 158.8 (8) | O1x—Ta1—O1xi | 87.2 (3) |
| O1i—(Sm,Ca)—O1v | 74.0 (2) | O1viii—Ta1—O1i | 87.2 (3) |
| O1ii—(Sm,Ca)—O1v | 124.5 (4) | O1—Ta1—O1i | 87.2 (3) |
| O1iii—(Sm,Ca)—O1v | 95.4 (2) | O1ix—Ta1—O1i | 180.000 (2) |
| O1iv—(Sm,Ca)—O1v | 67.2 (2) | O1x—Ta1—O1i | 92.8 (3) |
| O1i—(Sm,Ca)—O1 | 67.9 (8) | O1xi—Ta1—O1i | 92.8 (3) |
| O1ii—(Sm,Ca)—O1 | 95.4 (2) | O1xii—Li1—O1 | 113.7 (6) |
| O1iii—(Sm,Ca)—O1 | 124.5 (4) | O1xii—Li1—O1xiii | 101.7 (3) |
| O1iv—(Sm,Ca)—O1 | 74.0 (2) | O1—Li1—O1xiii | 113.7 (6) |
| O1v—(Sm,Ca)—O1 | 108.13 (16) | O1xii—Li1—O1xiv | 113.7 (6) |
| O1i—(Sm,Ca)—O1vi | 124.5 (4) | O1—Li1—O1xiv | 101.7 (3) |
| O1ii—(Sm,Ca)—O1vi | 74.0 (2) | O1xiii—Li1—O1xiv | 113.7 (6) |
| O1iii—(Sm,Ca)—O1vi | 67.2 (2) | O1xv—Li2—O1i | 110 (3) |
| O1iv—(Sm,Ca)—O1vi | 95.4 (2) | O1xi—Li2—O1i | 87 (2) |
| O1v—(Sm,Ca)—O1vi | 73.41 (16) | O1xv—Li2—O1iii | 106 (3) |
| O1—(Sm,Ca)—O1vi | 167.0 (2) | O1xi—Li2—O1iii | 150 (10) |
| O1i—(Sm,Ca)—O1vii | 95.4 (2) | O1i—Li2—O1iii | 83 (2) |
| O1ii—(Sm,Ca)—O1vii | 67.2 (2) | O1xv—Li2—O1xvi | 87 (10) |
| O1iii—(Sm,Ca)—O1vii | 74.0 (2) | O1xi—Li2—O1xvi | 82 (2) |
| O1iv—(Sm,Ca)—O1vii | 124.5 (4) | O1i—Li2—O1xvi | 165 (10) |
| O1v—(Sm,Ca)—O1vii | 167.0 (2) | O1iii—Li2—O1xvi | 101 (2) |
| O1—(Sm,Ca)—O1vii | 73.41 (16) | O1xv—Li2—O1vii | 148 (10) |
| O1vi—(Sm,Ca)—O1vii | 108.13 (16) | O1xi—Li2—O1vii | 78.4 (19) |
| O1viii—Ta1—O1 | 87.2 (3) | O1i—Li2—O1vii | 98 (10) |
| O1viii—Ta1—O1ix | 92.8 (3) | O1iii—Li2—O1vii | 74.8 (18) |
| O1—Ta1—O1ix | 92.8 (3) | O1xvi—Li2—O1vii | 67 (6) |
| O1viii—Ta1—O1x | 92.8 (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. |
Table 1
Selected geometric parameters (Å) top| (Sm,Ca)—O1 | 2.561 (17) | Li2—O1iii | 2.12 (6) |
| Ta1—O1 | 2.014 (6) | Li2—O1iv | 2.20 (6) |
| Li1—O1 | 1.843 (7) | Li2—O1v | 2.55 (6) |
| Li2—O1i | 1.63 (6) | Li2—O1vi | 2.69 (6) |
| Li2—O1ii | 2.14 (6) | | |
| Symmetry codes: (i) −z+3/4, −y+1/4, x+1/4; (ii) z−1/2, x, −y+1/2; (iii) −y, z−1/2, −x+1/2; (iv) y, −z+1, −x+1/2; (v) y+1/4, −x+1/4, z−1/4; (vi) −y+1/4, −x+1/4, −z+5/4. |
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.
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![[Figure 1]](./wm2261fig1thm.gif)
![[Figure 2]](./wm2261fig2thm.gif)
![[Figure 3]](./wm2261fig3thm.gif)
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).