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.

Related literature

For a general description of structures and physical properties of garnets, see: Geller (1967). Recently, high Li-ion conductivity was discovered in garnet-related compounds such as Li₅La₃M₂O₁₂ (M = Nb, Ta), see: Thangadurai et al. (2003); Cussen (2006). For studies focused on the substitution of La³⁺ by divalent alkaline earth ions (Ca, Sr, Ba), see: Murugan et al. (2007); Thangadurai & Weppner (2005a,b); O'Callaghan & Cussen (2007); Percival & Slater (2007). For indexing the powder diffraction pattern, see: Boultif & Louër (2004).

Experimental

Crystal data

• Li₆CaSm₂Ta₂O₁₂

• M_r = 936.45

• Cubic,

• a = 12.55128 (7) Å

• V = 1977.26 (2) ų

• 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

• 2θ_min = 10.0, 2θ_max = 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

Table 1
Selected bond lengths (Å)

(Sm,Ca)—O1	2.561 (17)
Ta1—O1	2.014 (6)
Li1—O1	1.843 (7)
Li2—O1^i	1.63 (6)
Li2—O1^ii	2.14 (6)
Li2—O1^iii	2.12 (6)
Li2—O1^iv	2.20 (6)
Li2—O1^v	2.55 (6)
Li2—O1^vi	2.69 (6)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) .

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

Supporting information

S1. 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)(Ca_1/3Sm_2/3)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).

Related literature

For a general description of structures and physical properties of garnets, see: Geller (1967). Recently, high Li-ion conductivity was discovered in garnet-related compounds such as Li₅La₃M₂O₁₂ (M = Nb, Ta), see: Thangadurai et al. (2003); Cussen (2006). For studies focused on the substitution of La³⁺ by divalent alkaline earth ions (Ca, Sr, Ba), see: Murugan et al. (2007); Thangadurai & Weppner (2005a,b); O'Callaghan & Cussen (2007); Percival & Slater (2007). For indexing the powder diffraction pattern, see: Boultif & Louër (2004).

S2. 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).

S3. 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, 'F_obs') 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.

Computing details

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).

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.

Hexalithium calcium disamarium(III) ditantalum(V) dodecaoxide

Crystal data

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) Å^3	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

Pohang Light Source 8C2 HRPD Beamline diffractometer	Data collection mode: reflection
Radiation source: Synchrotron	Scan method: step
Si 111 monochromator	2θmin = 10.000°, 2θmax = 131.000°, 2θstep = 0.010°
Specimen mounting: 'packed powder pellet'

Refinement

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
12100 data points	Background function: manual background
Excluded region(s): None	Preferred orientation correction: 'None'

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

	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, º)

(Sm,Ca)—O1^i	2.441 (18)	Li1—O1^xiv	1.843 (7)
(Sm,Ca)—O1^ii	2.441 (18)	Li2—O1^xv	1.63 (6)
(Sm,Ca)—O1^iii	2.441 (18)	Li2—O1^xi	2.14 (6)
(Sm,Ca)—O1^iv	2.441 (18)	Li2—O1^i	2.12 (6)
(Sm,Ca)—O1^v	2.561 (17)	Li2—O1^iii	2.20 (6)
(Sm,Ca)—O1	2.561 (17)	Li2—O1^xvi	2.55 (6)
(Sm,Ca)—O1^vi	2.561 (17)	Li2—O1^vii	2.69 (6)
(Sm,Ca)—O1^vii	2.561 (17)	Li1—Li2^viii	1.53 (6)
Ta1—O1^viii	2.014 (6)	Li1—Li2^xvii	1.53 (6)
Ta1—O1	2.014 (6)	Li1—Li2^xviii	1.53 (6)
Ta1—O1^ix	2.014 (6)	Li1—Li2^xix	1.53 (6)
Ta1—O1^x	2.014 (6)	Li1—Li2^xx	2.33 (6)
Ta1—O1^xi	2.014 (6)	Li1—Li2^ix	2.33 (6)
Ta1—O1^i	2.014 (6)	Li1—Li2^vii	2.33 (6)
Li1—O1^xii	1.843 (7)	Li1—Li2^xxi	2.33 (6)
Li1—O1	1.843 (7)	Li2—Li2^xxii	2.27 (9)
Li1—O1^xiii	1.843 (7)	Li2—Li2^xxiii	2.27 (9)
O1^i—(Sm,Ca)—O1^ii	158.8 (8)	O1—Ta1—O1^x	180.000 (1)
O1^i—(Sm,Ca)—O1^iii	72.8 (2)	O1^ix—Ta1—O1^x	87.2 (3)
O1^ii—(Sm,Ca)—O1^iii	111.2 (2)	O1^viii—Ta1—O1^xi	180.0 (4)
O1^i—(Sm,Ca)—O1^iv	111.2 (2)	O1—Ta1—O1^xi	92.8 (3)
O1^ii—(Sm,Ca)—O1^iv	72.8 (2)	O1^ix—Ta1—O1^xi	87.2 (3)
O1^iii—(Sm,Ca)—O1^iv	158.8 (8)	O1^x—Ta1—O1^xi	87.2 (3)
O1^i—(Sm,Ca)—O1^v	74.0 (2)	O1^viii—Ta1—O1^i	87.2 (3)
O1^ii—(Sm,Ca)—O1^v	124.5 (4)	O1—Ta1—O1^i	87.2 (3)
O1^iii—(Sm,Ca)—O1^v	95.4 (2)	O1^ix—Ta1—O1^i	180.000 (2)
O1^iv—(Sm,Ca)—O1^v	67.2 (2)	O1^x—Ta1—O1^i	92.8 (3)
O1^i—(Sm,Ca)—O1	67.9 (8)	O1^xi—Ta1—O1^i	92.8 (3)
O1^ii—(Sm,Ca)—O1	95.4 (2)	O1^xii—Li1—O1	113.7 (6)
O1^iii—(Sm,Ca)—O1	124.5 (4)	O1^xii—Li1—O1^xiii	101.7 (3)
O1^iv—(Sm,Ca)—O1	74.0 (2)	O1—Li1—O1^xiii	113.7 (6)
O1^v—(Sm,Ca)—O1	108.13 (16)	O1^xii—Li1—O1^xiv	113.7 (6)
O1^i—(Sm,Ca)—O1^vi	124.5 (4)	O1—Li1—O1^xiv	101.7 (3)
O1^ii—(Sm,Ca)—O1^vi	74.0 (2)	O1^xiii—Li1—O1^xiv	113.7 (6)
O1^iii—(Sm,Ca)—O1^vi	67.2 (2)	O1^xv—Li2—O1^i	110 (3)
O1^iv—(Sm,Ca)—O1^vi	95.4 (2)	O1^xi—Li2—O1^i	87 (2)
O1^v—(Sm,Ca)—O1^vi	73.41 (16)	O1^xv—Li2—O1^iii	106 (3)
O1—(Sm,Ca)—O1^vi	167.0 (2)	O1^xi—Li2—O1^iii	150 (10)
O1^i—(Sm,Ca)—O1^vii	95.4 (2)	O1^i—Li2—O1^iii	83 (2)
O1^ii—(Sm,Ca)—O1^vii	67.2 (2)	O1^xv—Li2—O1^xvi	87 (10)
O1^iii—(Sm,Ca)—O1^vii	74.0 (2)	O1^xi—Li2—O1^xvi	82 (2)
O1^iv—(Sm,Ca)—O1^vii	124.5 (4)	O1^i—Li2—O1^xvi	165 (10)
O1^v—(Sm,Ca)—O1^vii	167.0 (2)	O1^iii—Li2—O1^xvi	101 (2)
O1—(Sm,Ca)—O1^vii	73.41 (16)	O1^xv—Li2—O1^vii	148 (10)
O1^vi—(Sm,Ca)—O1^vii	108.13 (16)	O1^xi—Li2—O1^vii	78.4 (19)
O1^viii—Ta1—O1	87.2 (3)	O1^i—Li2—O1^vii	98 (10)
O1^viii—Ta1—O1^ix	92.8 (3)	O1^iii—Li2—O1^vii	74.8 (18)
O1—Ta1—O1^ix	92.8 (3)	O1^xvi—Li2—O1^vii	67 (6)
O1^viii—Ta1—O1^x	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.