Li₂Ca_1.5Nb₃O₁₀ from X-ray powder data

Lithium calcium niobium oxide (2/1.5/3/10), Li₂Ca_1.5Nb₃O₁₀, has been synthesized by conventional solid-state reaction. Its structure consists of triple-layer perovskite slabs of corner-sharing NbO₆ octa­hedra inter­leaved with lithium ions; Ca cations partially occupy the perovskite A sites at 75% occupancy probability. All eight atoms in the asymmetric unit are on special positions: one Nb atom has site symmetry 4/mmm; the second Nb, both K, the Sr and two O atoms have site symmetry 4mm; the remaining two O atoms have site symmetries 2mm. and mmm., respectively.

Related literature

For background to Ruddlesden–Popper layered perovskites, see: Schaak & Mallouk (2002). Structures of related crystal A-site deficient three-layer Ruddlesden–Popper phases have been reported for K₂Sr_1.5Ta₃O₁₀ (Le Berre et al., 2002), Li₄Sr₃Nb₆O₂₀ (Bhuvanesh et al., 1999a), Li₂La_1.78Nb_0.66Ti_2.34O₁₀ (Bhuvanesh et al., 1999b) and Li₂CaTa₂O₇ (Liang et al., 2008). For crystallographic background, see: Howard (1982); Thompson et al. (1987).

Experimental

Crystal data

• Li₂Ca_1.5Nb₃O₁₀

• M_r = 512.71

• Tetragonal, I 4/m m m

• a = 3.87880 (6) Å

• c = 26.2669 (4) Å

• V = 395.19 (1) ų

• Z = 2

• Cu Kα radiation, λ = 1.54060, 1.54443 Å

• T = 298 K

• flat sheet, 20 × 20 mm

Data collection

• PANalytical X'pert PRO diffractometer

• Specimen mounting: packed powder pellet

• Data collection mode: reflection

• Scan method: continuous

• 2θ_min = 10.004°, 2θ_max = 129.939°, 2θ_step = 0.017°

Refinement

• R_p = 0.050

• R_wp = 0.076

• R_exp = 0.009

• R(F²) = 0.068

• χ² = 0.706

• 7056 data points

• 51 parameters

Table 1
Selected bond lengths (Å)

Nb1—O1^i	1.9394 (1)
Nb1—O4	2.027 (11)
Nb2—O2	1.689 (8)
Nb2—O3^i	1.9704 (11)
Nb2—O4	2.029 (11)
Ca1—O1^ii	2.805 (4)
Ca1—O3^ii	2.567 (4)
Ca1—O4^iii	2.7427 (1)
Li1—O2	1.599 (4)
Symmetry codes: (i) x, y-1, z; (ii) ; (iii) .

Data collection: X'pert Data Collector (PANalytical, 2003); cell refinement: GSAS (Larson & Von Dreele, 2004) and EXPGUI (Toby, 2001); data reduction: X'pert Highscore (PANalytical, 2003); method used to solve structure: coordinates taken from an isotypic compound (Bhuvanesh et al., 1999a; Liang et al., 2008); program(s) used to refine structure: GSAS and EXPGUI; molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

Layered perovskites that belong to the Ruddlesden-Popper family have a general formula A'₂[A_n-1B_nO_3n+1] (Schaak et al., 2002), where B is a small transition metal cation, A is a larger s-, d-, or f-block cation and A' is always an alkali cation. The Ruddlesden-Popper phases which are intergrowths of the perovskite and rocksalt structures posses a wide variety of interesting properties including superconductivity, colossal magnetoresistance, ferroelectricity, and catalytic activity. Related crystal structures of A sites deficiency three-layer Ruddlesden-Popper phases have been reported for K₂Sr_1.5Ta₃O₁₀(Le Berre et al., 2002), Li₂La_1.78Nb_0.66Ti_2.34O₁₀ ( Bhuvanesh et al., 1999b), and Li₄Sr₃Nb₆O₂₀ ( Bhuvanesh et al., 1999a).

Fig. 1 shows the observed, calculated and difference plots of the Rietveld refinement. We applied the March-Dollase formalism for a correction of the 00l preferential orientation which is frequently observed in Rietveld refinement of layered perovskites.

The structure of the compound is illustrated in Fig. 2. It is formed from two differently stacked NbO₆ octahedra thick slabs cut along the c direction. Two successive layers are shifted by (a+b)/2 with Ca cations partially occupying the 12-coordinated sites. The Li cations occupy the interlayer spacing at Wyckoff site 8f and not the 4e site since the distance between two adjacent layers is short. Ca cations partially occupy the perovskite A sites at 75% occupancy probability. The Nb cations are coordinated by six oxygen atoms to form NbO₆ octahedra with Nb—O distances ranging from 1.689 (8) to 2.029 (11) Å. The octahedra forming the outer layer of the slabs are characterized by off-centering of the Nb atoms, leading to four equal equatorial Nb—O distances within the perovskite layers [1.9704 (11) Å], a short Nb—O bond toward the interlayer spacing [1.689 (8) Å], and a long opposite Nb—O bond [2.029 (11) Å]. The octahedra forming the inner layer are less distorted with four equal equatorial Nb—O distances[1.9394 (1) Å] and other two equal Nb—O distances [2.027 (11) Å] parallel to the c axis. These type of distorsions are well known in triple-layer perovskites.

Related literature

For background to Ruddlesden–Popper layered perovskites, see: Schaak & Mallouk (2002). Structures of related crystal A-site deficient three-layer Ruddlesden–Popper phases have been reported for K₂Sr_1.5Ta₃O₁₀ (Le Berre et al., 2002), Li₄Sr₃Nb₆O₂₀ (Bhuvanesh et al., 1999a), Li₂La_1.78Nb_0.66Ti_2.34O₁₀ (Bhuvanesh et al., 1999b) and Li₂CaTa₂O₇ (Liang et al., 2008). Profile coefficients for Simpson's rule integration of the pseudo-Voigt function were taked from Howard (1982) and Thompson et al. (1987).

S2. Experimental

The sample was prepared by conventional solid-state reaction. Stoichiometric amounts of Li₂CO₃,CaCO₃ and Nb₂O₅ were mixed, ground, and calcined at 1423 K for 6 h with one intermediate grid. An excess amount of Li₂CO₃(20 mol%) was added to compensate for the loss due to the volatilization of alkali metal carbonate.

S3. Refinement

The crystal structures of Li₄Sr₃Nb₆O₂₀ (Bhuvanesh et al., 1999a) and Li₂CaTa₂O₇ (Liang et al., 2008) were used as a starting model for the Rietveld refinement. The X-ray powder diffraction patterns of Li₂Ca_1.5Nb₃O₁₀ were indexed in a body-centered tetragonal space group I4/mmm. Structure refinement was carried out by the Rietveld method using the GSAS profile refinement program (Larson & Von Dreele, 2004). The site occupancy factors of Ca and Li were set at 0.75 and 0.50, respectively in view of the close ressemblance of the cell parameters with those of the related structures and they were not further refined. The corresponding isotropic atomic displacement parameters of all oxygen atoms and niobium atoms were constrained to be equal, respectively. The March-Dollase option in the EXPGUI program (Toby, 2001) was applied to correct 00l preferential orientation.

Computing details

Data collection: X'pert Data Collector (PANalytical, 2003); cell refinement: GSAS (Larson & Von Dreele, 2004) and EXPGUI (Toby, 2001); data reduction: X'pert Highscore (PANalytical, 2003); program(s) used to solve structure: coordinates taken from an isotypic compound (Bhuvanesh et al., 1999a) and (Liang et al., 2008); program(s) used to refine structure: GSAS (Larson & Von Dreele, 2004) and EXPGUI (Toby, 2001); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. Experimental and calculated X-ray diffraction pattern of Li2Ca1.5Nb3O10. The difference profile is given at the bottom. The Bragg positions are indicated by the vertical markers below the observed pattern.
	Fig. 2. The crystal structure of Li2Ca1.5Nb3O10 in a projection along [010].

Lithium calcium niobium oxide (2/1.5/3/10)

Crystal data

Li2Ca1.5Nb3O10	Z = 2
Mr = 512.71	Dx = 4.309 Mg m^−3
Tetragonal, I4/mmm	Cu Kα radiation, λ = 1.540600, 1.544430 Å
Hall symbol: -I 4 2	T = 298 K
a = 3.87880 (6) Å	white
c = 26.2669 (4) Å	flat sheet, 20 × 20 mm
V = 395.19 (1) Å^3	Specimen preparation: Prepared at 1423 K

Data collection

PANalytical X'pert PRO diffractometer	Data collection mode: reflection
Radiation source: sealed tube	Scan method: continuous
Graphite monochromator	2θmin = 10.004°, 2θmax = 129.939°, 2θstep = 0.017°
Specimen mounting: packed powder pellet

Refinement

Refinement on F^2	Profile function: CW Profile function number 2 with 18 terms Profile coefficients for Simpson's rule integration of pseudovoigt function C.J. Howard (1982). J. Appl. Cryst.,15,615-620. P. Thompson, D.E. Cox & J.B. Hastings (1987). J. Appl. Cryst.,20,79-83. #1(GU) = 149.621 #2(GV) = -120.364 #3(GW) = 31.573 #4(LX) = 1.000 #5(LY) = 17.840 #6(trns) = 0.000 #7(asym) = 0.0000 #8(shft) = 0.0000 #9(GP) = 0.000 #10(stec)= 0.00 #11(ptec)= 0.00 #12(sfec)= 0.00 #13(L11) = 0.000 #14(L22) = 0.000 #15(L33) = 0.000 #16(L12) = 0.000 #17(L13) = 0.000 #18(L23) = 0.000 Peak tails are ignored where the intensity is below 0.0010 times the peak Aniso. broadening axis 0.0 0.0 1.0
Least-squares matrix: full	51 parameters
Rp = 0.050	0 restraints
Rwp = 0.076	4 constraints
Rexp = 0.009	w = 1/[σ^2(Fo^2) + (0.0677P)^2] where P = (Fo^2 + 2Fc^2)/3
R(F^2) = 0.06796	(Δ/σ)max = 0.01
χ^2 = 0.706	Background function: GSAS Background function number 1 with 36 terms. Shifted Chebyshev function of 1st kind 1: 10229.8 2: -3178.07 3: 2423.18 4: -808.112 5: 540.944 6: -198.924 7: 271.065 8: 94.4177 9: 234.644 10: 188.507 11: 146.243 12: 265.504 13: -11.6147 14: 51.8836 15: 137.742 16: 26.3316 17: -53.6065 18: 3.80136 19: 279.859 20: -56.8162 21: -60.3405 22: 50.5886 23: 41.8504 24: 9.38150 25: -48.8258 26: -20.5686 27: -49.8098 28: 74.7145 29: -37.5745 30: 90.5252 31: -21.2918 32: -56.1545 33: 0.932266 34: -17.8446 35: -27.9120 36: -2.66006
7056 data points	Preferred orientation correction: March-Dollase AXIS 1 Ratio= 0.89341 h= 0.000 k= 0.000 l= 1.000 Prefered orientation correction range: Min= 0.84444, Max= 1.40236
Excluded region(s): none

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Nb1	0.0	0.0	0.0	0.0112 (3)*
Nb2	0.0	0.0	0.15442 (5)	0.0112 (3)*
Ca1	0.0	0.0	0.5771 (2)	0.0157 (3)*	0.75
O1	0.0	0.5	0.0	0.0132 (3)*
O2	0.0	0.0	0.2187 (3)	0.0132 (3)*
O3	0.0	0.5	0.1412 (2)	0.0132 (3)*
O4	0.0	0.0	0.0772 (4)	0.0132 (3)*
Li1	0.25	0.25	0.25	0.0182 (3)*	0.5

Geometric parameters (Å, º)

Nb1—O1^i	1.9394 (1)	Ca1—O1^ii	2.805 (4)
Nb1—O4	2.027 (11)	Ca1—O3^ii	2.567 (4)
Nb2—O2	1.689 (8)	Ca1—O4^iii	2.7427 (1)
Nb2—O3^i	1.9704 (11)	Li1—O2	1.599 (4)
Nb2—O4	2.029 (11)
O1^i—Nb1—O1	180.0	O1^ii—Ca1—O3^vi	118.25 (6)
O1^i—Nb1—O1^iv	90.0	O1^ii—Ca1—O4^ii	60.7 (2)
O1^i—Nb1—O4	90.0	O1^ii—Ca1—O4^v	119.3 (3)
O2—Nb2—O3^i	100.17 (18)	O1^v—Ca1—O3^v	87.19 (9)
O2—Nb2—O4	180.0	O3^ii—Ca1—O3^v	98.1 (2)
O3^i—Nb2—O3	159.7 (4)	O3^ii—Ca1—O3^vi	64.58 (12)
O3^i—Nb2—O3^iv	88.21 (6)	O3^ii—Ca1—O4^vii	57.70 (19)
O3^i—Nb2—O4	79.83 (18)	O3^ii—Ca1—O4^v	122.3 (3)
O1^ii—Ca1—O1^v	87.49 (16)	O4^ii—Ca1—O4^vii	90.0000 (2)
O1^ii—Ca1—O1^vi	58.54 (9)	O4^ii—Ca1—O4^iii	180.0000 (3)
O1^ii—Ca1—O3^v	174.68 (17)	O2—Li1—O2^viii	180.0

Symmetry codes: (i) x, y−1, z; (ii) x−1/2, y−1/2, z+1/2; (iii) x+1/2, y+1/2, z+1/2; (iv) −y, x, z; (v) x+1/2, y−1/2, z+1/2; (vi) −y+1/2, x−1/2, z+1/2; (vii) x−1/2, y+1/2, z+1/2; (viii) −x−1/2, −y−1/2, −z−1/2.