Oxygen-deficient strontium cobaltate, SrCoO_2.64

Single crystals of strontium cobaltate, SrCoO_3-x, have been grown by the floating-zone method in an oxy­gen flow. The compound crystallizes with the cubic perovskite structure, with Pmm symmetry, as determined by X-ray diffraction. Refinement of the O-atom site occupancy yields the chemical composition SrCoO_2.64 [x = 0.36 (3)]. The anisotropic displacement ellipsoids of the O atoms suggest that their positional disorder occurs in a direction perpendicular to that of the Co-O bonds.

S1. Comment

Strontium cobaltate, SrCoO_{3 − x}, exists over a wide range of oxygen non-stoichiometry and several phases with different oxygen contents have been reported (Watanabe & Takeda, 1970). The cubic perovskite-type structure, with Pm3 m symmetry, is stable over the composition range x = 0.0–0.4 (Takeda et al., 1986). SrCoO₃ contains low-spin Co⁴⁺ ions and is a ferromagnetic oxide, with a Curie temperature of 222 K (Taguchi et al., 1979), and a metallic conductor (Taguchi et al., 1980; Bezdicka et al., 1993). Co³⁺ and Co⁴⁺ ions coexist in the oxygen-deficient SrCoO_{3 − x} phase. The magnetic and electric properties of that phase are sensitive to the oxygen deficiency (x), and the Curie temperature and the electric resistance both decrease with increasing x (Taguchi et al., 1979, 1980). Hence, investigating the structural variation of SrCoO_{3 − x} as a function of x is important for the understanding of its physical properties. To date, only lattice parameters determined using powder samples have been reported and a full structure refinement, including displacement parameters, has not been published. The determination of these displacement parameters is important for understanding the behavior of oxygen vacancies in the structure.

The present structure refinement of SrCoO_{3 − x} converged to an occupancy of 0.88 (1) for the O-atom site, yielding a chemical composition of Sr(Co⁴⁺_0.28Co³⁺_0.72)O_2.64 [x = 0.36 (3)]. This result agrees well with the composition of Sr(Co⁴⁺_0.34Co³⁺_0.66)O_2.67 obtained by chemical analysis, and also with the composition estimated from the correlation between lattice parameter and oxygen deficiency proposed by Takeda et al. (1986). The presence of oxygen vacancies is reflected in the larger displacement parameter of the O atom [U_eq = 0.025 (2) Ų]. The same observation has also been reported in other non-stoichiometric perovskites (e.g. Harrison et al., 1995). The strong anisotropy of the displacement ellipsoid in a direction perpendicular to the Co–O bonds (Fig. 1) suggests that positional disorder of the O atoms occurs in the same direction.

S2. Experimental

A single-crystal of the title compound was grown by the floating-zone method. The starting materials were powders of SrCO₃ and CoO (99.9%). Stoichiometric amounts of the starting materials were mixed well, and the mixture was placed in a platinum crucible and heated at 1300 K for 10 h under a flow of oxygen. The reaction product was reground and pressed hydrostatically into a rod of 8 mm in diameter and 40 mm in length, which was then sintered at 1500 K for 1 h in an oxygen atmosphere. For the crystal growth, the sintered rod was put into an image-type floating-zone furnace, with a halogen lamp as a heat source. The crystal growth was carried out under oxygen at the flow rate of 1 l min⁻¹. Upper and lower shafts were counter-rotated at the rate of 25 r.p.m. and the growth rate was 2 mm h⁻¹. The number of Co⁴⁺ ions was analyzed by iodometric oxidation–reduction titration. The analytical results gave a chemical composition of 54.8 wt% SrO, 16.1 wt% CoO₂ and 29.2 wt% Co₂O₃, with a total of 100.1 wt%, corresponding to a composition of Sr(Co⁴⁺_0.34Co³⁺_0.66)O_2.67.

S3. Refinement

The maximum and minimum residual electron densities are located at the coordinates (0, 0, 0) and (0.46, 0.46, 0.46), respectively.

Computing details

Data collection: WinAFC (Rigaku Corporation, 1999); cell refinement: WinAFC (Rigaku Corporation, 1999); data reduction: RADY (Sasaki, 1987); program(s) used to refine structure: RADY (Sasaki, 1987); molecular graphics: ATOMS for Windows (Dowty, 2000).

Figures

Fig. 1. Displacement ellipsoids in SrCoO2.64 perovskite. The ellipsoids are drawn at the 50% probability level.

strontium cobalt(III, IV) oxide

Crystal data

SrCoO2.64	Dx = 5.483 Mg m^−3
Mr = 188.79	Mo Kα radiation, λ = 0.71069 Å
Cubic, Pm3m	Cell parameters from 25 reflections
Hall symbol: -P 4 2 3	θ = 20.0–22.5°
a = 3.8530 (4) Å	µ = 30.22 mm^−1
V = 57.20 (2) Å^3	T = 296 K
Z = 1	Sphere, black
F(000) = 86.12	0.08 mm (radius)

Data collection

Rigaku AFC-7R diffractometer	Rint = 0.011
ω–2θ scans	θmax = 60.0°
Absorption correction: for a sphere (Sasaki, 1987)	h = 0→9
Tmin = 0.008, Tmax = 0.011	k = 0→9
539 measured reflections	l = 0→9
120 independent reflections	3 standard reflections every 100 reflections
96 reflections with F > 3σ(F)	intensity decay: none

Refinement

Refinement on F	σ w = 1/σ^2(F)
R[F^2 > 2σ(F^2)] = 0.021	(Δ/σ)max = 0.0001
wR(F^2) = 0.011	Δρmax = 2.80 e Å^−3
S = 1.88	Δρmin = −2.64 e Å^−3
96 reflections	Extinction correction: isotropic Type I (Becker & Coppens, 1974a and 1974b)
7 parameters	Extinction coefficient: 0.054 (2) 10^-4

Special details

Experimental. The composition of the grown crystal was examined by means of a Jeol JCMA-733II electron microprobe analyzer and edta titration at a pH of 3.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sr	0	0	0	0.01400 (9)
Co	0.5	0.5	0.5	0.01009 (8)
O	0.5	0.5	0	0.025 (2)	0.88 (1)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr	0.0140 (2)	0.0140 (2)	0.0140 (2)	0.0	0.0	0.0
Co	0.0101 (2)	0.0101 (2)	0.0101 (2)	0.0	0.0	0.0
O	0.049 (4)	0.0126 (11)	0.0126 (11)	0.0	0.0	0.0

Geometric parameters (Å, º)

Sr—O	2.7245 (2)	O—O^i	2.7245 (2)
Co—O	1.9265 (2)
O—Sr—O^ii	180.00	O—Sr—O^ix	120.00
O—Sr—O^iii	90.00	O—Sr—O^x	60.00
O—Sr—O^iv	90.00	O—Sr—O^xi	60.00
O—Sr—O^v	120.00	O—Co—O^i	90.00
O—Sr—O^vi	120.00	O—Co—O^xi	90.00
O—Sr—O^vii	60.00	O—Co—O^xii	180.00
O—Sr—O^i	60.00	O—Co—O^xiii	90.00
O—Sr—O^viii	120.00	O—Co—O^xiv	90.00

Symmetry codes: (i) z, x, y; (ii) −x, −y, z; (iii) −x, y, −z; (iv) x, −y, −z; (v) z, −x, −y; (vi) −z, −x, y; (vii) −z, x, −y; (viii) −y, z, −x; (ix) −y, −z, x; (x) y, −z, −x; (xi) y, z, x; (xii) x, y, z+1; (xiii) z+1, x, y; (xiv) y, z+1, x.