Disordered misfit [Ca₂CoO₃][CoO₂]_1.62 structure revisited via a new intrinsic modulation

The structure of the thermoelectric lamellar misfit cobalt oxide [Ca₂CoO₃][CoO₂]_1.62 was refined again using single-crystal X-ray diffraction data. A new commensurate intrinsic modulation was observed involving a modulation vector orthogonal to the misfit direction (⅔,0,−⅓). The five-dimensional superspace group is C2/m(1δ0)(α0γ)gm and the structure was solved using a commensurate approximation. A new model is given involving an occupation modulation of the split sites of the [CoO] layer. This [CoO] layer can be described by triple chains running along b. The residual disorder along b can then be explained by the assumption of a local ordering with two types of clusters: CoO₂ and Co₅O₄. A powder neutron diffraction experiment confirmed the ordering evidenced by the single-crystal X-ray diffraction study, but was not sufficient by itself to deal with this double modulated scheme. The new intrinsic modulation is destroyed by partial metal substitutions in the [CoO] layer. The structural modifications of this layer directly influence the physical properties which are related to the electronic structure of the [CoO₂] layers.

S1. Experimental

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S2. Refinement

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Computing details

Program(s) used to refine structure: Jana2000 (Petricek, Dusek & Palatinus, 2000); software used to prepare material for publication: Jana2000 (Petricek, Dusek & Palatinus, 2000).

Figures

(I)

Crystal data

Ca2Co2.496O6.117	Z = 2
Mr = 325.1	F(000) = 313
Monoclinic, X2/m†	Dx = 4.557 Mg m^−3
q1 = 0.66666a* + -0.33333c*; q2 = 1.62500b*‡	Mo Kα radiation, λ = 0.71069 Å
a = 4.8395 Å	µ = 10.80 mm^−1
b = 4.5531 Å	T = 300 K
c = 10.8583 Å	Plate
β = 98.124°	0.20 × 0.15 × 0.03 mm
V = 236.86 Å^3

† Symmetry operations: (1) x₁, x₂, x₃, x₄, x₅; (2) −x₁, x₂, −x₃, −x₄, 1/2+x₅; (3) −x₁, −x₂, −x₃, −x₄, −x₅; (4) x₁, −x₂, x₃, x₄, 1/2−x₅; (5) 1/2+x₁, 1/2+x₂, x₃, x₄, 1/2+x₅; (6) 1/2−x₁, 1/2+x₂, −x₃, −x₄, x₅; (7) 1/2−x₁, 1/2−x₂, −x₃, −x₄, 1/2−x₅; (8) 1/2+x₁, 1/2−x₂, x₃, x₄, −x₅.

‡ ;

Data collection

Bruker Kappa CCD diffractometer	θmax = 45.0°, θmin = 5.8°
4288 measured reflections	h = −9→9
2492 independent reflections	k = 0→9
1673 reflections with I > 3σ(I)	l = 0→21
Rint = 0.005

Refinement

Refinement on F	78 parameters
R[F^2 > 2σ(F^2)] = 0.033	Weighting scheme based on measured s.u.'s w = 1/σ^2(F)
wR(F^2) = 0.032	(Δ/σ)max = 0.016
S = 1.66	Δρmax = 0.67 e Å^−3
2492 reflections	Δρmin = −0.71 e Å^−3

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Ca	0.43009 (6)	0	0.27488 (3)	0.00852 (6)
O1	0.0527 (2)	0	0.66413 (10)	0.0109 (2)
Co1a	0	0	0.5	0.0039 (4)*	0.279 (5)
Co1b	0.0766 (7)	0.0666 (8)	0.5	0.0044 (2)*	0.1685 (14)
O2a	0.1072 (19)	0.5	0.4996 (4)	0.0099 (14)*	0.268 (10)
O2b	0	0.421 (2)	0.5	0.0080 (15)*	0.205 (8)
Co2	0.75	0.75	0	0.00315 (4)	0.9498
O3	0.38597 (15)	0.75	−0.09328 (7)	0.00587 (14)	0.9757

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca	0.00730 (10)	0.00866 (12)	0.00953 (10)	0	0.00093 (8)	0
O1	0.0109 (4)	0.0109 (5)	0.0109 (4)	0	0.0011 (3)	0
Co2	0.00219 (6)	0.00285 (7)	0.00444 (7)	0	0.00057 (5)	0
O3	0.0047 (2)	0.0054 (3)	0.0076 (3)	0	0.00127 (19)	0

Geometric parameters (Å, º)

	Average	Minimum	Maximum
Co2—O3	1.9040 (12)	1.9028 (17)	1.9053 (18)
Co2—O3^i	1.9003 (9)	1.8868 (6)	1.9142 (6)
Co2—O3^ii	1.9003 (9)	1.8868 (6)	1.9142 (6)
Co2—O31^iii	1.9003 (6)	1.8866 (6)	1.9143 (6)
Co2—O31	1.9003 (6)	1.8866 (6)	1.9143 (6)
Co2—O31^iv	1.9040 (7)	1.9028 (8)	1.9053 (8)
O3—Co2—O3^i	85.37 (4)	84.11 (3)	86.65 (3)
O3—Co2—O3^ii	85.37 (4)	84.11 (3)	86.65 (3)
O3—Co2—O31^iii	94.62 (4)	93.02 (3)	96.26 (3)
O3—Co2—O31	94.62 (4)	93.02 (3)	96.26 (3)
O3—Co2—O31^iv	179.43 (5)	179.11 (7)	179.89 (3)
O3^i—Co2—O3	85.37 (4)	84.11 (3)	86.65 (4)
O3^i—Co2—O3^ii	95.02 (4)	94.17 (3)	95.86 (3)
O3^i—Co2—O31^iii	84.97 (3)	84.95 (3)	84.99 (4)
O3^i—Co2—O31	178.73 (4)	178.01 (4)	179.76 (5)
O3^i—Co2—O31^iv	94.62 (4)	93.00 (3)	96.23 (3)
O3^ii—Co2—O3	85.37 (4)	84.11 (3)	86.65 (4)
O3^ii—Co2—O3^i	95.02 (4)	94.17 (3)	95.86 (3)
O3^ii—Co2—O31^iii	178.73 (4)	178.01 (4)	179.76 (5)
O3^ii—Co2—O31	84.97 (3)	84.95 (3)	84.99 (4)
O3^ii—Co2—O31^iv	94.62 (4)	93.00 (3)	96.23 (3)
O31^iii—Co2—O31	95.02 (3)	94.14 (3)	95.84 (3)
O31^iii—Co2—O31^iv	85.37 (3)	84.10 (3)	86.63 (3)
O31—Co2—O31^iii	95.02 (3)	94.14 (3)	95.84 (3)
O31—Co2—O31^iv	85.37 (3)	84.10 (3)	86.63 (3)
O31^iv—Co2—O31^iii	85.37 (3)	84.10 (3)	86.63 (3)
O31^iv—Co2—O31	85.37 (3)	84.10 (3)	86.63 (3)

Symmetry codes: (i) −x₁+1, −x₂+1, −x₃, −x₄, −x₅; (ii) −x₁+1, −x₂+2, −x₃, −x₄, −x₅; (iii) x₁, x₂−1, x₃, x₄, x₅; (iv) −x₁+2, −x₂+2, −x₃, −x₄, −x₅.