New insight on bismuth cuprates with incommensurate modulated structures

The incommensurate modulated crystal structure of Bi_2.27Sr_1.73CuO_6 + δ (2201) phase [a = 5.3874 (5), b = 5.3869 (4), c = 24.579 (3) Å; β = 90.01 (1)°, q = 0.2105 (3)a^* + 0.538 (4)c^*, Z = 4, the (3 + 1)-dimensional monoclinic A2/a(α0γ) group] has been refined with R = 0.041, wR = 0.052 from X-ray single-crystal data including up to third-order satellite reflections. The same structure has also been considered as incommensurate composite with a₂ = 2.437, b₂ = 5.387, c₂ = 24.614, β₂ = 93.06, q₂ = 0.4524a₂^*−0.243c₂^* and the (3 + 1)-dimensional A2/m(α0γ)0s group for the second component. Both approaches give quite similar results. The structure possesses oxygen disorder in the oxygen-rich region of the BiO layer. An extra O atom is determined in the bridging position shifted ∼ 0.6 Å from BiO towards the SrO layer. Its presence is the cause of the tremendous increase of the bismuth U¹¹ atomic displacement parameter in ∼ 20% of the unit cells (t = −0.05–0.15). Vacancies are determined in the oxygen site of the SrO layer, which may result in the oxygen content variation with annealing at different oxygen pressures. The total refined oxygen content 6.18 (1) corresponds to the results of chemical analysis.

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).

(I)

Crystal data

Bi2.27CuO6.18Sr1.73	Z = 4
Mr = 788.5	F(000) = 1331
Monoclinic, A2/a(α0γ)00†	Dx = 7.340 Mg m^−3
q = 0.21050a* + 0.53800c*	Mo Kα radiation, λ = 0.71069 Å
a = 5.3869 (5) Å	µ = 71.58 mm^−1
b = 5.3874 (4) Å	T = 293 K
c = 24.579 (3) Å	, black
β = 90.01 (1)°	0.46 × 0.1 × 0.01 mm
V = 713.32 (12) Å^3

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

Data collection

CAD4 diffractometer	7115 independent reflections
Radiation source: sealed tube	4194 reflections with I > 3σ(I)
Graphite monochromator

Refinement

Refinement on F	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.000025F^2)
R[F^2 > 2σ(F^2)] = 0.041	(Δ/σ)max = 0.010
wR(F^2) = 0.052	Δρmax = 7.01 e Å^−3
S = 3.78	Δρmin = −7.82 e Å^−3
4194 reflections	Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
241 parameters	Extinction coefficient: 0

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Bi1	−0.00100 (5)	0.27368 (5)	0.065418 (11)	0.01539 (8)
Sr1	0.49973 (10)	0.24779 (9)	0.17907 (3)	0.01935 (19)	0.865 (3)
Bi2	0.499728	0.247792	0.179067	0.01935 (19)	0.135 (3)
Cu	0	0.25	0.25	0.0078 (2)
O1	0.2505 (8)	0.5014 (8)	0.2488 (2)	0.0138 (11)
O2	0.0005 (15)	0.2231 (11)	0.1465 (2)	0.0176 (14)
O3	0.510 (2)	0.3424 (11)	0.0694 (3)	0.0261 (18)
O4	0.236 (4)	0.519 (7)	0.0873 (13)	0.060 (13)	0.123 (12)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Bi1	0.02252 (17)	0.01233 (11)	0.01130 (11)	−0.00011 (12)	−0.00143 (9)	−0.0005 (1)
Sr1	0.0115 (3)	0.0089 (3)	0.0377 (4)	−0.0003 (2)	−0.0010 (2)	−0.0029 (2)
Bi2	0.0115 (3)	0.0089 (3)	0.0377 (4)	−0.0003 (2)	−0.0010 (2)	−0.0029 (2)
Cu	0.0046 (3)	0.0033 (4)	0.0157 (5)	0.0001 (3)	−0.0006 (3)	0.0005 (4)
O1	0.0076 (16)	0.0051 (17)	0.029 (3)	−0.0042 (13)	0.0001 (17)	−0.0004 (18)
O2	0.022 (2)	0.021 (3)	0.010 (2)	−0.001 (2)	−0.003 (2)	0.003 (2)
O3	0.040 (4)	0.011 (2)	0.027 (3)	0.004 (2)	−0.005 (3)	0.000 (2)
O4	0.020 (13)	0.11 (3)	0.05 (2)	0.005 (13)	−0.011 (12)	−0.03 (2)

Bond lengths (Å)

	Average	Minimum	Maximum
Bi1—O2	2.03 (3)	1.98 (3)	2.12 (4)
Bi1—O3^i	2.67 (4)	1.75 (6)	4.25 (6)
Bi1—O3	2.80 (4)	1.84 (6)	4.34 (6)
Bi1—O3^ii	3.40 (2)	2.99 (3)	3.75 (2)
Bi1—O3^iii	2.15 (2)	2.05 (2)	2.34 (4)
Bi1—O4	2.13 (3)	2.05 (3)	2.21 (3)
Bi1—O4^iii	2.01 (3)	1.89 (3)	2.13 (3)
Sr1—O1	2.577 (19)	2.456 (18)	2.645 (19)
Sr1—O1^iv	2.607 (19)	2.537 (19)	2.688 (19)
Sr1—O1^v	2.599 (19)	2.480 (18)	2.639 (18)
Sr1—O1^vi	2.575 (19)	2.483 (19)	2.648 (17)
Sr1—O2	2.83 (6)	2.31 (8)	3.86 (8)
Sr1—O2^vii	2.82 (6)	2.41 (8)	3.83 (8)
Sr1—O2^viii	2.68 (4)	2.57 (4)	2.77 (3)
Sr1—O2^vi	2.97 (4)	2.83 (4)	3.26 (4)
Sr1—O3	2.82 (3)	2.74 (2)	3.09 (3)
Sr1—O4	2.84 (3)	2.78 (3)	2.90 (3)
Sr1—O4^vi	2.77 (3)	2.66 (3)	2.88 (3)
Cu—O1	1.916 (14)	1.906 (13)	1.932 (15)
Cu—O1^ix	1.916 (14)	1.906 (14)	1.931 (15)
Cu—O1^v	1.901 (14)	1.867 (15)	1.923 (13)
Cu—O1^iii	1.901 (14)	1.867 (15)	1.923 (14)
Cu—O2	2.58 (3)	2.45 (3)	2.69 (3)
Cu—O2^ix	2.58 (3)	2.45 (3)	2.69 (3)
O1—O1^v	2.69 (2)	2.664 (19)	2.73 (2)
O1—O1^x	2.69 (2)	2.66 (2)	2.725 (19)
O1—O1^iii	2.70 (2)	2.65 (2)	2.74 (2)
O1—O1^vi	2.70 (2)	2.65 (2)	2.74 (2)
O1—O2^ix	3.18 (4)	2.81 (4)	3.44 (6)
O1—O2^x	3.17 (4)	2.81 (4)	3.51 (6)
O2—O3^i	3.33 (6)	2.85 (7)	5.05 (9)
O2—O3	3.43 (6)	2.94 (7)	5.11 (9)
O2—O3^iii	3.04 (4)	2.97 (4)	3.14 (4)
O2—O4	2.85 (7)	2.73 (6)	2.98 (7)
O2—O4^iii	2.67 (7)	2.02 (5)	2.94 (7)
O3—O3^iii	3.24 (5)	2.90 (7)	5.33 (8)
O3—O3^vi	3.24 (5)	2.91 (6)	5.34 (8)
O3—O4	2.63 (5)	1.19 (4)	2.94 (6)
O3—O4^vi	2.66 (6)	2.52 (5)	2.80 (6)
O4—O4^iii	2.70 (3)	2.70 (3)	2.70 (3)
O4—O4^vi	2.70 (3)	2.70 (3)	2.70 (3)

Symmetry codes: (i) x₁−1, x₂, x₃, x₄; (ii) −x₁+1/2, x₂, −x₃, −x₄; (iii) x₁−1/2, −x₂+1, x₃, x₄; (iv) −x₁+1, −x₂+1/2, −x₃+1/2, −x₄; (v) −x₁+1/2, x₂−1/2, −x₃+1/2, −x₄; (vi) x₁+1/2, −x₂+1, x₃, x₄; (vii) x₁+1, x₂, x₃, x₄; (viii) x₁+1/2, −x₂, x₃, x₄; (ix) −x₁, −x₂+1/2, −x₃+1/2, −x₄; (x) −x₁+1/2, x₂+1/2, −x₃+1/2, −x₄.