Structure of russellite (Bi2WO6): origin of ferro­electricity and the effect of the stereoactive lone electron pair on the structure

Okudera, H.; Sakai, Y.; Yamagata, K.; Takeda, H.

doi:10.1107/S2052520618006133

Structure of russellite (Bi₂WO₆): origin of ferroelectricity and the effect of the stereoactive lone electron pair on the structure

H. Okudera, Y. Sakai, K. Yamagata and H. Takeda

The structure of the low-temperature polar (orthorhombic) phase of russellite (Bi₂WO₆) was examined on artificial specimens with precise single-crystal X-ray diffraction experiments. The final atomic arrangement thus obtained was identical to that reported by Knight [Miner. Mag. (1992), 56, 399–409] with powder neutron diffraction. The residual density attributable to a stereochemically-active lone pair of electrons of bismuth was prominent at approximately the centre of a larger cap of BiO₈ square antiprisms, that is on the line from the Bi sites to an adjacent WO₄²⁻ slab along the b-axis direction. Quite uneven Bi—O distances and the formation of a vacant coordination hemisphere (within 3 Å) should, therefore, be ascribed to the strong demand of bismuth to form shorter Bi—O bonds to use up its electrostatic charge within its coordination environment. The shift of bismuth along −c propagates via the correlated shift of the W site and these cooperative shifts cause ferroelectricity in the compound. This propagation was easily effected by the intrusion of molecules such as acetone into the structure.

Keywords: russellite; Bi₂WO₆; ferroelectrics; lone electron pair; single-crystal X-ray diffraction.

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Supporting information

	Crystallographic Information File (CIF) https://doi.org/10.1107/S2052520618006133/lo5019sup1.cif Contains datablock I
	Structure factor file (CIF format) https://doi.org/10.1107/S2052520618006133/lo5019Isup2.hkl Contains datablock I

CCDC reference: 1838887

Computing details top

Data collection: AFC-5S control; cell refinement: Rigaku Control software, corrected with Si; data reduction: AFCPHI, SORT6 (parts of LSGCEX); program(s) used to refine structure: LSGCEX (Kihara, 1990); molecular graphics: VESTA (Momma & Izumi, 2008); software used to prepare material for publication: none.

(I) top

Crystal data top

Bi₂WO₆	D_x = 9.513 Mg m⁻³
M_r = 697.80	Mo Kα radiation, λ = 0.71069 Å
Orthorhombic, Pca2₁	Cell parameters from 16 reflections
a = 5.4345 (12) Å	θ = 40–53°
b = 16.4324 (12) Å	µ = 95.56 mm⁻¹
c = 5.4558 (12) Å	T = 296 K
V = 487.21 (16) Å³	Sphere, pale yellow
Z = 4	0.10 × 0.10 × 0.10 × 0.05 (radius) mm
F(000) = 1152

Data collection top

Rigaku AFC5S diffractometer	844 reflections with F > 3u(F) & (\|Fobs.max\|−\|Fobs.min\|)/\|Fobs.min\| < 0.5 among equivalents
Radiation source: X-ray tube, 50kV 25mA	R_int = 0.038
Graphite (002) monochromator	θ_max = 35.0°, θ_min = 4.0°
integrated with \v/2θ scans	h = −8→8
Absorption correction: for a sphere spherical absorption correction	k = −26→26
T_min = 0.004, T_max = 0.005	l = −8→8
8160 measured reflections	3 standard reflections every 200 reflections
2163 independent reflections	intensity decay: none

Refinement top

Refinement on F	9 restraints
Least-squares matrix: full	5 constraints
R[F² > 2σ(F²)] = 0.017	Weighting scheme based on measured s.u.'s weight proportional to sigma^-2
wR(F²) = 0.020	Δρ_max = 3.18 e Å⁻³
S = 0.52	Δρ_min = −2.76 e Å⁻³
784 reflections	Extinction correction: B-C type 1 Lorenzian isotropic
42 parameters	Extinction coefficient: 0.022173 (1262)

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	B_iso*/B_eq
Bi1	0.5261 (6)	0.4197 (9)	0.9730 (3)	0.39 (7)*
Bi2	0.4859 (6)	0.0747 (10)	0.9850 (3)	0.56 (9)*
W	−0.0016 (9)	0.2488 (3)	0.0	0.282 (12)*
O1	0.045 (4)	0.1388 (13)	0.080 (3)	0.59 (7)*
O2	0.262 (6)	0.999 (2)	0.267 (7)	0.59*
O3	0.241 (7)	0.5032 (18)	0.258 (6)	0.59*
O4	0.705 (2)	0.2349 (7)	0.256 (2)	0.59*
O5	0.212 (3)	0.2650 (7)	0.333 (2)	0.59*
O6	0.571 (4)	0.3565 (13)	0.563 (3)	0.59*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Bi1	0.0053 (5)	0.0035 (13)	0.0062 (7)	−0.0001 (8)	−0.0002 (5)	0.0011 (10)
Bi2	0.0071 (7)	0.0058 (19)	0.0085 (8)	0.0014 (5)	−0.0026 (7)	0.0000 (8)
W	0.0036 (2)	0.0036	0.0036	0.00000	0.00000	0.00000
O1	0.0075 (9)	0.0075	0.0075	0.00000	0.00000	0.00000
O2	0.0075	0.0075	0.0075	0.00000	0.00000	0.00000
O3	0.0075	0.0075	0.0075	0.00000	0.00000	0.00000
O4	0.0075	0.0075	0.0075	0.00000	0.00000	0.00000
O5	0.0075	0.0075	0.0075	0.00000	0.00000	0.00000
O6	0.0075	0.0075	0.0075	0.00000	0.00000	0.00000

Geometric parameters (Å, º) top

Bi1—O3ⁱ	2.59 (4)	Bi2—O1^iv	2.45 (2)
Bi1—O3ⁱⁱ	2.14 (4)	Bi2—O2^viii	2.32 (4)
Bi1—O3ⁱⁱⁱ	2.32 (4)	Bi2—O2ⁱⁱ	2.19 (4)
Bi1—O3^iv	2.32 (4)	Bi2—O2ⁱⁱⁱ	2.47 (4)
Bi1—O6ⁱ	3.39 (2)	Bi2—O2^ix	2.18 (4)
Bi1—O6	2.48 (2)	W—O1	1.88 (2)
Bi1—O6^v	2.47 (2)	W—O4^x	2.134 (13)
Bi1—O6^iv	3.44 (2)	W—O4^xi	1.743 (13)
Bi2—O1ⁱ	2.67 (2)	W—O5	2.173 (14)
Bi2—O1^vi	3.26 (2)	W—O5^xi	1.838 (15)
Bi2—O1^vii	3.417 (19)	W—O6^xi	1.84 (2)

O3ⁱ—Bi1—O3ⁱⁱ	111.7 (13)	O1^vii—Bi2—O2^viii	60.6 (10)
O3ⁱ—Bi1—O3ⁱⁱⁱ	67.1 (13)	O1^vii—Bi2—O2ⁱⁱⁱ	65.8 (9)
O3ⁱ—Bi1—O3^iv	67.4 (13)	O1^iv—Bi2—O2ⁱⁱ	78.0 (10)
O3ⁱ—Bi1—O6ⁱ	68.6 (8)	O1^iv—Bi2—O2^ix	73.1 (10)
O3ⁱ—Bi1—O6^iv	60.7 (9)	O2^viii—Bi2—O2ⁱⁱ	113.2 (14)
O3ⁱⁱ—Bi1—O3ⁱⁱⁱ	75.4 (13)	O2^viii—Bi2—O2ⁱⁱⁱ	69.1 (12)
O3ⁱⁱ—Bi1—O3^iv	75.0 (14)	O2^viii—Bi2—O2^ix	74.7 (13)
O3ⁱⁱ—Bi1—O6	72.2 (10)	O2ⁱⁱ—Bi2—O2ⁱⁱⁱ	71.6 (13)
O3ⁱⁱ—Bi1—O6^v	80.5 (11)	O2ⁱⁱ—Bi2—O2^ix	76.9 (14)
O3ⁱⁱⁱ—Bi1—O3^iv	109.4 (13)	O2ⁱⁱⁱ—Bi2—O2^ix	115.8 (14)
O3ⁱⁱⁱ—Bi1—O6ⁱ	59.7 (9)	O1^xii—W^xii—O4	81.1 (7)
O3ⁱⁱⁱ—Bi1—O6^v	69.5 (10)	O1^xii—W^xii—O4^xiii	97.9 (7)
O3^iv—Bi1—O6	81.5 (9)	O1^xii—W^xii—O5^xii	81.5 (7)
O3^iv—Bi1—O6^iv	70.2 (10)	O1^xii—W^xii—O5^xiii	98.0 (7)
O6ⁱ—Bi1—O6^v	67.6 (6)	O1^xii—W^xii—O6^xiii	155.4 (17)
O6ⁱ—Bi1—O6^iv	80.7 (5)	O4—W^xii—O4^xiii	90.7 (6)
O6—Bi1—O6^v	85.2 (7)	O4—W^xii—O5^xii	82.2 (5)
O6—Bi1—O6^iv	95.4 (6)	O4—W^xii—O5^xiii	169 (2)
O1ⁱ—Bi2—O1^vii	69.5 (6)	O4—W^xii—O6^xiii	80.1 (7)
O1ⁱ—Bi2—O1^iv	86.8 (7)	O4^xiii—W^xii—O5^xii	173 (3)
O1ⁱ—Bi2—O2^viii	67.0 (10)	O4^xiii—W^xii—O5^xiii	100.6 (6)
O1ⁱ—Bi2—O2^ix	77.1 (10)	O4^xiii—W^xii—O6^xiii	98.0 (7)
O1^vi—Bi2—O1^vii	78.1 (5)	O5^xii—W^xii—O5^xiii	86.5 (6)
O1^vi—Bi2—O1^iv	93.9 (6)	O5^xii—W^xii—O6^xiii	80.4 (7)
O1^vi—Bi2—O2ⁱⁱ	71.5 (10)	O5^xiii—W^xii—O6^xiii	97.4 (7)
O1^vi—Bi2—O2ⁱⁱⁱ	59.6 (10)

Symmetry codes: (i) x, y, z+1; (ii) −x+1, −y+1, z+1/2; (iii) x+1/2, −y+1, z+1; (iv) −x+1/2, y, z+1/2; (v) −x+3/2, y, z+1/2; (vi) x+1, y, z+1; (vii) −x+1/2, y, z+3/2; (viii) x, y−1, z+1; (ix) −x+1/2, y−1, z+1/2; (x) x−1, y, z; (xi) −x+1/2, y, z−1/2; (xii) x+1, y, z; (xiii) −x+3/2, y, z−1/2.

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Structure of russellite (Bi2WO6): origin of ferro­electricity and the effect of the stereoactive lone electron pair on the structure

Supporting information

Structure of russellite (Bi₂WO₆): origin of ferroelectricity and the effect of the stereoactive lone electron pair on the structure