Neutron diffraction study of φ-Bi₈Pb₅O₁₇: structure refinement and analysis of cationic ordering

The triclinic crystal structure of the φ phase in the structure of the Bi₂O₃–PbO phase diagram has been recently solved by the synergetic use of electron and X-ray diffraction on a polycrystalline Bi₈Pb₅O₁₇ sample. In the present work the problem is re-examined on the basis of powder neutron diffraction data: the structure has been confirmed and refined by the Rietveld method. The increased accuracy of the O-atom positions allowed the study of the cationic ordering in this structure by means of bond-valence calculations and charge distribution analysis. The results, confirmed by the refinement of the site occupancies, indicate that the structure is ordered to a large extent, with Bi and Pb atoms occupying preferentially different lattice sites. In this frame, the φ phase being the most ordered one in the considered region of the Bi₂O₃–PbO phase diagram, it should represent the thermodynamically stable low-temperature polymorph.

Computing details

Program(s) used to refine structure: GSAS; molecular graphics: Cerius2.

Figures

bismuth lead oxide

Crystal data

O17Pb5Bi8	γ = 80.6428 (13)°
Mr = 2979.83	V = 659.55 (1) Å^3
Triclinic, P1	Z = 2
a = 7.45067 (12) Å	Neutron radiation, λ = 1.594 Å
b = 14.1768 (2) Å	T = 295 K
c = 7.21007 (12) Å	yellow
α = 97.1900 (13)°	cylinder, ? × ? × ? mm
β = 118.4728 (11)°	Specimen preparation: Prepared at 833 K, cooled at 8 K min^−1

Data collection

D2B high-res. diffractometer	Data collection mode: transmission
Radiation source: nuclear reactor, thermal beam	Scan method: step
Ge[335] composite crystals monochromator	2θmin = 10.00°, 2θmax = 150.00°, 2θstep = 0.05°

Refinement

Least-squares matrix: full matrix	Excluded region(s): 0.00-10.00,150.00-155.00
Rp = 0.038	Profile function: pseudo-voigt
Rwp = 0.049	82 parameters
Rexp = ?	2 constraints
R(F^2) = 0.0480
χ^2 = 1.904	Background function: shifted chebychev
2799 data points

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Bi1	0.2277 (9)	0.0276 (4)	0.2937 (9)	0.0081 (13)
Bi2	0.0586 (9)	0.1900 (4)	0.7163 (8)	0.0117 (13)	0.54 (06)
Pb2	0.0586 (9)	0.1900 (4)	0.7163 (8)	0.0117 (13)	0.46 (06)
Pb3	0.2143 (8)	0.5560 (4)	0.2380 (8)	0.0076 (11)
Bi4	0.4825 (9)	0.3249 (4)	0.2355 (10)	0.0101 (13)
Bi5	0.6857 (8)	0.0719 (4)	0.1979 (8)	0.0068 (12)
Bi6	0.7577 (9)	0.5449 (4)	0.2723 (9)	0.0108 (12)
Pb7	0.9851 (8)	0.2928 (4)	0.2585 (8)	0.0142 (12)
Bi8	0.5309 (9)	0.2039 (4)	0.7298 (9)	0.0121 (15)	0.37 (10)
Pb8	0.5309 (9)	0.2039 (4)	0.7298 (9)	0.0121 (15)	0.63 (10)
O1	0.9645 (12)	0.9773 (6)	0.2761 (14)	0.0123 (17)
O2	0.2618 (13)	0.1207 (6)	0.5961 (14)	0.0130 (18)
O3	0.4035 (13)	0.2125 (6)	0.9856 (14)	0.0158 (19)
O4	0.0546 (12)	0.4699 (6)	0.3355 (13)	0.0079 (17)
O5	0.5	0.5	0.5	0.037 (4)
O6	0.5995 (13)	0.0847 (6)	0.5069 (13)	0.0144 (19)
O7	0.6794 (13)	0.3742 (6)	0.1417 (14)	0.016 (2)
O8	0.2371 (14)	0.4270 (6)	0.0293 (14)	0.0134 (19)
O9	0.2513 (14)	0.8979 (7)	0.0556 (16)	0.023 (2)
O10	0.8279 (13)	0.1912 (6)	0.3603 (14)	0.016 (2)
O11	0.0811 (15)	0.3257 (7)	0.5939 (15)	0.022 (2)

Geometric parameters (Å, º)

Bi1—O1^i	2.137 (10)	Bi5—O1^iii	2.145 (9)
Bi1—O2	2.328 (10)	Bi5—O3^vi	2.659 (10)
Bi1—O6	2.655 (10)	Bi5—O6	2.576 (10)
Bi1—O6^ii	2.098 (10)	Bi5—O9^vii	2.196 (10)
Bi1—O9^iii	2.399 (12)	Bi5—O10	2.064 (11)
Bi2—O1^iv	2.414 (11)	Bi6—O4^viii	2.151 (10)
Bi2—O2	2.136 (10)	Bi6—O4^iv	2.505 (10)
Bi2—O3	2.415 (10)	Bi6—O7	2.539 (11)
Bi2—O10^v	2.311 (10)	Bi6—O8^vii	2.278 (11)
Bi2—O11	2.277 (11)	Bi6—O11^iv	2.215 (12)
Pb2—O1^iv	2.414 (11)	Pb7—O4^viii	2.586 (11)
Pb2—O2	2.136 (10)	Pb7—O7	2.188 (10)
Pb2—O3	2.415 (10)	Pb7—O10	2.369 (10)
Pb2—O10^v	2.311 (10)	Pb7—O11^viii	2.181 (10)
Pb2—O11	2.277 (11)	Pb8—O2	2.233 (10)
Pb3—O4	2.209 (10)	Pb8—O3	2.428 (10)
Pb3—O5	2.173 (5)	Pb8—O6	2.350 (11)
Pb3—O8	2.256 (11)	Pb8—O9^iv	2.125 (11)
Bi4—O3^vi	2.169 (10)	Bi8—O2	2.233 (10)
Bi4—O7	2.120 (10)	Bi8—O3	2.428 (10)
Bi4—O8	2.176 (11)	Bi8—O6	2.350 (11)
Bi4—O10	2.767 (10)	Bi8—O9^iv	2.125 (11)
O6^ii—Bi1—O9^iii	76.5 (4)	O6—Bi5—O10	81.1 (4)
O2—Bi1—O9^iii	163.4 (4)	O6—Bi5—O9^vii	164.9 (5)
O2—Bi1—O6^ii	87.0 (4)	O1^iii—Bi5—O10	95.6 (4)
O1^i—Bi1—O9^iii	90.3 (4)	O1^iii—Bi5—O9^vii	80.7 (4)
O1^i—Bi1—O6^ii	86.0 (4)	O1^iii—Bi5—O6	108.9 (4)
O1^i—Bi1—O2	88.8 (4)	O8^vii—Bi6—O11^iv	84.9 (4)
O10^v—Bi2—O11	72.6 (4)	O7—Bi6—O11^iv	163.0 (4)
O3—Bi2—O11	83.3 (4)	O7—Bi6—O8^vii	91.9 (4)
O3—Bi2—O10^v	146.3 (4)	O4^iv—Bi6—O11^iv	74.2 (3)
O2—Bi2—O11	85.5 (4)	O4^iv—Bi6—O8^vii	148.9 (4)
O2—Bi2—O10^v	81.7 (4)	O4^iv—Bi6—O7	101.36 (3)
O2—Bi2—O3	73.1 (4)	O4^viii—Bi6—O11^iv	85.8 (4)
O1^iv—Bi2—O11	158.4 (4)	O4^viii—Bi6—O8^vii	80.8 (4)
O1^iv—Bi2—O10^v	91.6 (4)	O4^viii—Bi6—O7	77.2 (4)
O1^iv—Bi2—O3	104.2 (4)	O4^viii—Bi6—O4^iv	75.0 (4)
O1^iv—Bi2—O2	77.6 (3)	O10—Pb7—O11^viii	73.1 (4)
O10^v—Pb2—O11	72.6 (4)	O7—Pb7—O11^viii	96.6 (4)
O3—Pb2—O11	83.3 (4)	O7—Pb7—O10	81.5 (4)
O3—Pb2—O10^v	146.3 (4)	O4^viii—Pb7—O11^viii	73.09 (4)
O2—Pb2—O11	85.5 (4)	O4^viii—Pb7—O10	136.1 (4)
O2—Pb2—O10^v	81.7 (4)	O4^viii—Pb7—O7	75.5 (4)
O2—Pb2—O3	73.1 (4)	O6—Pb8—O9^iv	77.0 (4)
O1^iv—Pb2—O11	158.4 (4)	O3—Pb8—O9^iv	79.1 (4)
O1^iv—Pb2—O10^v	91.6 (4)	O3—Pb8—O6	136.6 (4)
O1^iv—Pb2—O3	104.2 (4)	O2—Pb8—O9^iv	97.3 (4)
O1^iv—Pb2—O2	77.6 (3)	O2—Pb8—O6	76.45 (3)
O5—Pb3—O8	90.3 (3)	O2—Pb8—O3	71.29 (4)
O4—Pb3—O8	86.4 (4)	O6—Bi8—O9^iv	77.0 (4)
O4—Pb3—O5	87.5 (3)	O3—Bi8—O9^iv	79.1 (4)
O7—Bi4—O8	87.8 (4)	O3—Bi8—O6	136.6 (4)
O3^vi—Bi4—O8	92.1 (4)	O2—Bi8—O9^iv	97.3 (4)
O3^vi—Bi4—O7	91.4 (4)	O2—Bi8—O6	76.45 (3)
O9^vii—Bi5—O10	86.6 (4)	O2—Bi8—O3	71.29 (4)

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