Synthetic hydro­cerussite, 2PbCO₃·Pb(OH)₂, by X-ray powder diffraction

Synthetic hydro­cerussite [trilead dihydroxide dicarbonate, Pb₃(CO₃)₂(OH)₂] can be easily obtained, as a white powder, by the action of carbon dioxide and water on either lead or litharge at pH 4-5. This compound is also found in lead corrosion technological products as a fine-grained phase. Ab initio crystal structure determination was carried out on X-ray powder diffraction data. The heavy-atom method and the Patterson function helped determine the crystallographic model and the atom locations. The Rietveld fitting procedure was used for the final refinement. The atomic arrangement is closely related to the structures of other lead hydro­xide carbonates. The hydro­cerussite structure can be viewed as a sequence of two types of layers stacked along [001]. Layer A is composed of Pb and CO₃, and layer B is composed of Pb and OH. The stacking sequence is ...BAABAA...

S1. Comment

Although the basic lead carbonate naturally occurs as the rare mineral hydrocerussite, it has been synthesized since early historical times and has been used extensively for artistic and cosmetic purposes (`lead white' pigment). Theophrastus, Pliny and Vitruvius all described its preparation from metallic lead and vinegar. In addition to its artistic importance, lead hydroxide carbonates play a significant role in geology (Krivovichev & Burns, 2000a) and in lead acid battery chemistry (Steele et al., 1998), and they have attracted much attention in recent years. We report here the crystal structure of hydrocerussite, 2PbCO₃·Pb(OH)₂, carried out on a commercially available synthetic powder.

In 1966, Olby published a review on the basic lead carbonates. He showed that the existence of two closely related basic lead carbonates was the cause for the publication of a large range of values for the unit-cell constants of hydrocerussite (Kokkoros & Vassiliadis, 1953; Cowley, 1956; Voronova & Vainshtein, 1964). The action of carbon dioxide and water on either lead or litharge produces the hydrocerussite 2PbCO₃·Pb(OH)₂ [or Pb₃(CO₃)₂(OH)₂] and the plumbonacrite 6PbCO₃·3Pb(OH)₂·PbO [or Pb₁₀(CO₃)₆PbO(OH)₆]. This latter compound was recently revised by Krivovichev & Burns (2000b), who suggested a slightly different chemical formula, i.e. Pb₅O(OH)₂(CO₃)₃. In 1964, Voronova & Vainshtein presented an electron-diffraction study of the crystal structure of PbCO₃·PbO·H₂O. The phase is trigonal (space group R3m) and was indexed on the basis of a hexagonal unit cell with cell parameters a = 5.23 Å and c = 23.82 Å. But their method of preparation produces a mixture of litharge and plumbonacrite in addition to the trigonal phase; hence, the composition of the trigonal phase could not be established by analysis. Therefore, they attempted to resolve the structure with a wrong chemical formula. Later on, Olby (1966) proved that the phase of interest was, in fact, the hydrocerussite compound.

The structure of hydrocerussite is layered and based on hexagonal sheets of Pb atoms (labelled A and B in Fig. 1). These two distinct types of layers are stacked along [001] as ···BAABAA··· Layer A is composed of Pb (Pb1 site) and CO₃ (C1 and O1 sites), and layer B is composed of Pb (Pb2 site) and OH (O2 site). Layer A can be considered as interpenetrating hexagonal nets of Pb and CO₃ groups. Pb atoms (Pb1) lie on the 3-axis and are surrounded by six O atoms (2.674 Å) belonging to CO₃ groups in the plane of this layer. Furthermore, they are coordinated to one OH (2.36 Å) and to three O1 (3.261 Å), above and below this layer. The Pb—OH bond length is in agreement with the usually accepted average value (2.3 Å; Steele et al., 1998). In the B layer, the Pb atoms (Pb2) are split on the 18 h sites, with occupation factors of 1/6. The splitting of the Pb atoms reveals some static disorder which also occurs for the six (OH) groups (O2 sites) surrounding the Pb2 site. These groups are bonded to the Pb2 atoms, forming a thick layer of approximately 1 Å. The Pb—OH bond lengths are presumably equal to 2.43 and 2.51 Å, the two other values (1.95 and 2.90 Å) obtained for this disordered structural model are not realistic. Above and below this layer, the Pb2 atoms are coordinated to three O1 atoms (2.558 and 2.740 Å). The disorder observed in the B layer is probably due to the long-range order built from the two layers A and B somewhat similar, but of different density (Pb-3O per unit cell in A and Pb-2O in B). The double-layer AA forms a structural backbone, which basically reproduces a slab of the cerussite structure. This grouping, which might be considered as an `anchor unit', is also found in other lead hydroxide carbonate structures, viz. NaPb₂(OH)(CO₃)₂, macphersonite, plumbonacrite, leadhillite, susannite, etc, (Krivovichev & Burns, 2000a,b,c; Steele et al., 1998). This layered crystal structure could explain some physical properties of the lead white pigment, viz. the easy spreading and high covering power most appreciated by painters.

S2. Experimental

Synthetic basic lead carbonate is commercially available from Merck Eurolab SAS, KgaA, Darmstadt, Germany. The powder was used as received.

S3. Refinement

In order to perform the structure determination, we used the synchrotron X-ray diffraction data set of a synthetic compound which corresponds to a mixture of 48% hydrocerussite and 52% cerussite (mass fractions deduced afterwards from our Rietveld refinements). The structure of cerussite is known and has not been refined (Sahl, 1974; Chevrier et al., 1992). The two phases were fed into the refinement process, the already known full atomic structure of the cerussite along with the unknown hydrocerussite phase, which was introduced by a full-profile cell-constrained refinement procedure (Rodriguez-Carjaval, 1990) in order to extract observed integrated Bragg intensities for direct-methods and Patterson-function purposes. The starting cell parameters of hydrocerussite are those suggested by Voronova & Vainshtein (1964). The systematic absences of some reflections confirmed the trigonal symmetry. The structure solution was carried out in parallel for the several different plausible space groups and centrosymmetric space group R3 m was eventually retained. This stage was also used to check the cell correctness and the proposed space group. The obtained structure factors were then used to first generate a Patterson map, where the positions of the Pb atoms were clearly visible. After Rietveld refinements of the positions of the found Pb atoms and comparison with parent structures, a difference Fourier synthesis revealed the positions of the remaining O and C atoms in the asymmetric unit, which were subsequently introduced in the refinements. We imposed the same isotropic atomic displacement parameter for all atoms in the CO₃ group.

Computing details

Data collection: SPEC package ESRF; cell refinement: FULLPROF (Rodriguez-Carjaval, 1990); data reduction: BINIT BM16 software ESRF; program(s) used to solve structure: Gfourier (Gonzalez-Platas & Rodriguez-Carjaval, 2002); program(s) used to refine structure: FULLPROF; molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: FULLPROF.

Figures

	Fig. 1. (a) The crystal structure of 2PbCO3·Pb(OH)2, (b) projection of the A layers along [001] and (c) projection of the A layers along [001].
	Fig. 2. The Rietveld plot for 2PbCO3·Pb(OH)2. Comparison of observed (circles) and calculated (solid line) profiles; the bottom curve is the difference pattern. The markers indicate reflection positions for the two phases cerussite and hydrocerussite.

lead carbonate hydroxide

Crystal data

2PbCO3·Pb(OH)2	F(000) = 480
Mr = 775.6	Dx = 6.84 Mg m^−3 Dm = 6.82 Mg m^−3 Dm measured by from Olby (1966)
Trigonal, R3m	Synchrotron radiation, λ = 0.35324 Å
a = 5.2465 (6) Å	T = 295 K
c = 23.702 (3) Å	Particle morphology: thin powder
V = 565.00 (1) Å^3	white
Z = 3	cylinder, 50 × 0.4 mm

Data collection

Two-circle diffractometer	Data collection mode: transmission
Radiation source: synchrotron	Scan method: step
Monochromator crystal Si(111)	2θmin = 3.05°, 2θmax = 38°, 2θstep = 0.005°
Specimen mounting: glass capillary

Refinement

Refinement on Inet	Profile function: Pseudo-Voigt convoluted with axial divergence asymmetry (Finger et al., 1994)
Least-squares matrix: full with fixed elements per cycle	23 parameters
Rp = 0.072	H-atom parameters not refined
Rwp = 0.091	Weighting scheme based on measured s.u.'s 1/Yi
Rexp = 0.055
χ^2 = 2.756	Background function: linear interpolation between 33 given points
6990 data points	Preferred orientation correction: March-Dollase correction (Dollase, 1986)
Excluded region(s): none

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Pb1	0.00000 (1)	0.00000 (1)	0.21510 (5)	0.0172 (2)*
Pb2	0.9158 (3)	−0.9158 (3)	0.0016 (3)	0.0215 (9)*	0.16666
C1	0.00000 (1)	0.00000 (1)	0.4304 (15)	0.028 (2)*
O1	0.8568 (11)	−0.8568 (11)	0.4318 (5)	0.028 (2)*
O2	−0.293 (4)	0.293 (4)	0.0200 (8)	0.010 (8)*	0.3333

Geometric parameters (Å, º)

Pb1—O1^i	2.674 (6)	Pb2—O2^vi	2.51 (2)
Pb1—O1^ii	3.261 (10)	Pb2—O2^vii	2.430 (17)
Pb1—O2^iii	2.36 (2)	Pb2—O2^viii	1.95 (2)
Pb2—O1^iv	2.558 (12)	Pb2—O2^ix	2.90 (2)
Pb2—O1^v	2.740 (11)	C1—O1^x	1.302 (6)
O1^xi—Pb1—O1^xii	157.7 (4)	O2^vi—Pb2—O2^vii	71.8 (10)
O1^xi—Pb1—O1^i	49.8 (2)	O2^vii—Pb2—O2^xvii	143.5 (12)
O1^xi—Pb1—O1^xiii	117.0 (4)	O1^iv—Pb2—O2^vi	78.0 (8)
O1^xi—Pb1—O1^xiv	68.1 (3)	O1^v—Pb2—O2^vii	133.3 (9)
O1^xi—Pb1—O2^xv	79.3 (8)	O1^iv—Pb2—O1^v	139.6 (7)
O1^ii—Pb1—O2^xvi	142.8 (10)	O1^x—C1—O1^xviii	119.9 (7)

Symmetry codes: (i) y+1/3, −x+y+5/3, −z+2/3; (ii) x−2/3, y+2/3, z−1/3; (iii) x−y+2/3, x+1/3, −z+1/3; (iv) −x+5/3, −y−5/3, −z+1/3; (v) −x+y+7/3, −x−1/3, z−1/3; (vi) −y+1, x−y, z; (vii) y+1, −x+y−1, −z; (viii) x+1, y−1, z; (ix) −x, −y−1, −z; (x) x−1, y+1, z; (xi) −x+1/3, −y−4/3, −z+2/3; (xii) −x+4/3, −y−1/3, −z+2/3; (xiii) y+4/3, −x+y+5/3, −z+2/3; (xiv) x−y−5/3, x−4/3, −z+2/3; (xv) −x−1/3, −y+1/3, −z+1/3; (xvi) y−1/3, −x+y−2/3, −z+1/3; (xvii) x−y+1, x−1, −z; (xviii) −y−1, x−y−2, z.