Glass-Induced Lead Corrosion of Heritage Objects: Structural

May 1, 2017 - Synopsis. Corrosion of lead in alkaline, potassium-bearing media leads to K(OH)·2PbCO3. The structure consists of Pb(OH) (CO3)3[CO3]3 l...
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Glass-Induced Lead Corrosion of Heritage Objects: Structural Characterization of K(OH)·2PbCO3 Sebastian Bette,*,† Gerhard Eggert,‡ Andrea Fischer,‡ and Robert E. Dinnebier† †

Max-Planck-Institute for Solid State Research, Heisenbergstr. 1 70569 Stuttgart, Germany State Academy of Art and Design, Am Weißenhof 1, 70191 Stuttgart, Germany

‡

S Supporting Information *

ABSTRACT: The investigation of the corrosion of a lid made from a tin−lead alloy of a 200 years old beer jug induced by the degradation of the potash based glass revealed SnO, Cerussite (PbCO3) and K(OH)·2PbCO3 as main corrosion product. A model experiment, simulating the corrosion of lead at room temperature confirmed the formation of K(OH)·2PbCO3 as a corrosion product in alkaline, potassium containing medium. For detailed characterization K(OH)·2PbCO3 was prepared by hydrothermal synthesis, as well. K(OH)·2PbCO3 crystallizes in space group P63/mmc with lattice parameters of a = 5.3389(1) Å and c = 13.9295(5) Å. The structure consists of Pb(OH)1/2(CO3)6/9[CO3]3/91/2‑ layers and intercalated K+ and exhibits a close relationship to the crystal structure of hydrocerussite (Pb(OH)2·2PbCO3), also known as “lead white”. A novel structure family, Mn+(OH)n·2PbCO3 (with n = 1,2), was identified by structure solution of K(OH)·2PbCO3, which can be assigned to a 2H-type subspecies and detailed comparison to Pb(OH)2·2PbCO3, which represents a 3R-type subspecies.

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INTRODUCTION The corrosion of lead metal and lead-based alloys by contact with water containing physically dissolved oxygen and salt solution, for example, in old lead made water pipes, leads to the formation of lead(II) carbonate and carbonate hydroxide phases. Those carbonates and carbonate hydroxide salts are also formed as secondary products of lead and lead oxide carbonization1 or oxidation of primary galena (PbS).2 In addition, lead(II) carbonate hydroxides and oxides have been used as pigments, “lead white”, for artistic and cosmetic application since antiquity.3−5 During the last 70 years many of these phases were synthesized artificially or found as naturally occurring minerals: PbCO3 (cerussite), Pb(OH)2·2PbCO3 (hydrocerussite), PbO·Pb(OH)2·3PbCO3 (plumbonacrite), PbO·PbCO3 (shannonite), and 2PbO·PbCO3 (unnamed mineral, found in association with shannonite and litharge [PbO]).1,2,6−13 All crystal structures have already been determined.14−18 When lead metal, lead-based alloys, or lead white pigments are exposed to air and humidity in alkali-metal-bearing and alkaline media, more complex alkali-metal (mostly sodium and potassium)-containing lead(II) carbonate hydroxide phases are formed. Those media evolve during the degradation of glasses, especially historic glasses, which are not completely stable. During the exposure to air moisture over long periods of time, for example, in museums, alkaline surface films are formed by ion exchange with alkali metal ions from the glass network. If no other acidic pollutant gases or their precursors (e.g., formaldehyde, H2CO) are present, alkali carbonate solutions © 2017 American Chemical Society

emerge by uptake of carbon dioxide. Hence, glass degradation provides corrosive media for metals at the metal−glass contact areas and leads to “glass-induced metal-corrosion on museum exhibits” (GIMME) that recently has been shown to be quite common.19,20 By investigation of glass beads made from soda lime glass that were coated on the inside with molten lead, a lead(II) sodium carbonate hydroxide, Na(OH)·2PbCO3, could be identified as a corrosion product.21 As degradation of sodalime glasses in contact with lead metal and lead alloys leads to the formation of Na(OH)·2PbCO3, potash-based glasses will most likely yield the also-known potassium analogue K(OH)· 2PbCO3.22 The occurrence of these alkali metal lead(II) carbonate hydroxides is not limited to metal corrosion: lead white pigments used in wall paintings are exposed to solutions of migrating soluble salts in the wall, for example, KNO3, or during cleaning treatments. Laboratory experiments by Kotulanová et al.23 (pigment samples stored in dilute salt solution up to one year) showed that cerussite, hydrocerussite, massicot (orthorhombic PbO), and minium (Pb3O4) are at least partially transformed in potassium carbonate or hydrogen carbonate solution to K(OH)·2PbCO3. In addition Na(OH)· 2PbCO3 and K(OH)·2PbCO3 can be synthesized by hydrothermal conversion of cerussite, hydrocerussite or litharge and cerussite mixtures in aqueous sodium carbonate, sodium hydrogen carbonate, respectively, potassium carbonate solReceived: February 13, 2017 Published: May 1, 2017 5762

DOI: 10.1021/acs.inorgchem.7b00391 Inorg. Chem. 2017, 56, 5762−5770

Article

Inorganic Chemistry

Figure 1. (a) Beer jug, ca. 1800 A.D., Kunstsammlungen der Veste Coburg (a.S.777). (b) Detail of the degraded glass surface in contact with the severely corroded mounting made of a tin−lead alloy. cleaned with a glass fiber brush and degreased with acetone before, was immersed in a 2 mol·L−1 K2CO3 solution, obtained by dissolution of K2CO3 (Alfa Aesar, p.a.) in deionized water at room temperature for half a year. For crystal structure solution and solid phase characterization a sample of pure K(OH)·2PbCO3 was required. According to Brooker et al.22 K(OH)·2PbCO3 was prepared by hydrothermal conversion of PbCO3 in concentrated K2CO3 solution. In detail, 2.0 g of PbCO3 (VEB Appolda Chemie, p.a.) was added to 40 g of a 5 mol· (kg[H2O])−1 K2CO3 solution, that is, 5 mol K2CO3 (Merck, p.a.) per kilogram of H2O. After the suspension was transferred into a TiPd autoclave, equipped with a Teflon insert, the sealed autoclave was tempered at 110 °C in an oven for 6 d. The device was cooled at room temperature for 30 min and subsequently opened, and the solid was filtered off. For removal of the adherent mother liquor, the product was suspended in 100 mL of cold (T < 4 °C), deionized water, twice, and finally suspended in 50 mL of cold (T < 4 °C) ethanol (VWR, p.a.). Drying of the solid was conducted at room temperature for 2 d. Phase Characterization. Both the glass and the corroded section of the tin−lead alloy of the beer jug were investigated by energydispersive X-ray spectroscopy (EDX) using a Zeiss EVO 60 microprobe and an accelerating voltage of 20 kV. In addition, the corroded metal at the glass−metal contact was analyzed by XRPD using a Siemens D 5000 diffractometer in Bragg−Brentano geometry within a scan range from 4.0 to 70° 2θ, employing a step size of 0.02 and a scan time of 15 s/step. The XRPD pattern of the white efflorescence crystals was collected at room temperature from 3.0 to 40° 2θ employing a total scan time of 14 h on a laboratory powder diffractometer in Debye−Scherrer geometry (Stadi P-Diffraktometer (Stoe), Mo Kα1 radiation from primary Ge(111)-Johannson-type monochromator, Mythen 1 K detector (Dectris)). The sample was sealed in a 0.3 mm diameter borosilicate glass capillary (Hilgenberg glass No. 14), which was spun during the measurement. μ-Raman spectroscopy of the corrosion products was conducted using a Renishaw inVia Raman spectrometer with a Leica DMLM microscope and a RenCam CCD detector. The spectrometer was equipped with a He−Ne laser operating at 632.8 nm, with power kept below 400 μW on the sample surface. The corrosion products formed on the lead coupon during the model experiments were identified by XRPD-measurements using the device described above with identical scan range and step width and a scan time of 1 s/step. Raman spectra of the resulting solid were taken with the devise described above. XRPD patterns for phase identification of synthesized K(OH)· 2PbCO3 were taken at room temperature with a laboratory powder diffractometer in Bragg−Brentano geometry (D8 Discover (Bruker), Cu Kα1 radiation, Vantec 1 detector), within a scan range from 4.0 to 70° 2θ, employing a step size of 0.02 and a scan time of 2 s/step. The

utions.17,22,24 Both phases were characterized in detail by Brooker et al.22 From the vibrational spectra and the X-ray powder diffraction (XRPD) patterns the authors expect that the lead(II) potassium carbonate hydroxide phase is the structural analogue of the sodium salt. The crystal structure of Na(OH)· 2PbCO3 (hexagonal lattice, space group P63mc) was originally determined by Krivovichev et al.17 This data set, however, is not included in the Inorganic Crystal Structure Database (ICSD).25 Instead a slightly different structure model (trigonal lattice, space group P31c) was published by Belokoneva et al.,26 completely ignoring the work of Krivovichev et al.,17 and submitted to the structural database. In contrast, K(OH)· 2PbCO3 has not been investigated in more detail, and its crystal structure is still unknown. In this study the corrosion of the lid made from a lead−tin alloy of an over 200 year-old beer jug of the collection of the “Kunstsammlung der Veste Coburg” (Bavaria, Germany) induced by the degradation of potash-based glass was investigated. The formation of K(OH)·2PbCO3 as a corrosion product was proven for the first time and demonstrated by a model experiment simulating glass-induced metal corrosion, as well. For a detailed characterization a sample of pure K(OH)· 2PbCO3 was prepared by hydrothermal synthesis, and the crystal structure was solved from high-resolution laboratory XRPD data. These results are crucial, as they can be used for unambiguous identification of corrosion products of usually more complex natural samples. In addition profound knowledge of crystal structures is the basis for quantitative phase analysis by X-ray powder diffraction, which provides a favorable way to monitor corrosion processes and for diagnosis of the degradation state of historic art- and craftwork.

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EXPERIMENTAL SECTION

Sample Collection and Phase Preparation. In the collection of the “Kunstsammlung der Veste Coburg” (Bavaria, Germany) two corrosion samples containing K(OH)·2PbCO3 were collected from a beer jug (inventory No. KVC-a.S.777), made at approximately A.D. 1800. A lid made of a tin−lead alloy was mounted on the handle and showed some corrosion in contact to glass (Figure 1). One sample was taken directly at the metal−glass contact, and for another sample the white efflorescence crystals on the lid mounting near the metal glass contact were collected. In a model experiment simulating glass-induced metal corrosion, a lead coupon (Merck, p.a., No. 1.07365, Cu ≤ 0.002%) that had been 5763

DOI: 10.1021/acs.inorgchem.7b00391 Inorg. Chem. 2017, 56, 5762−5770

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Inorganic Chemistry

Figure 2. XRPD patterns, including assignments of reflections according to the PDF-4 database, of (a) the severely corroded mounting made of a tin−lead alloy, collected directly at the metal glass contact, (b) the lead coupon after immerging into 2.0 M K2CO3 solution for six months, and (c) synthesized K(OH)·2PbCO3 after removal of adherent mother liquor. samples were prepared as flat plates. Thermal analysis was performed using a TG/DTA 22 of Seiko instruments (reference substance: Al2O3, open platinum crucible, nitrogen flow 300 mL·min−1, heating rate 2 °C·min−1). Elemental analyses of carbon, hydrogen, sulfur, and nitrogen were performed with a Vario Micro Cube analyzer (Elementar). Infrared spectra were recorded from KBr blanks using a Fourier transform infrared (FT-IR) spectrometer Nicolet 380X (Thermo Electron Company) with DLaTGS-Detector. The Raman spectrum was recorded on an FT-spectrometer (RFG 100/S, Bruker, Nd:YAG laser, wavelength 1064 nm, laser power 200−300 mW). Scanning electron microscopy (SEM) images were taken with a TESCAN Vega 5130 SB (20 kV accelerating voltage), after the sample was coated with gold. The synthetic sample of pure K(OH)·2PbCO3 was used for crystal structure determination. Therefore, an XRPD pattern of the solid phase was collected at room temperature on a laboratory powder diffractometer in Debye−Scherrer geometry (Stadi P-Diffraktometer (Stoe), Ag Kα1 radiation from primary Ge(111)-Johannson-type monochromator, Mythen 1 K detector (Dectris)). The sample was sealed in a 0.3 mm diameter borosilicate glass capillary (Hilgenberg glass No. 14), which was spun during the measurement. Crystal Structure Solution. The program TOPAS 6.027 was used to determine and refine the crystal structure of K(OH)·2PbCO3. Indexing of the phase was performed by an iterative use of singular value decomposition (LSI)28 leading to a primitive trigonal unit cell with lattice parameters given in Table 3 and in Table S1 (Supporting Information) and P31c (159), P3̅1c (163), P6cc (184), P63mc (186), P63/mcc (192), and P63/mmc (194) estimated as the most probable space groups from the observed extinction rules. The peak profile and the precise lattice parameters were determined by LeBail29 fits applying the fundamental parameter approach of TOPAS.30 The background was modeled by employing Chebychev polynomials of sixth order. The refinement converged quickly. The crystal structure of K(OH)·2PbCO3 was solved by applying the global optimization method of simulated annealing (SA) in real space as it is implemented in TOPAS.31 Atoms located on identical positions and occupying special positions were identified by using a merging radius of 0.7 Å.32 A rigid body of the carbonate unit (CO3) was defined in z-matrix notation and freely rotated and translated within the unit cell. After few hours the positions of all atoms were found. The structure solution was performed both in space group P31c (159) and the centrosymmetric counterpart, P3̅1c (163), and led to identical structure models and agreement factors (R values). Transforming the

non-centrosymmetric cell into the centrosymmetric one requires a shift of the unit cell origin of (0x, 0y, 0.05z). By detailed inspection of the atomic positions additional potential symmetry elements were observed for the general O1 site (12i). The fractional coordinates of this site were close to a (x, 2x, z) relation. Accordingly all minimal non-isomorphic supergroups of P3̅1c were checked, as well. Space group P63/mmc (194) led to a successful structure refinement with O1 occupying a 12k-special position, whereas the Wyckoff sequences of all other atoms were not changed. The structure refinement using the hexagonal lattice led to an almost identical structure model and to identical agreement factors as using the trigonal space groups. Structure refinement was performed again, using the non-centrosymmetric subgroup P63mc (186). This did not yield a significant improvement of neither the agreement factors nor the fit of the pattern. In addition the refined positions of all sites that were split by removal of the symmetry center matched within the estimated standard deviation with their centrosymmetric counterparts. According to the atomic positions the crystal structure of K(OH)·2PbCO3 is clearly centrosymmetric, only treatment of the s2 lone-electron pair of Pb2+ as a pseudo ligand would break centrosymmetry. To minimize the number of refineable parameters for the final refinement, structure description of K(OH)·2PbCO3 using space group P63/mmc (194) was favored. For the final Rietveld refinement, all profile and lattice parameters were released iteratively, and all atomic positions were subjected to free unconstrained refinement; the rigid body constraining the bond lengths of the carbonate ion was removed, as well. Thermal displacement of the potassium ion and the carbon and oxygen atoms was modeled by isotropic thermal displacement parameters. The lone-electron pair of Pb2+ required modeling of thermal displacement by using anisotropic displacement parameters according to Leineweber and Dinnebier.33 Some residual electron density was discovered in direct surrounding of the K+ site by checking the difference Fourier map, which pointed to a disordered cation site, similar to the disordered Pb(2) site in the crystal structure of hydrocerussite.18 Accordingly an attempt was made to move K+ from the 2c special position to a close 12k or 12j position and to refine this site using an site occupancy factor (S.O.F.) of 1/6. In result the K+split position almost merged at the 2c special position, and the agreement factors did not show any significant improvement. Hence K+ was kept at the 2c special position. In a second approach the crystal structure of Na(OH)·2PbCO326 was used as a starting model, with Na+ replaced by K+. Carbonate ions were constrained by defining rigid body, and the refinement converged 5764

DOI: 10.1021/acs.inorgchem.7b00391 Inorg. Chem. 2017, 56, 5762−5770

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Figure 3. (a) Lead coupon after immersion into 2.0 M K2CO3(aq) at room temperature for six months; the bottom part was polished to remove corrosion products for investigation, (b) pure K(OH)·2PbCO3 obtained by hydrothermal synthesis, and (c) SEM image of K(OH)·2PbCO3 obtained by hydrothermal synthesis.

having diameters of ∼1 to 3 μm shaped as cut hexagonal bipyramids (Figure 3c).

quickly. Both the ab inito structure solution (global + local optimization) and the structure refinement using Na(OH)·2PbCO3 as a starting model (local optimization only) led to identical noncentrosymmetric crystal structures in a trigonal lattice and to comparable agreement factors (R values). The structure model obtained using Na(OH)·2PbCO3 as a starting model could be described by the higher symmetric space group P63mc (186) or P63/ mmc (194) without any deterioration of the agreement factors, as well. The final agreement factors are listed in Table S1, the atomic coordinates and selected bond distances are given in Table S2 and Table S3, and the fit of the whole powder pattern is shown in Figure S2 in the Supporting Information. The crystallographic data were deposited at ICSD and CSD No. 432587.

Table 1. Elemental Analysis of Synthetic K(OH)·2PbCO3 wt % (measured) wt % (calculated)

N

C

H

S

0.03(3) 0.00

4.03(3) 4.07

0.13(3) 0.17

0.00(3) 0.00

In Figure 4 the IR (d) and Raman spectrum of synthetic K(OH)·2PbCO3 (c) is presented and compared to the Raman spectra of the corrosion product of the lead coupon (b) and the lid mounting of the beer jug (a). The number of the observed bands, as well as the band positions in the vibrational spectra of synthetic K(OH)·2PbCO3 (Supporting Information, Table S4), are in good agreement to the data reported by Brooker et al.;22 hence, the purity of the synthesized phase is confirmed by vibrational spectroscopy as well. The Raman spectra of the corrosion product of the lead coupon (Figure 4b) and the lid mounting of the beer jug (a) exhibit identical characteristic internal vibrations of the carbonate ions (12), (11), (10), (8), and (7) and lattice vibrations (3), (2), and (1) as the synthetic K(OH)·2PbCO3; therefore, it can be assumed the crystallization from aqueous solution during the hydrothermal synthesis and the formation during the corrosion of lead, respectively, a lead−tin alloy at room temperature led to identical lead potassium carbonate hydroxide phases. In particular, the position of the sharp ν1(CO32−) band ((10), Supporting Information Figure S4) is very characteristic for K(OH)·2PbCO3, as for other potentially occurring solid carbonates this band occurs at a significantly different position (Supporting Information Table S5) or their appearance can be excluded by the media in which the corrosion took place (e.g., for Na(OH)·2PbCO3) or by the diffraction pattern (e.g., for plumbonacrite, PbO·Pb(OH)2·3PbCO3). In the Raman spectrum of the corroded lid mounting of the beer jug (Figure 4c) the lattice vibration (3) exhibits an unusual high intensity and a slight upshift. These effects are mainly attributed to the A1g (211 cm−1) lattice vibration of SnO.34,35 Thermal Analysis and in Situ X-ray Powder Diffraction. The thermal decomposition of synthesized K(OH)· 2PbCO3 was investigated by thermogravimetric−differential thermal analysis (TG-DTA) analyses and by temperaturedependent in situ XRPD measurements. The decomposition process starts at ∼250 °C. In the TG curve (Figure 5, black line) only one decomposition step is visible between 250 and

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RESULTS Phase Identification and Characterization. The glass of the beer jug exhibits craquelure (Figure 1a), which is a clearly visible sign for instability, and the mounting of the lid, made of a tin−lead alloy, shows gray and white efflorescence (Figure 1b), which indicates an advanced stage of corrosion of the metal. XRPD analyses of a sample collected directly at the metal−glass contact (Figure 2a) revealed mainly SnO (blue squares) besides cerrusite (PbCO3, green triangles) and K(OH)·2PbCO3 (red bars) as corrosion products. An investigation of the white efflorescence crystals also confirmed the occurrence of cerrusite (PbCO3), SnO, and K(OH)· 2PbCO 3 as the main corrosion products (Supporting Information, Figure S3). The opaque, blue glass could be identified as the source for potassium, as SEM-EDX analyses point to significant amounts of potash in the glass matrix (Supporting Information, Table S3). In a model experiment to simulate glass-induced lead corrosion a lead coupon was immersed in a 2.0 M K2CO3 solution at room temperature. After six months the coupon was almost completely coated with a white powder (Figure 3a). The powder consists mainly of K(OH)·2PbCO3 (Figure 2b), residual lead (magenta circles), and an unidentified byproduct (black stars). Because of the presence of byproducts both the corrosion products of the lid mounting of the beer jug and of the lead coupon were not suitable for a detailed characterization of K(OH)·2PbCO3. Hence this phase was prepared by hydrothermal synthesis in concentrated K2CO3 solution using PbCO3 as starting material.22 The purity of the obtained solid was confirmed by XRPD (Figure 2c) as well as elemental analyses (Table 1). Hydrothermal synthesis led to a white powder (Figure 3b) consisting of crystals of K(OH)·2PbCO3 5765

DOI: 10.1021/acs.inorgchem.7b00391 Inorg. Chem. 2017, 56, 5762−5770

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Figure 6. Temperature-dependent in situ XRPD measurements (Cu Kα1 radiation, 1.5406 Å) of synthesized K(OH)·2PbCO3, including assignment of solid phases to related powder patterns: (a) pure K(OH)·2PbCO3, (b) K(OH)·2PbCO3 + PbO·PbCO3 (shannonite, magenta ellipses: characteristic 112 and 121 reflections) + monoclinic K2CO3, (c) faulted α-PbO + monoclinic K2CO3.

2PbCO3 are still apparent after the first step. During further heating of the sample all lead(II)−carbonate containing phases are decomposed. The observable reflections of the residue can be assigned to monoclinic K2CO3 and to faulted, orthorhombic α-PbO.41 Further heating to 500 °C does not result in any additional mass loss (Figure 5). The decomposition process of K(OH)·2 PbCO3 up to 350 °C is summarized by reaction I. According to this reaction a mass loss of 12.7 wt % was calculated, which is in very good agreement with the measured mass loss of 12.8 wt %.

Figure 4. Raman spectra of (a) the severely corroded mounting made of a tin−lead alloy, (b) the lead coupon after immerging into 2.0 M K2CO3 solution for six months, and (c) synthesized K(OH)·2PbCO3 after removal of adherent mother liquor and (d) IR spectrum of the latter solid.

K(OH)· 2PbCO3(s) 250 − 350 ° C

step(1 − 2)

⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 0.5K 2CO3(s) + 2PbO(s) + 0.5H 2O(g) ↑ +1.5CO2(g)↑

(I)

Crystal Structure Description of K(OH)·2PbCO3. K(OH)·2PbCO3 was found to crystallize in a primitive hexagonal lattice with lattice parameters given in Table 3. The atomic positions are in accordance with the centrosymmetric space group P63/mmc (194); if, however, the s2 electron-lone pair of Pb2+ is treated as a pseudo ligand, then the space group symmetry must be lowered to a pseudo centrosymmetric setting of P63mc (186). In the crystal structure each carbonate ion is located on the 63-screw axis, which results in an O−C−O angle of exactly 120°. The measured C−O distance of 1.312(6) Å is in good agreement with related carbonates, for example, hydrocerrusite (Pb(OH)2·2PbCO3), d(C−O) = 1.301(7) Å.18 All other ions (OH−, Pb2+, K+) are located on six-fold symmetry axes, as well. The lead ions are coordinated by one hydroxide ion, three bidentate coordinating carbonate ions (η2CO3) in direct surrounding with Pb−O distances of 6 × 2.703(1) Å and by three additional monodentate coordinating carbonate ions (η1CO3) in the periphery (d(Pb−O) = 3 × 3.082 Å; Figure 7b). Each Pb(OH)1/2(CO3)6/9[CO3]3/90.5− polyhedron shares three rectangular faces with other polyhedra

Figure 5. Thermal analysis of synthesized K(OH)·2PbCO3.

350 °C, whereas the DTA (blue line) and differential thermo gravimetric analysis curves (green line) reveal a two-step process in this temperature range. The mass losses strongly overlap, and each decomposition step is associated with an endothermic effect. In situ XRPD measurements also indicate a two-step decomposition process between 290 and 320 °C (Figure 6a−c). During the first decomposition step K(OH)· 2PbCO3 is partially decomposed to monoclinic K2CO339 and to PbO·PbCO3 (shannonite).15 Reflections from residual K(OH)· 5766

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parameters, especially of the c-axis, do not correspond to the difference of the ionic radii. The structure of the Na phase was determined by Krivovichev et al.17 and Belokoneva et al.26 from single crystals. According to the latter data set, Na(OH)· 2PbCO3 crystallizes in a non-centrosymmetric, trigonal lattice. There is, however, a pseudosymmetry center in the crystal structure. Nevertheless the authors refined the crystal structure using a non-centrosymmetric space group (P31c), as this led to better agreement factors. As structure solution and refinement of K(OH)·2PbCO3 led to a centrosymmetric, hexagonal lattice the published crystal structures of Na(OH)·2PbCO3 was reviewed (see Supporting Information), and the choice of the space group was proven as wrong. In the structural model of Krivovichev et al. 17 a pseudosymmetry center is apparent, as well. By shifting the unit cell origin to this symmetry center, the space group symmetry can be raised from P63mc to P63/mmc. The authors only used the non-centrosymmetric unit cell setting, as they treated the s2 electron lone pair of Pb2+ as a pseudo ligand. To reveal structural relationships and to obtain further structural insights the crystal structures of M(OH)·2PbCO3 (M = Na,26 K) should be compared to the structure of hydrocerrusite (Pb(OH)2·2PbCO3).18 A comparison of these crystal structures is presented in Figure 8. In the latter crystal

Figure 7. Packing diagram Pb(OH)1/2(CO3)6/9[CO3]3/90.5− units (yellow polyhedra) in the crystal structure of K(OH)·2PbCO3 (a), illustration of the Pb2+ (b) and K+ (c) coordination.

that results in Pb(OH)1/2(CO3)6/9[CO3]3/90.5− polyhedra sheets perpendicular to c-axis (Figure 7a). In addition the hydroxide ions (μ2-OH−) bridge two Pb(OH)1/2(CO3)6/9[CO3]3/90.5− polyhedra of adjacent layers. The potassium ions are located in the interlayer space and are exclusively coordinated by carbonate forming a trigonal prismatic coordination sphere (Figure 7c). As a trigonal prismatic coordination of K+ is apparent in neither the low-38 nor the high-temperature39,40 modifications of K2CO3 and due to its interlayer position, the potassium appears to be an intercalated species.

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DISCUSSION Phase Formation and Thermal Behavior of K(OH)· 2PbCO3 in Relation to the Crystal Structure. The formation of K(OH)·2PbCO3 could be observed after exposure of lead to K2CO3 solution at room temperature. K(OH)· 2PbCO3 is also formed in aqueous solution at 200 °C by conversion of PbCO3 in concentrated K2CO3 solution. This implies that the corrosion of lead in aqueous solution leads to the formation of K(OH)·2PbCO3 at elevated temperatures, as well, as long as a sufficient amount of dissolved potassium is apparent. Even small amounts of dissolved potassium, released by the contact of instable, historic glasses, lead to the partial formation of K(OH)·2PbCO3 during the corrosion process. In the crystal structure of this lead potassium carbonate hydroxide phase, the alkali metal is exclusively coordinated by carbonate ions. High temperatures, T > 250 °C, easily lead to a segregation of K2CO3 and PbO·PbCO3 (shannonite), and when those temperatures are applied during the hydrothermal synthesis, the conversion of lead(II) oxide, respectively, carbonate in aqueous K2CO3 yields pure lead(II) oxocarbontes, that is, shannonite15 or oxo-hydroxide carbonates, that is, plumbonacrite (PbO·Pb(OH)2·3PbCO3). Taking these facts into account, the sum formula of the potassium lead carbonate hydroxide phase can be alternatively expressed as K2CO3· Pb(OH)2·3PbCO3. Structural Relations between Pb(OH)2·2PbCO3 and M(OH)·2PbCO3 (M = Na, K). Brooker et al.22 expected the structures of Na(OH)·2PbCO3 and K(OH)·2 PbCO3 to be isotypic based on the nearly identical XRPD pattern and vibrational spectra. Surprisingly the differences of the lattice

Figure 8. Comparison of the stacking order in the crystal structures of Pb(OH)2·2PbCO3 (hydrocerussite) and K(OH)·2PbCO3, with OH− layers denoted as a′, b′, ..., CO32− layers as a, b, ..., Pb2+ layers as α, β, γ, and layers of intercalated cations denoted as α′, β′, γ′.

structure one Pb2+ site (Pb2) and the OH− site exhibit a positional and an occupational disorder; instead of three disordered hydroxide (oxygen) sites, only one oxygen site is shown on its idealized position for clarity. In the crystal structure of hydrocerrusite one of the lead sites (Pb1) exhibits a coordination sphere analogous to K(OH)· 2PbCO3 with one hydroxide and three bidentate carbonate ions in direct surrounding and the monodentate carbonate ions in the periphery. The Pb(OH) (CO3)6/9[CO3]3/9− polyhedra share rectangular faces and form layers perpendicular to the caxis, as well (Figure 8). Like K+ or Na+, the second, disordered lead site (Pb2) is situated in the interlayer space. In contrast to the alkali metal lead carbonate hydroxides, the hydroxide ions in hydrocerussite are not bridging (μ1); therefore, the layers are 5767

DOI: 10.1021/acs.inorgchem.7b00391 Inorg. Chem. 2017, 56, 5762−5770

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Inorganic Chemistry not interconnected. For a detailed comparison of the stacking sequences the carbonate layers shall be denoted as a, b, etc., the hydroxide layer as a′, b′, etc., the lead layers as α, β, etc., and the layers of intercalated ions as α′, β′, γ′ (Figure 8). H y d r o c e r u s s i t e e x h i b i t s a n α ′ ( a ′ a β α a̅ b ′ ) β ′ (c′bγβb̅ a ′)γ′(b′cαγc c′) stacking pattern with Pb(OH) ̅ (CO3)6/9[CO3]3/9− layers indicated by brackets. Each Pb(OH) (CO3)6/9[CO3]3/9− layer consists of two hydroxide, two lead, and two carbonate layers. The latter are situated on identical positions but have inverted orientation, indicated by a bar on top of the layer symbol. When the stacking of the Pb(OH) (CO3)6/9[CO3]3/9− layers is summarized by using a capital Latin letter then the stacking sequence of hydrocerussite simplifies to α′Aβ′Bγ′C, which refers to a 3R-type stacking pattern. The layers of hydrocerussite and the alkali metal lead carbonate hydroxide phases are constituted identically; nevertheless, there are two main differences in the crystal structures: I. the Pb(OH)1/2(CO3)6/9[CO3]3/90.5− layers of K(OH)· 2PbCO3 and Na(OH)·2PbCO3 are interconnected by a μ2hydroxide and II. the stacking order differs (Figure 8). In the crystal structure of the alkali metal lead carbonate hydroxide phases an α′[a′](aβαa)β′[b′](a αβa) stacking pattern can be ̅ ̅ observed; here, each hydroxide layer, indicated by squared brackets, belongs to two adjacent Pb(OH)1/2(CO3)6/9[CO3]3/90.5− layers. In this stacking pattern each layer always has the reverse stacking sequence in relation to the preceding one, which can be summarized as an α′Aβ′A̅ stacking sequence that refers to a 2H-type stacking pattern. This type of stacking is often found in crystal structures having space group P63/mmc (194) like 2H-heterogenite (CoOOH).36 In result the relationship between the crystal structures of hydrocerussite and the alkali metal lead carbonate hydroxide phases becomes obvious. By replacing Pb2+ by Na+/K+ one hydroxide group is removed from the structure to maintain charge balance. As the total constitution of the layers and the coordination of lead within the layer is not affected by the cation substitution, the residual hydroxide ion bridges two Pb(OH)1/2(CO3)6/9[CO3]3/90.5− polyhedra, which causes a shift from 3R-type stacking to 2H-type stacking. Because of the larger ionic radius of K+, the substitution of Pb2+ by K+ leads to an extension of the unit cell in a- and b-directions (Table 2). The expansion (∼0.09 Å), however, is smaller than the difference of the ionic radii (∼0.15 Å37). This indicates that the voids in the crystal structure of hydrocerussite are not completely filled by the Pb2+ cation. In addition the extension of the voids in the hydrocerussite structure in c-direction (Figure 9) is far larger than in the crystal structure of K(OH)· 2PbCO3 despite the smaller ionic radius of Pb2+ (Table 2). Therefore, the overextension of the void in the crystal structure of hydrocerussite is most likely the origin of the vast positional disorder of the interlayer Pb2+ site. In contrast the Pb(OH) 1/2 (CO3 )6/9[CO 3]3/9 0.5−, respectively, the Pb(OH) (CO3)6/9[CO3]3/9− coordination polyhedron is almost unaffected by the substitution of Pb2+ by K+ or Na+, which additionally confirms the relationship between Pb(OH)2· 2PbCO3 and K(OH)·2PbCO3, respectively, Na(OH)·2PbCO3 and the layered character of these structures. The slight disorder of the K+ site in the crystal structure and the elongation of the c-axis after substitution of Na+ by K+ in the crystal structure of M(OH)·2PbCO3 with M = Na, K that is clearly smaller (0.45 Å) than two times (as there are two M+

Table 2. Comparison of Lattice Parameters, Structural Motifs, and Selected Atomic Distances in the Crystal Structures of Pb(OH)2·2PbCO3 (Hydrocerussite),35 K(OH)· 2PbCO3, and Na(OH)·2PbCO326 Pb(OH)2· 2PbCO3 space group R3̅m V, Å3 565.01(15) a, Å 5.2465(6) c, Å 23.702(3) No. of layers per unit cell 3 stacking sequence α′Aβ′Bγ′C = 3 (Figure 8) R-type stacking r (intercalated ion),37 Å Pb2+: 1.40 d(Pb−O(η2CO3)), Å 6 × 2.67

K(OH)· 2PbCO3

Na(OH)· 2PbCO3

P63/mmc 343.96(2) 5.3389(1) 13.9300(4) 2 α′Aβ′A̅ = 2Htype stacking K+: 1.55 6 × 2.70

P63/mmca 323.98(55) 5.268(4) 13.48(1) 2 α′Aβ′A̅ = 2Htype stacking Na+: 1.24 (3 × 2.62, 3 × 2.70)b (3 × 2.72, 3 × 2.74)b (3 × 3.08)b (3 × 3.44)b 2.28b 2.18b 3.79

d(Pb−O(η1CO3)), Å

3 × 3.26

3 × 3.20

d(Pb−O(OH)), Å

2.33

1 × 2.33

basal distance of interlayer voids, Å d(Mn+void-O(OH)), Å

4.60

3.95

6 × 2.87c

6 × 2.65

d(Mn+void-O(CO3)), Å

6 × 3.06c

3 × 3.08

3 × 2.59,b 3 × 2.57b 3 × 3.04b

a

The reported space group of Na(OH)·2PbCO3, P31c, was proven to be wrong. The most probable space group is P63/mmc. bThese interatomic distances should be corrected by a redetermination of the crystal structure. cThese distances are given as distances from the geometrical center of the disordered lead site to the oxygen sites.

Figure 9. Illustration and extension of the interlayer voids in the crystal structures of Pb(OH)2·2PbCO3 (hydrocerussite)18 and K(OH)·2PbCO3.

sites in the unit cell) the difference of the ionic radii (2 × 0.31 Å = 0.62 Å) can be understood in this way, as well. As it is evident that the Pb(OH)n/2(CO3)6/9[CO3]3/9n/2− layers own the ability to host intercalated cations, the chemical formulas of hydrocerussite (Pb(OH)2·2PbCO3), Na(OH)· 2PbCO 3 , and K(OH)·2PbCO 3 can be generalized as Mn+(OH)n·2PbCO3 with n = 1, 2; M = Pb, K, Na. In addition these layers have most likely the ability to intercalate other mono- and divalent cations; hence, the generalized sum formula could be extended to M = NH4, Rb, Cs, Mg, Ca, Sr, Ba, and divalent transition metals. Investigations of clay minerals seem to point to this fact. Hayase et al.42 assumed that Ca2+ replaces interlayer Pb2+ in hydrocerussite like interlayers of surite ((Pb1.89Cu0.01Ca0.79Na0.23)2.92(CO3)2.00(OH)1.15· (Al1.72Fe3+0.05Mg0.30)2.07(Si3.66Al0.34)4.00O10(OH)2·0.35H2O). A 5768

DOI: 10.1021/acs.inorgchem.7b00391 Inorg. Chem. 2017, 56, 5762−5770

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Inorganic Chemistry more detailed investigation of Uehara et al.43 confirmed this and additionally revealed that interlayer Pb2+ is partially replaced by Cu2+, as well. Accordingly hydrocerussite belongs to a 3R-subspecies and Na(OH)·2PbCO3 as well as K(OH)· 2PbCO3 to a 2H-subspecies of a Mn+(OH)n·2PbCO3 structure family.

[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Author

*E-mail: [email protected].

CONCLUSION The investigation of the corrosion of a lid made from a tin− lead alloy of a 200 years old beer jug of the collection of the “Kunstsammlung der Veste Coburg” (Bavaria, Germany) that was induced by the degradation of the potash-based glass coating revealed SnO, cerussite (PbCO3), and K(OH)·2PbCO3 as main corrosion products. A model experiment in which lead metal was immersed in aqueous K2CO3 solution demonstrated that the corrosion of lead in alkaline, potassium-bearing medium leads to the formation of K(OH)·2PbCO3. Preparation of this lead(II) potassium carbonate hydroxide as pure phase by hydrothermal synthesis allowed a detailed characterization by spectral and thermal analysis as well as by temperature-dependent XRPD measurements and the determination of the crystal structure from high-resolution laboratory XRPD data applying the methods of global and local optimization. K(OH)·2PbCO3 was found to crystallize in a hexagonal lattice with atomic positions being in accordance with P63/mmc (194) space group symmetry. The crystal structure consists of hydroxide-bridged Pb(OH)1/2(CO3)6/9[CO3]3/90.5− layers and intercalated potassium ions. By a detailed comparison a strong relationship between the crystal structures of hydrocerussite (Pb(OH)2·2PbCO3), Na(OH)·2PbCO3, and K(OH)·2PbCO3 was revealed. Lead(II) sodium and potassium carbonate hydroxide belong to the 2Htype and the pure lead(II) carbonate hydroxide (hydrocerussite) to the 3R-type subspecies of an Mn+(OH)n· 2PbCO3 (with n = 1,2) structure family. In consequence the corrosion of lead metal and lead-based alloys in alkaline medium potentially leads to additional subspecies when other mono- and divalent metals, for example, from glasses, are present.

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AUTHOR INFORMATION

ORCID

Sebastian Bette: 0000-0003-3575-0517 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Mrs. C. Stefani from the Max-Planck-Institute for Solid State research is acknowledged for performing the XRPD measurements for structure determination, and Mrs. R. Moßig from the Institute for Inorganic Chemistry of the TU Bergakademie Freiberg is acknowledged for measuring the Raman spectrum of synthesised K(OH)·2PbCO3. The authors also thank H. Grieb from the Kunstsammlungen der Veste Coburg for providing the beer jug and taking the samples of corrosion products and H. Euler from the Univ. of Bonn for measuring the XRPD pattern of the corrosion products of the beer jug, collected at the metal−glass contact, and of the lead coupon. Funding by DFG for the project “In search of structure” (Grant No. EG 137/9-1) is gratefully acknowledged.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00391. Review of the crystal structure of Na(OH)·2PbCO3 published by Belokoneve et al.26, crystallographic and Rietveld refinement data, bond lengths and atomic coordinates for K(OH)·2PbCO3 at room temperature, measured band positions and assignments in the IR and Raman spectrum of synthetic K(OH)·2PbCO3 compared to the data22 given by Brooker et al., results of the EDX analysis of the glass and the quantitative phase analysis of the white efflorescence crystals near the metal−glass contact, excerpt of the Raman spectrum of the corroded lid mounting at the glass−metal contact, and comparison of the measured position of the ν1(CO32−) band with related carbonate phases (PDF) Accession Codes

CCDC 1545303 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ 5769

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DOI: 10.1021/acs.inorgchem.7b00391 Inorg. Chem. 2017, 56, 5762−5770