Mixed Pb Research Articles

Study of the structure and properties of Pb–Zn borophosphate glasses

Multicomponent oxide glasses of composition 25PbO–25ZnO– xB 2O 3–(50− x)P 2O 5, series A, and yPbO–(50− y)ZnO–20B 2O 3–30P 2O 5, series B, are investigated. The role of compositional changes on their density, chemical durability, thermomechanical properties and Raman scattering is examined. Structural changes are found to be more pronounced in the series A glasses affecting most thermomechanical properties of the glasses. Raman spectra reveal a continuous shift in frequency of the main vibrational band from 1170 cm −1 at x=0, to 960 cm −1 at x=40, explained in terms of a successive transformation of metaphosphate (Q 2) units into pyrophosphate (Q 1) and orthophosphate (Q 0) units. In the series B of glasses, results show that mixed Pb–Zn borophosphate glasses ( y=25) have better chemical durability than end point (y=0, y=50) composition single-cation glasses. With an increasing PbO content, T g is found to slowly decrease and the thermal expansion coefficient to increase. The frequency shift of the main Raman band from 1092 cm −1 (at y=0) to 1011 cm −1 (at y=50) is ascribed to differences in chemical bonding, coordination numbers and field strength of both cations.

Electrodeposition studies in the MnO2 + PbO2 system: formation of Pb3Mn7O15

Metal oxides are the object of continuous research due to their importance in many electrochemical applications like batteries [1] and electrocatalysis [2]. As well as pure oxides, many mixed oxides have been obtained and characterised. For instance PbO2 doped with minor amounts of elements like Ag [3], Fe [4], Bi [5±8] and As [7, 9], has been extensively studied as an electrocatalytic material for oxygen transfer reactions. Mixed oxides obtained directly by electrodeposition are uncommon. One instance is Pb8Tl5O24 which has been obtained by anodic oxidation of a Pb2‡ + Tl‡ alkaline solution at room temperature [10] and used as an anode for oxygen evolution and oxidation of organics [11]. Deposition of ternary Pb±Tl±O compounds with modulation of stoichiometry has been investigated with the aim of producing ceramic superlattices [12±16]. To the best of our knowledge, no attempt has been reported to prepare mixed Pb + Mn oxides by anodic electrodeposition, although some mixed oxides like Pb3Mn7O15 are known to be stable in acid medium and may be useful as electrode materials [17]. Therefore, we have undertaken a study of the anodic codeposition of PbO2 and MnO2, in order to assess whether mixed oxides are formed either at room temperature or after thermal annealing.

Investigation of matrix effects for Pb isotope ratio measurements by multiple collector ICP-MS: verification and application of optimized analytical protocols

Previous studies have demonstrated the potential of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for precise Pb isotopic measurements using admixed Tl for 'external normalization' of instrumental mass discrimination. The Micromass IsoProbe is a new, single-focusing MC-ICP-MS instrument that does not employ an electrostatic sector for energy-focusing. Instead, a hexapole collision cell is used to thermalize the ion beam. This study presents the first in-depth investigation of the application of the IsoProbe to Pb isotope ratio measurements. With a Tl-based mass bias correction, multiple analyses of mixed Pb–Tl standard solutions typically display reproducibilities (±2σ) of about 50 ppm for 207Pb∶206Pb, 100 ppm for 208Pb∶206Pb and 206Pb∶204Pb, 150 ppm for 207Pb∶204Pb and 175 ppm for 208Pb∶204Pb. Using an empirically optimized exponential law normalization, the Pb isotope data show excellent agreement with reference values obtained by thermal ionization mass spectrometry. The cross-calibration between NIST SRM 981 and 982 demonstrates that accurate results can be obtained with this technique over a wide range of Pb isotopic compositions. The Tl-corrected Pb data obtained on the IsoProbe, however, display correlations with the Pb∶Tl ratio of the analyzed solutions. The addition of large amounts of Tl to the samples is unfavorable for the accurate measurement of the low-intensity 204Pb ion beam. Analytical artifacts may also be generated if high concentrations of concomitant elements are present in the sample solutions. Matrix effects and instrumental memory, however, can be readily overcome by the adoption of appropriate analytical protocols and it is demonstrated that sample measurements can achieve the same levels of precision and accuracy as are routinely obtained for analyses of pure standard solutions.

Formation of lead/sulfur binary cluster ions by laser ablation

Lead/sulfur binary cluster ions were produced by laser single ablation and laser double ablation, respectively, photolyzed by a UV laser and detected with a tandem time-of-flight (TOF) mass spectrometer. The distribution of cluster ions produced by laser single ablation is different from that produced by laser double ablation. By laser single ablation on a mixed Pb + S target, Pb n S n−1 + and Pb n S n − are formed as stable compositions that serve as structural skeletons of the binary clusters. All other cluster ions can be indicated as Pb n S m + with m ≥ n − 1 or Pb n S m − with m ≥ n, produced by the attachment of excess S atoms to Pb n S n−1 + or Pb n S n −. The stable compositions shown above are also found in the fragments from the main photodissociation channels of cluster cations, confirming their special stability. The structural models of the cluster ions with stable compositions are proposed. On the other hand, by laser double ablation on a separated Pb target and S target, the Pb clusters and S clusters are formed independently and then react with each other to generate binary cluster ions Pb n S m + and Pb n S m − with n = 1–7 and m ≥ 0. These two different formation pathways, by laser single ablation and laser double ablation, respectively, can lead to distinct differences in the compositions and distributions of the cluster ions. The different properties, such as internal energies or structural isomers, for a single stoichiometry from the different formation pathways are also discussed.

Monte-carlo computer simulation of the phase transitions in Sn(Pb)2P2S(Se)6 crystals

In Sn2P2S6 crystals ferroelectric second order phase transition (PT) occurs. In this case the number of formula units in elementary cell stays unchanged. In Sn2P2Se6 crystals the intermediate incommensurate phase exists. A dipole Ising model of Sn2P2S(Se)6 ferroelectrics has been developed. To consider the dipole long-range interaction around the dipole an area of simple form is singled out. Within this area the interaction between the dipoles is calculated precisely. The interaction with the other dipoles is considered by introduction of a local field. For better accordance with the real experiment an amendment on the short-range interaction with two nearest neighbours has been introduced. As a result of calculations made by Monte-Carlo method it has been found that when a certain short-range interction coefficient in Sn2P2S6 is selected the intermediate modulated phase disappears. In this case the temperatures of PT are as follows: for Sn2P2S6 model To ∼ 10.3; for Sn2P2Se6 model Tc ∼ 8.7, Ti ∼ 9.7 (in the effective temperatures). These data are in satisfactory agreement with the results of the experiment. In mixed (Pb y Sn1−y )2P2Se6 crystals increase of (Pb) concentration causes reduction of temperatures of PT to 0 K. In this case the character of PT stays unchanged. By the method of diluted Ising lattice the concentration dependences of temperatures of PT in this crystals are simulated.

Formation,Reaction and Photodessociation of Lead/Sulfur Clusters

The formation and photodissociation of Pb/S cluster ions, produced by two different ways of laser ablation, were studied with a tandem time-of-flight mass spectrometer. By laser direct ablation on a mixed Pb+S sample, the cluster ions with the compositions of PbnSn-1+ or PbnSn- were the most abundant species and can be identified as the stable components of Pb/S cluster ions. By laser double ablation on separated Pb sample and S sample, the products were PbnSm+(n=1-3, m=0-9) and PbnSm- (n=1-7, m=0-9) formed through reactions between Pb clusters and S clusters. From the distributions of cluster compositions and the photodissociation of clusters, we can confirm two different formation mechanisms of Pb/S clusters according to the producing ways of clusters.

Adsorption of Pb(II) and benzenecarboxylates onto corundum

An experimental investigation of the adsorption of Pb(II) and three benzenecarboxylate compounds (terephthalate, phthalate and pyromellitate) was conducted using the corundum surface. The experiments were carried out in 0.01 M NaNO 3 at 25°C between pH 3 and 11, for separate and mixed Pb and benzenecarboxylate solutions. The experimental data were used to quantify equilibrium surface complexation reactions using the constant capacitance model (CCM) and the extended constant capacitance model (ECCM). The adsorption of Pb was quantified using the complexes >AlOHPb +2–(NO 3 −1) 2 and >AlOPb +1–NO 3 −1 with Pb as an inner-sphere complex, and with outer-sphere nitrate counterions. The adsorption of the benzenecarboxylates was best described with outer-sphere surface complexes, differing in degrees of protonation to account for adsorption over the entire pH range that was investigated. Pb and phthalate adsorption from mixed Pb–phthalate solutions were modeled using the surface ternary complexes >AlOHPbPht 0 with >AlOHPbHPht +1–NO 3 −1, where the Pb ion serves as a bridge between the mineral surface and a phthalate molecule. Modeling could not be performed for the mixed Pb–terephthalate and Pb–pyromellitate systems because stability constants for aqueous Pb–terephthalate and Pb–pyromellitate complexation have not been determined. Model calculations show that the reduction of surface charge upon benzenecarboxylate adsorption is a function of both the number of functional groups on the adsorbate and on the binding strengths. Results also indicate that the number and the position of functional groups on the benzenecarboxylate compounds have important effects on the adsorption of Pb at low pH. However, at high pH, aqueous Pb–benzenecarboxylate complexation is not strong enough to compete with Pb surface species. The reduction of surface charge, however, was not sufficient to explain Pb adsorption from mixed Pb–phthalate solutions, providing evidence for the formation of the ternary surface complexes.

Charge Transfer and Surface Diffusion of Lead on Ge(111)

Small amounts of Pb significantly enhance the surface diffusion on the Ge(111)c(2 × 8) surface. Scanning tunneling microscopy at different sample bias polarities has been used to study the local geometric and electronic properties of the mixed Pb and Ge adatom structure. It was found that all Ge adatoms transfer charge to the Ge rest atoms whereas the Pb atoms exist in two different electronic configurations. Pb atoms which do not transfer charge to Ge rest atoms allow the existence of various characteristic Pb-induced defects in the c(2 × 8) reconstruction. During the formation of these defects vacancies are created which facilitate the diffusion of adatoms over large distances on the surface.

The electronic structure of the mixed valence compound Pb 3O 4

Ab initio self-consistent calculations of the electronic structure of Pb 3O 4 are presented. The calculations show that Pb 3O 4 is a semiconductor. The calculated bandgap of 1.1 eV is smaller than the observed gap of 2.1–2.2 eV. The calculations show strong hybridization between Pb(6s) and O(2p) states. For one type of lead atom, this leads to a distribution of Pb(6s) states over two occupied energy bands, similar to the situation in PbO. For the other type of lead atom in Pb 3O 4 the Pb(6s) states are distributed over an occupied and an unoccupied band, similar to the situation in β-PbO 2. This clearly demonstrates that Pb 3O 4 is a mixed valence compound, i.e. Pb(II) 2Pb(IV)O 4. According to the calculations, the conductivity of p-type Pb 3O 4 is due to holes in a Pb(II)(6s) valence band and the conductivity in n-type Pb 3O 4 is due to electrons in a Pb(IV)(6s) conduction band.

Microstructural instability in single-crystal thin films

Epitaxial PbTiO3 thin films were produced from a mixed Pb–Ti double-alkoxide precursor by spin-coating onto single crystal (001) SrTiO3 substrates. Heat treatment at 800 °C produces a dense and continuous, epitaxial lead titanate film through an intermediate Pb-Ti fluorite structure. A microstructural instability occurred when very thin single crystal films were fabricated; this instability caused the films to become discontinuous. Scanning electron microscopy and atomic force microscopy observations show that single crystal films with a thickness less than ∼80 nm developed holes that expose the substrate; thinner films broke up into isolated, single crystal islands. The walls of the holes were found to be (111) perovskite planes. A free energy function, which considered the anisotropic surface energies of different planes, was developed to describe the microstructural changes in the film and to understand the instability phenomenon. The function predicted that pre-existing holes greater than a critical size are necessary to initiate hole growth, and it predicted the observed morphological changes in the current system. Morphological stability diagrams that explain the stability fields for different film configurations, i.e., either completely covered, with holes, or single crystal islands, can be calculated for any film/substrate system.

Electronic structure of beta -PbO2 and its relation with BaPbO3.

Self-consistent calculations of the electronic structure of \ensuremath{\beta}-${\mathrm{PbO}}_{2}$, using the augmented-spherical-wave method and the ab initio pseudopotential method, are presented. The band structure consists of a set of nonbonding oxygen 2p bands and a very broad band, formed by strong hybridization of Pb 6s and O 2p orbitals. The calculations indicate that \ensuremath{\beta}-${\mathrm{PbO}}_{2}$ is a semimetal, with holes of O 2p character and electrons of mixed Pb 6s--O 2p character. The relation with the electronic structure and the properties of ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Bi}}_{\mathit{x}}$${\mathrm{O}}_{3}$ is discussed.

Influence of tin on the anodic behaviour of lead in sulphuric acid solutions—I. Voltammetric, photoelectrochemical and AC impedance measurements on a Pb—10%Sn alloy

The influence of Sn on the formation of the corrosion layer on Pb has been studied with a Pb—10%Sn electrode in 0.5 and 4.5 M sulphuric acid using voltammetry, rrde and potential decay measurements, as well as photoelectrochemistry and ac impedance. The results obtained support the conclusion that in the presence of tin the amount of non-stoichiometric oxide formed in the corrosion layer in the vicinity of the equilibrium potential of the system PbO 2/PbSO 4 is increased. According to the results of the impedance measurements tin catalyses the oxidation of Pb(II) to Pb(IV) by 0.1–0.2V. It is suggested that the resulting, electronically conducting oxide is a mixed PbSn oxide containing mainly Pb(IV).

Tricritical point behaviour in the mixed system Pb(1−x)CaxTiO3

Abstract Experimental data on the mixed system Pb(1−x)CaxTiO3 for 0 ≤ x ≤ 0.40 (molar concentration) indicate a trend from a clearly first order transition at x = 0 to a second order (tricritical point) transition at x ≃ 0.40, characterized by a decrease with x of the thermal hysteresis. ΔT = T* −T c, where T* is the transition temperature and T c the extrapolated Curie temperature. An analysis of these data in terms of the generalized effective field approximation (E eff = E + βP + γP 3 + δP 5 …) for the mixed system leads to a quantitatively satisfactory prediction of the tricritical point concentration, x c ≃ 0.40, from data pertaining to the pure system (PbTiO3) alone.

Potential-dependent surface segregation in lead + ruthenium pyrochlore Pb 2Ru 2O 7-y

The effect of anodic polarization on the surface composition of Pb+Ru pyrochlore Pb 2Ru 2O 7-y in acid and acid+alcohol solutions has been studied using cyclic voltammetry, scanning electron microscopy, energy dispersive analysis and X-ray photoelectron spectroscopy. In acidic solutions the surface is observed to be unstable to oxygen evolution even at reasonably low overpotentials. Lead is found to diffuse from the pyrochlore matrix resulting in the formation of small crystallises of PbO 2 on the surface of a mixed Pb+Ru oxide substrate. RuO 2 does not appear to form on the surface.

Valence states in Pb 2Sr 2Y 1−xCa xCu 3O y

The valence states of Cu and Pb in the Pb 2Sr 2Y 1−xCa xCu 3O y system have been investigated by the use of a combination of the volumetric measurement, iodometric titration, and gravimetric measurements. It was found that the Cu 3+ions were absent in the entire regime from X = 0 to X = 1 and from pure oxygen treatment to pure nitrogen treatment. Mixed valent Pb 4+/Pb 2+ exist in a wide range of composition and heat treatment conditions. The amount of Pb 4+ increases with decreasing Y content for samples treated in the same oxygen partial pressure, and it reaches a maximum of 75% of Pb for a sample treated in pure oxygen at 450°C. Na doping promotes superconductivity.

On the structural mechanism of the Reaction Pb(H2PO4)2 → PbHPO4 + H3PO4

AbstractDecomposition of Pb(H2PO4)2 into PbHPO4 and amorphous H3PO4 proceeds crystallographically oriented. In order to prove a topotactic model for the mechanism of the reaction formerly derived from comparison of the structures, the As/P ratios of arsenatephosphate mixed crystals Pb[H2(As,P)O4]2 and of their reaction products PbH(As,P)O4 and H3(As,P)O4 were determined. The crystalline PbH(As,P)O4 showed in each case a higher As‐content than the educt and the amorphous H3(As,P)O4. Comparison of these results with the expectation values according to probability theory for the As/P‐distribution confirm the topotactic mechanism of the reaction.

Structural origin of the martensitic phase transitions in the insulating mixed system Pb3P2x V2(1-x)O8

Abstract Lead orthovanadate Pb3V2O8 is rhombohedral R3m at high temperature and transforms at 373 K and 273 K into monoclinic phases whose space groups are respectively P21/c and A2. Lead orthophosphate is isotypic at high temperature to lead orthovanadate but transforms at 453 K into a different monoclinic cell C2/c which remains stable down to 5 K. This system is particularly interesting because: i. The transitions of the pure compounds Pb3P2O8 and Pb3V2O8 are highly first-order and are characterized by important shears.ii. It is possible to obtain mixed compounds in the whole range of concentrations 0 ≤ x ≤ 1. The critical temperatures are strongly depressed with the substitution. The volumic discountinuities remain as high as in Pb3P2O8 and Pb3V2O8, but during the transformations a huge phase coexistence appears and thermal hysteresis as large as 100°C can be observed for compositions with x near 0.28 and 0.65. For 0.28 < x < 0.65 no transformations can be evidenced and the compounds have rhombohedral symmetry down to 5 K.iii. For the phosphorus-rich compounds the transitions display an unusually strong critical regime; we have shown that it is characterized by the existence of quasielastic incommensurable peaks with misfit parameters depending on the Brillouin zone (Kiat and Calvarin, 1989).

The effect of antimony on the anodic behaviour of lead in sulphuric acid solutions—I. Voltammetric measurements

The influence of Sb on the formation of the corrosion layer on Pb has been studied with a Pb—12%Sb alloy electrode in sulphuric acid solution using the ring-disk technique decay measurement and galvanostatic oxidation. It was found that in voltammograms of the PbSb electrode a new cathodic peak appears, compared to pure Pb, after oxidation at constant potentials > 0.8V, showing that a new oxidation product is formed. The stoichiometry of this product changes gradually with potential approaching that PbO 2 at high anodic potentials. This product also improves the electronic conductivity of the corrosion layer. The effect of Sb(III) or Sb(V) dissolved in the electrolyte on the formation of this Sb containing product on pure Pb was similar to that of Sb as an alloying agent. It is suggested that this new product is an oxide of Pb (most probably nonstoichiometric) doped with Sb or a mixed PbSb oxide.

Accumulation of Cu, Zn, Cd, and Pb by aquatic macrophytes

The levels of Cu, Zn, Cd, and Pb were determined in water, sediment, and aquatic plant samples taken from the Nile River and its branches at selected sites characterized by heavy industrialization and dense population. The results revealed that concentration factors in the sediments were, in most cases, much higher than in plants. Metals content in the plants collected from downstream wastewater discharge points was usually higher than that in the plants collected from sites upstream of these points. In laboratory scale models of the piston and completely mixed continuous flow hydraulic systems, Cu, Cd, and Pb removal by a test plant ( Eichhornia crassipes) was assessed. The results showed that the root of the test plant acted as a scavenger for the metals up to a certain capacity. The metals were taken up by different rates, the least of them was that of cadmium. In a batch system metals accumulation from waters of different compositions was investigated using the same test plant. The results are summarized in the following: (1) Cu, Zn, Cd, or Pb accumulation was a function of metal: plant root exposure ratio on dry weight basis. (2) The rates of metals accumulation from both the natural medium (Nile water) and the synthetic medium (mM NaHCO 3 solution) were quite close. (3) Metal accumulation was much lower in the presence of EDTA. (4) The presence of Ca 2+ and Mg 2+ in a concentration equivalent to that of EDTA did not eliminate the retarding effect of the latter on metal accumulation. (5) Metal translocation from the root to the shoot occurred at a low rate compared to metal accumulation by the root.

A polarographic study of the mixed complexes of Pb(II) with oxalate and malonate ions

The coordinated mixed Pb(II)-oxalate-malonate system has been polarographically studied in aqueous medium. The ionic strength has been maintained constant at μ = 1.0 by using NaClO 4. Application of the Schaap and McMasters method detected only the existence of the [Pb-ox-mal] 2− complex, the overall stability constant of which was β 11 = 4.1 x 10 5. The various coordination equilibria are discussed on the basis of statistical aspects.