Tetragonal PbO

Fig. 10. Analysis of the atomic lattice images of the lead compound entering CNTs by capillary forces (a)detailed view of the high resolution image of the filling material, (b)tetragonal PbO atomic arrangement, note the layered structure and (c)tetragonal PbO observed in the [111] direction, note that the distribution of lead atoms follows the contrast pattern observable in (a), (d)bidimensional projection of the deduced PbO filling orientation inside CNTs as viewed in the tube axis direction, note that PbO layers are parallel to the cylindrical CNT cavity.

Calcium hydroxide leached from incompletely cured concrete causes serious corrosion of lead (see Section 9.3). This is because carbon dioxide reacts with the lime solution to form calcium carbonate, which is practically insoluble. Carbonate ions are therefore not available to form a passive film on the surface of the lead . Typically, thick layers of PbO are formed, which may show seasonal rings of litharge (tetragonal PbO) and massicot (orthorhombic PbO) . 

Figure 6.17. (a) A projection of the tetragonal PbO cell on a face. Large circles are Pb. (b) A view of the PbO structure showing the square pyramidal Pb04 units. 

Single crystal flat surface. Air at 150-350°C The axes of the face centered Electropolished. tetragonal PbO are parallel to... 

FIG. 12.14. The crystal structure of tetragonal PbO (and SnO). The small shaded circles represent metal atoms. The arrangement of bonds from a metal atom is shown at the right, where the two dots represent the inert pair of electrons (see p. 937). 

PbO PbO exists in two polymorphic forms - tetragonal -PbO and orthorhombic jS-PbO they are photosensitive substances. Basic electrochemical properties of PbO have been studied in acidic [105, 111], neutral [165,] and alkaline [153] media. In acidic solutions, the p-PbO formation is followed by transformation to a- PbO [166, 167]. In sulfuric acidic... 

A further variation results from a combination of the above-described structures with those of the tetragonal PbO-type structure. A typical example of the resulting class of compounds., referred to as Aurivillius phases, is Bi2Nb205F, where the bismuth atoms combine with the oxygen atoms to form a highly altered polymeric network (28). Other examples of both ordered and disordered intergrowth phenomena from Aurivillius phases have been given by Rao et al. (24). 

Conclusive proof of the identity of the photoactive film on lead follows from the analysis of the photocurrent spectra. Figure 9(b) compares the spectrum of the anodic photocurrent with the absorption spectrum of the tetragonal form of PbO. The oxide film is sufficiently thin for the photocurrent to be a linear function of the absorption coefficient so that direct comparison of the photocurrent and absorption spectra is possible and it is clear from the coincidence of the two spectra that the film consists of tetragonal PbO. The results illustrate the sensitivity of photocurrent spectroscopy as a method for the identification of thin surface phases. In-situ Raman spectroscopy [24] and in-situ X-ray measurements [25] of the same system show no evidence for the formation of PbO unless the electrode is held at a constant potential for a time sufficient for a much thicker layer of oxide to be formed. By contrast, photocurrent spectroscopy is sufficiently sensitive to detect the formation of PbO on the much shorter timescale of a linear sweep measurement and quantitative estimates of the film thickness are feasible. Similar results have been obtained for the reduction of a-Pb02 in alkaline solution, where the existence of the tetragonal form has also been established from the photocurrent spectra [26]. 

The oxidation of lead in various electrolyte solutions has been studied [736-738]. In a tetraborate solution, the photocurrent spectrum measured with white light showed a marked dependence on the positive (anodic) potential limit of the potential scans. With the most positive limit, the photocurrent-potential relationship displayed in Fig. 5.128 was obtained. The formation of tetragonal PbO by reduction of 0 -PbO2 was monitored. The onset of the photocurrent indicates the appearance of the formation of PbO and the electrode potential coincides with the sudden onset of the photocurrent in the anodic scan. In the positive-going scan, the photocur-... 

PbO—Pb02 as derived by Roy [37] is shown in Fig. 2.20. Pbi20i9 has its own thermodynamic range of stability. An equilibrium exists between the rhombic (or tetragonal) PbO and Pb304. In order to reach this equilibrium, however, tens or in some cases hundreds of hours are required. The kinetics of this process depends on the experimental conditions and the history of the oxide. 

Changes in phase composition of the paste during stirring. Initial oxide 100% tetragonal PbO H2SO4 solution and 1 wt% sodium lignosulfonate (SL) [13]. 

Tetragonal PbO is a chemical product. It can be seen that first 3Pb0-PbS04-H20 and orthorhomb-PbO are formed. After 30 min, the paste contains mainly 4PbO PbS04-... 

The only compound truly isomorphous with red PbO is the common modification of SnO, crystalline GeO being unknown. X-ray studies of the structure of SnO at room temperature and pressure up to lOOkbar were reported by Vereshchagin et al [247]. A linear decrease of the lattice constants as well as of the axial ratio c/a was found up to 40 kbar where a first-order phase transition accompanied by a 7% volume decrease was observed. The high-pressure phase has not, as one might perhaps expect, the orthorhombic PbO structure (which appears to be about 2% denser than the tetragonal PbO structure) but is claimed to adopt the hexagonal wurtzite structure. A tetrahedral coordination for a lone pair cation, however, appears to be rather peculiar. The assumption of a second high-pressure phase with a rocksalt structure, on the other hand, seems to be quite plausible. 

The structure of Ag2Pb02 [251] bears some relation to that of red PbO. The coordination figure of the Pb atom is of the same type as in tetragonal PbO, but somewhat distorted and instead of forming a chess-board like array, the [Pb04] pyramids are connected in chains. These (Pb02)oo chains are linked by Ag atoms to a three-dimensional network. 

Orthorhombic and tetragonal PbO are mixed conductors conducting oxygen via charged oxygen interstitials. For orthorhombic PbO, is aborrt 2 x 1(T S/cm at 500°C, while O5.1 is slightly larger. Both electron and hole conductivities are observed. 

Ultrapure PbO is made by precipitation from lead acetate solution by anunonium hydroxide in polyethylene vessels. In such wet preparations of lead(II) oxide, the yellow orthorhombic form is first produced which undergoes transformation to the red tetragonal PbO. This transformation is particularly sensitive to impurities, and the presence of elements such as silicon, germanium, phosphorus, arsenic, antimony, selenium, tellurium, molybdenum and tungsten in concentrations as low as 10 ppm prevents the transformation. The use of polythene vessels for the preparation of ultrapure red lead(II) oxide is emphasized, because sufficient silica is released from glass vessels to prevent the yellow to red con-versions s. ... 

The layer in rhombohedral PbO is a slightly puckered version of that in tetragonal PbO, where four equal Pb-0 bonds (2-30 A) replace the two short (2-21 A) and two long (2-49 A)... 

Pb-0 bonds. This close similarity of structures presumably allows small impurities to stabilize the yellow rhombic form at ordinary temperatures. The two crystal forms can be obtained simultaneously by heating lead carbonate at 400° in a melt of potassium and sodium nitrates, when over a period of one day red prisms of tetragonal PbO and yellow plates of rhombic PbO are formed along with orange needles of Pb304. 

PbeOgH4 is obtained either by hydrolysis of lead(II) acetate solutions , or from reduced pressure evaporation of solutions of tetragonal PbO in large volumes of carbon dioxide free water7< Such a preparation and the proposed structure are in line with the hydroxy-Iead(U)... 

Table 14.4 Data for the solubility constant of the oxide phases of lead(ll), litharge (red, tetragonal) PbO(s), massicot (yellow, orthorhombic) PbO(s) (reaction (2.13), M = Pb , x= 1).

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