Thallium monolayer

Figure 2 Schematic representation of the ideal structures of the thallium monolayer cuprate superconductors.

Single crystals of the 1223 thallium monolayer phase were grown from a copper-rich melt with molar composition 1 2 2 4 (Tl Ba Ca Cu) the mixture was heated to 925°C, soaked 6 h, and cooled at l°C/min (57). Magnetic flux exclusion experiments indicated a sharp Tc onset of 110 K. 

Syntheses of near-single phases of the lead-substituted thallium monolayer phases with up to 6 Cu-O layers i.e., Pb-doped 1212, 1223, 1234, 1245, and 1256, have been recently reported (21). Reactant mixtures of various proportions of Tl2Os, PbO, CaO, Ba02, and CuO were pelletized, wrapped in gold foil, and sintered at 860-900°C under flowing oxygen for 10-30 h. The Tc value reached a maximum of 121 K for the 1234 compound and declined with further increase in the number of Cu-O layers. X-ray powder diffraction data for the different phases were refined using the Rietveld method and a consistent increase in the c-axis accompanied the increase in number of Cu-O layers. 

Fig. 6.9. Thallium monolayer near-neighbor distance as a function of electrode potential, aqueous electrolyte solution 0.1 M Na2S04 + 2.5 mM TI2SO4 (based on data in [72])...

As an example [13] we consider the underpotential deposition of thallium on silver (Fig. 15.13). At potentials above the onset of the upd of thallium the SHG signal decreases, at first slowly, then more rapidly. The adsorption of thallium causes a strong rise in a(o ), because the region in which the electronic density decays to zero becomes more extended with an angle of incidence of 45° this shows up as a drastic increase in the signal. A similar behavior is seen in other systems, and often even fractions of a monolayer can be detected. 

Waszczuk et al. [329] have carried out radiometric studies of UPD of thallium on single-crystal Ag electrode from perchloric acid solutions. Deposition of Tl on Ag(lOO) to obtain monolayer, bilayer, and bulk crystallites has been studied by Wang et al. [330]. These studies have shown that apart from the substrate geometry, the nature of the substrate-adatom interactions also influence the structure of the UPD metal adlayers. This is because of the fact that, contrary to Au and Pt electrodes, Tl forms a well-ordered bilayer phase before bulk deposition on Ag(lOO) surface occurs. 

The electrodeposition of thallium on Ag(l 10) is similar to that which takes place on the (111) face of silver [122]. The voltammogram shows well-defined structure in the formation of the first monolayer, and further deposition occurs before formation of the bulk deposit. Fig. 5.16c and d display the results for the isotropic and anisotropic response respectively. The magnitude and phase of o 2 were modeled by a constant contribution from the adatoms throughout the adsorption process (Eq. (5.4)). Values of x /Xin = 0-94 and a phase shift of 131° were obtained. As with Ag(lll), an enhancement in the anisotropic response was observed beyond 1 monolayer and was attributed to a similar resonance effect. 

Robinson and Richmond [117] also found that the time constant for adsorption of the first thallium overlayer depends on the final potential, E, of the potential step. They investigated the effect of underpotential and overpotential steps on the best fit values of z for exponential fits to /p p((). The under- and overpotentials, AE, are defined with respect to the maximum of the current peak, Ep, in the CVs for the first monolayer deposition. Fig. 5.22 displays the results for seven different potential steps. The time required to form the deposit increases at anodic potentials (A <0) closest to Ep, indicating that more time is required to deposit successively larger coverages. The form of the data suggests that z would reach a maximum near... 

Fig.5.25. s-polarized SH intensity ( ) and phase (O) for Au(111) as a function of thallium coverage (0) in monolayers. The inserts in the upper part of the figure represent the actual phase data for coverages of 0 = 0.0, 1.0, 1.5 monolayers labeled a, b, and c, respectively. Incident wavelength = 1064nm. The solid and dotted lines represent fits to the data (see text). From Ref. 121. 

In conclusion, we note that deposition of submono- and monolayers of adatoms is the most controllable and reliably predictable method of obtaining metallic nanodimension compositions. At least two or three kinds of adatoms can be deposited in a strictly layer-by-layer fashion on single-crystal substrates [217], and mixed adlayers can also be obtained. The combined deposition of adatoms and phase deposits of metals [217] is even more promising. Among the metals, HTSC components such as lead, thallium, bismuth, and copper rank among the most thoroughly studied adatomic systems. Electrodeposition methods are also applied to the technological preparation of conventional superconductors based on Nb-Sn alloys [218]. 

Tl,Bi)-1223 or (Tl,Pb)-1223 are selected for detailed description as representatives of the thallium bilayer and monolayer compound, respectively. Tl-2201 is selected because of its structural simplicity and continuously variable T, up to 84 K. It is the ideal candidate for fundamental studies. (Tl,Bi)-1223 and (Tl,Pb)-1223 offer a high irreversibility of field, which is very important for applications in high magnetic field. 

Formation of open adlayers via UPD is not a universal phenomena. Other metals such as lead, thallium and bismuth form close-packed mcommensurate monolayers and exhibit an effect which has been termed electrocompression. This refers to a gradual compression of the monolayer in the region between the monolayer deposition and bulk deposition regions. There is a slow reduction of the monolayer lattice parameter until it is about 3% compressed with respect to the bulk metal [22 - 23]. This type of behavior is expected for close-packed commensurate monolayers on the basis of effective medium theory [24]. 

Investigated systems include various metal deposits, like underpotentially deposited silver on Au(l 11) [69], thallium on a Pt(l 11) surface as a function of solution pH and bisulfate coadsorption [70] and other upd-systems [36, 59]. Evidence of dealloying of Cu3Au(lll) has been reported [71]. Near-neighbor distances between atoms in upd-monolayers of various transition metals deposited on Ag(lll) and Au(lll) surfaces have been measured as a function of electrode potential [72]. Typical results of a study of thallium-upd showing the changing Tl-Tl distance as a function of electrode potential are shown in Fig. 6.9. 

The first in-situ X-ray diffraction (XRD) investigations of phase transitions of adsorbed monolayers and multilayers [6] and reconstmction of a metal surface [7,8] were also reported by Fleischmann and Mao. The phase transitions were reported for the underpotential deposition (upd) and overpotential deposition (opd) of thallium onto a roughened silver electrode surfaces (similar to those used in surface-enhanced Raman spectroscopy (SERS) using the reflection mode of collection), and for upd of lead onto gold and silver... 

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