RE ions

The differential capacitance method cannot be used for reactive metals, such as transition metals in aqueous solutions, on which the formation of a surface oxide occurs over a wide potential re ion. An immersion method was thus developed by Jakuszewski et al. 3 With this technique the current transient during the first contact of a freshly prepared electrode surface with the electrolyte is measured for various immersion potentials. The electrode surface must be absolutely clean and discharged prior to immersion.182-18 A modification of this method has been described by Sokolowski et al. The values of obtained by this method have been found to be in reasonable agreement with those obtained by other methods, although for reactive metals this may not be a sufficient condition for reliability. 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

Divalent rare earth ions also have an outer electronic configuration of 4f"( including one more electron than for the equivalent trivalent rare earth). However, unlike that of (RE) + ions, the 4f " 5d excited configuration of divalent rare earth ions is not far from the 4f" fundamental configuration. As a result, 4f" 4f " 5d transitions can possibly occur in the optical range for divalent rare earth ions. They lead to intense (parity-allowed transitions) and broad absorption and emission bands. 

After X-ray irradiation of thermally annealed NaCl crystals, a small percentage of divalent europium ions are converted into trivalent europium ions (Aguilar et al, 1982). This is shown by the appearance of weak and narrow absorption lines at around 460 nm and 520 nm, related to the Fq D2 and Fq Di transitions of Eu + ions, respectively. For our purposes, this example allows us to compare the different band features between (RE) + and (RE) + ions Eu + ions show broad and intense optical bands (electric dipole allowed transitions), while Eu + ions present narrow and weak optical lines (forced electric dipole transitions). 

The nonradiative rate. Am, from a (RE) + ion level is also strongly related to the corresponding energy gap. Systematic studies performed over different (RE) + ions in different host crystals have experimentally shown that the rate of phonon emission, or multiphonon emission rate, from a given energy level decreases exponentially with the corresponding energy gap. This behavior can be expressed as follows ... 

F ure 6.6 The multiphonon nonradiative rate of (RE) ions as a function of the number of emitted effective phonons for LaCfi (260 cm ), LaEs (350 cm ), and Y2O3 (430-550 cm ). The numbers in brackets indicate the energies of the effective phonons. The shaded area indicates the range of typical radiative rates. 

Finally, it is important to recall that the simple nonradiative rate law described by Equations (6.1) and (6.2) is only vaUd for (RE) + ions. This is a consequence of the weak ion-lattice interactions for these ions, that leads to a Huang-Rhys parameter of... 

The 3d orbitals in TM ions have a relatively large radius and are unshielded by outer shells, so that strong ion-lattice coupling tend to occur in TM ions. As a result, the spectra of TM ions present both broad (S > 0) and sharp (S 0) bands, opposite to the spectra of (RE) + ions, discussed in section 6.2.1, which only showed sharp bands (S 0). 

In the first topic, we will briefly describe a semi-empirical method that is commonly nsed to estimate the radiative transition probabilities from energy levels of (RE) + ions in crystals. This is certainly very nsefnl in order to determine the efficiency of a (RE) + based system as a luminescent or laser material. In the previous chapter (Section 5.7), we have described a method for determining the qnantnm efficiency of a Inminescent system. However, the application of that method is limited to certain... 

We have seen that the absorption spectra of (RE) + ions (see Eignres 6.2 and 6.3) consist of several sets of lines corresponding to transitions between the Stark snblevels of 2S+1 j states within the 4f" electronic confignration. A typical absorption spectmm of a (RE) + ion in crystals is like the one sketched in Eignre 6.16. The different sets of transitions correspond to different J J transitions (/ acconnting for the gronnd state), which, in principle, are only permitted at magnetic dipole order the selection rnle is 2 / = 0, 1, with 0 -o- 0 forbidden. 

However, although f f transitions are, in principle, forbidden by the Laporte parity rule, most of the transitions in (RE) + ions occur at the electric dipole (ED) order. As we have already mentioned, this is an ED allowance due to the admixture of the 4f" states with opposite parity excited states 4f" 5d, as a result of the lack of inversion symmetry (ED forced transitions). The oscillator strength, /, for a / f absorption band can be estimated using expression (5.19). We now rewrite this expression as follows ... 

From the Judd-Ofelt theory, some general rules have been derived for ED transitions between 4f" states of (RE) + ions in crystals ... 

Doped manganite perovskites exhibiting CMR have the general formula REi fM f]V[n03 where RE represents a rare earth element and M a divalent metal such as Cu, Cr, Ba, or Pb. The divalent RE ions and divalent M ions occupy the A sites in the perovskite structure (Figure 9.14) and have 12-fold coordination to oxygen. The Mn ions occupy the octahedral B sites. (1- of the manganese ions... 

Infrared spectral studies of rare earth (RE) ion-exchanged faujasites have been reported by Rabo et al. (214), Christner et al. (217), Ward (211, 212), and Bolton (218). Distinct hydroxyl absorption bands are observed at 3740, 3640, and 3522 cm-1 after calcination at temperatures in the range of 340° to 450°C. As previously discussed, the hydroxyl groups at 3740 cm-1 are attributed to silanol groups either located at lattice termination sites or arising from amorphous silica associated with the structure. The hydroxyl groups that form the 3522 cm-1 band are nonacidic to pyridine or piperidine and are thought to be associated with the rare earth cations. 

Fig. 10. ass for different rare earth (RE) ions [147] only the shaded area has been studied. Compare the shaded plan Si3N4-YN 3AlN-4/3(AlN A1203) in Fig. 9... 

In the case where the catalyst lattice is doped with the rare-earth ion (represented by RE in a circle in Fig. 1), luminescence is ascribed to the electron transition within the RE ion according to the following reaction [7] ... 

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