Dopant ions

Sohd solutions of ceria with trivalent ions, eg, Y and La, can readily be formed. The ions substitute for the tetravalent Ce and introduce one oxygen vacancy for every two ions. The dopant ions and the oxygen vacancies form charge associates. The resulting defect-fluorites have good oxide... 

Electrochemical polymeriza tion of heterocycles is useful in the preparation of conducting composite materials. One technique employed involves the electro-polymerization of pyrrole into a swollen polymer previously deposited on the electrode surface (148—153). This method allows variation of the physical properties of the material by control of the amount of conducting polymer incorporated into the matrix film. If the matrix polymer is an ionomer such as Nation (154—158) it contributes the dopant ion for the oxidized conducting polymer and acts as an effective medium for ion transport during electrochemical switching of the material. 

Extrinsic Defects Extrinsic defects occur when an impurity atom or ion is incorporated into the lattice either by substitution onto the normal lattice site or by insertion into interstitial positions. Where the impurity is aliovalent with the host sublattice, a compensating charge must be found within the lattice to pre-serve elec-troneutality. For example, inclusion of Ca in the NaCl crystal lattice results in the creation of an equal number of cation vacancies. These defects therefore alter the composition of the solid. In many systems the concentration of the dopant ion can vary enormously and can be used to tailor specific properties. These systems are termed solid solutions and are discussed in more detail in Section 25.1.2. 

An example for chemical preparation that can be carried out within seconds in a beaker is this Dissolve pyrrole in dilute sulfuric acid. Add ferric chloride as an aqueous solution and watch the black polypyrrole precipitate. The oxidizing Fe ions are reduced to Fe, imparting one +-charge and donating their now excessive Cr ion as dopant ion to the polymer. 

Conduction in the raw doped polymer can be improved by devices such as compression and stretching and judicious after-synthesis dopant ion exchange. Matrix-guided electropolymerization also yields superior products. 

Some of the polymers can be processed like ordinary polymers even in the doped state, which is one of their virtues. Like ordinary polymers, blending of dilferent polymers (e.g., a conducting polymer and an ordinary polymer) is possible. The mutual compatibility of the two polymers can be improved by choosing in the conductive polymer a tenside-type dopant ion that has a tail having affinity to the non-conductive polymer. 

Within certain limits, the counterions serving as dopant ions can be exchanged, as in an ion exchanger. This is a useful action when synthesis is easier with one type of ion and charge storage is easier with another type of ion. 

Elemental analysis shows that the polymer generally contains four monomer units per dopant ion [20], and that there is also more hydrogen than would be expected (cf. polypyrrole) [20, 395], although this may vary depending on the starting material [409,410,414], Even in the neutral form, the polymer contains a small quantity of anions (0.5-1%) [19], although Waltman et al. [400] found that the extent to which the counter ion is incorporated into the polymer on polymerisation depends strongly on the nature of the /J-substituent (if present). 

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

Note that this simple formalism disguises the fact that a considerable amount of chemical skill is involved in ensuring that the dopant M only occupies the Ce4+ sites. For example, when the apparently suitable dopant ion Nd3+ is used, it occupies both the Ce and Ba sites, thus suppressing vacancy formation ... 

In general, low concentrations of the active ion, of the order of 1 %, are used. At these concentrations, the ions form point defects well isolated from each other. At higher concentrations, dopant ions tend to cluster and other energy loss mechanisms interfere with up-conversion. 

It has been noted that the conductivity and activation energy can be correlated with the ionic radius of the dopant ions, with a minimum in activation energy occurring for those dopants whose radius most closely matches that of Ce4+. Kilner et al. [83] suggested that it would be more appropriate to evaluate the relative ion mismatch of dopant and host by comparing the cubic lattice parameter of the relevant rare-earth oxide. Kim [84] extended this approach by a systematic analysis of the effect of dopant ionic radius upon the relevant host lattice and gave the following empirical relation between the lattice constant of doped-ceria solid solutions and the ionic radius of the dopants. 

Instead of considering how the incorporation of a dopant ion perturbs the electronic structure of the crystal, we will face the problem of understanding the optical features of a center by considering the energy levels of the dopant free ion (i.e., out of the crystal) and its local environment. In particular, we shall start by considering the energy levels of the dopant free ion and how these levels are affected by the presence of the next nearest neighbors in the lattice (the environment). In such a way, we can practically reduce our system to a one-body problem. 

Let us consider a dopant ion A (the central ion) placed at a lattice site, surrounded by an array of six regular lattice ions B ligand ions), separated by a distance a from the ion A. The ligand ions B are located at the corners of an octahedron, as shown in... 

So far, we have dealt with optically active centers based on dopant ions, which are generally introduced during crystal growth. Other typical optically active centers are associated with inhinsic lattice defects. These defects may be electrons or holes associated with vacancies or interstitials in ionic crystals, such as the alkali halide matrices. These centers are nsually called color centers, as they prodnce coloration in the perfect colorless crystals. 

Chapters 5 and 6 deal with the spectra of optically active centers. The term optically active center corresponds to a dopant ion and its environment (or to a color center), which produces absorption and/or emission bands that are different to those of the pure crystalline host. This is the case for a large variety of optical materials, such as phosphors, solid state lasers, and amplifiers. 

The activation energy for oxide ion conduction in the various zirconia-, thoria- and ceria-based materials is usually at least 0.8 eV. A significant fraction of this is due to the association of oxide vacancies and aliovalent dopants (ion trapping effects). Calculations have shown that the association enthalpy can be reduced and hence the conductivity optimised, when the ionic radius of the aliovalent substituting ion matches that of the host ion. A good example of this effect is seen in Gd-doped ceria in which Gd is the optimum size to substitute for Ce these materials are amongst the best oxide ion conductors. Fig. 2.11. 

In line with the terminology adopted, X is commonly called the dopant counter anion and y, which represents the ratio between dopant ion and polymer repeating unit, is commonly called the doping level. 

The perovskite structure is stable to relatively large amounts of dopant ions on either A or B sites. Oxygen vacancies are introduced into the lattice, either through transition-metal redox processes or by doping on the A or B sites with lower valence cations. 

These include ion-ion energy transfer, which can give rise to concentration quenching and non-exponential decay and relaxation by multiphonon emission, which is usually essential for completing the overall scheme, and can affect the quantum efficiency. For low concentration of rare earth dopant ions the principle nonradiative decay mechanism is a multiphonon emission. 

Figure 4.27 Measured SIMS and SSRM current profiles of a multiple-energy implanted 4H-SiC sample. The implanted dopant ion is Al and the activation after two annealing temperatures is shown. (From [122], 2003 Material Science and Engineering B. Reprinted with permission.)...

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