Interstitial ion

One feature of oxides is drat, like all substances, they contain point defects which are most usually found on the cation lattice as interstitial ions, vacancies or ions with a higher charge than dre bulk of the cations, refened to as positive holes because their effect of oxygen partial pressure on dre electrical conductivity is dre opposite of that on free electron conductivity. The interstitial ions are usually considered to have a lower valency than the normal lattice ions, e.g. Zn+ interstitial ions in the zinc oxide ZnO structure. 

If it is assumed that the ions and the interstitial ions are associated, i.e. not free to migrate in the lattice independently of one anotlrer, then we write for the equilibrium constant which involves the defect complex, — - U +... 

Fig. 1.76 Potential energy of an interstitial ion near the metal/oxide interface...

Pai Vemeker and Kannan [1273] observe that data for the decomposition of BaN6 single crystals fit the Avrami—Erofe ev equation [eqn. (6), n = 3] for 0.05 < a < 0.90. Arrhenius plots (393—463 K) showed a discontinuous rise in E value from 96 to 154 kJ mole-1 at a temperature that varied with type and concentration of dopant present Na+ and CO2-impurities increased the transition temperature and sensitized the rate, whereas Al3+ caused the opposite effects. It is concluded, on the basis of these and other observations, that the rate-determining step in BaN6 decomposition is diffusion of Ba2+ interstitial ions rather than a process involving electron transfer. 

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

Point defects in solids make it possible for ions to move through the structure. Ionic conductivity represents ion transport under the influence of an external electric field. The movement of ions through a lattice can be explained by two possible mechanisms. Figure 25.3 shows their schematic representation. The first, called the vacancy mechanism, represents an ion that hops or jumps from its normal position on the lattice to a neighboring equivalent but vacant site or the movement of a vacancy in the opposite direction. The second one is an interstitial mechanism where an interstitial ion jumps or hops to an adjacent equivalent site. These simple pictures of movement in an ionic lattice, known as the hopping model, ignore more complicated cooperative motions. 

Zn ->H +Zni, i.e. transition of electron from initially neutral H-atom into the interstitial ion of superstoichiometric zinc. According to Aishens and co-authors it is this formed Znt that is responsible for the... 

Depending on the electronegativity and the cation s oxidation state, the oxide can be either acidic or basic. Also, doping the ceria lattice does not automatically mean a substitution of a host cerium cation. Inomata and coworkers showed, in the case of Ce(i x)FexOy, that Fe3+ ions were located not only at Ce4+ sites, but also at interstitial sites (49). These interstitial ions bring about an increased barrier for electrons moving between Ce3+ and Ce4+, and influence the redox potential and the ease of reducibility of the material. 

The second possibility is that the second electron could interact with an interstitial ion to yield a second silver atom that would then diffuse to the first silver atom to form an identical cluster of two, namely ... 

One interstitial ion needs to be accommodated for each M3+ incorporated and two for each M4+ cation. 

Lattice defects in ionic crystals are interstitial ions and ion vacancies. In crystalline sodium chloride NaCl a cation vacancy Vn - is formed by producing a surface cation NaJ, (Nal - NaJ + Vua ) this is called the Schottky defect. On the other hand, in crystalline silver chloride AgCl a pair of cation vacancy Va,. and interstitial cation Ag is formed, (Ag - Agj + ) this is called the Frenkel... 

Fig. 3-12. Lattice defects and ion levels of ionic compound AB (a) ionnation of a pair of ion vacancy and interstitial ion, (b) A ion levels in ionic crystals. Va = A ion vacancy A] = intoatitial A ion Oa. = A ion level = unitary A ion level at lattice sites ...

Fig. 3 -13. (a) A ion levels at the surface and in the interior of ionic compound AB, and (b) concentration profile of lattice defects in a surface space charge layer since the energy scales of occupied and vacant ion levels are opposite to each other, ion vacancies accumulate and interstitial ions deplete in the space charge layer giving excess A ions on the surface. 

Conversely, when the surface ion level, a +, is higher than the interior ion level, aA>(AB)> surface charge is negative and an acoimulation layer of interstitial ions (a positive space charge layer) is formed. Obviously, when the siuface ion level, a +, is close to the interior ion level, almost no surface... 

A simple yet valuable starting point for treating ionic conductivity, tr, is as the product of the concentration, C(, of mobile species (interstitial ions or vacancies), their charge, q and their mobility, u, ... 

An important practical way of increasing the value of c, is by means of doping with aliovalent (or heterovalent) ions. This involves partial replacement of ions of one type by ions of different formal charge. In order to retain charge balance, either interstitial ions or vacancies must be generated at the same time. If the interstitials or vacancies are able to migrate, dramatic increases in conductivity can result. 

Whereas the ion does not compete strongly with Fe " ions for the A sites, the Cu" ion does. Moreover, the Cu ion can be fairly mobile in close-packed anion arrays, and we will suggest that accommodation of Cu in interstitial positions may be a source of some confusion about valence states. Therefore, let us consider first what evidence there is for the possibility of interstitial ions associated with the spinel structure. 

In point defect models, vacancies and interstitial ions may be responsible for point defects. These defects may be independent of both the composition and external conditions. 

As shown in Fig. 2.5, the cyclic voltammograms for Prussian blue attached to paraffin-impregnated graphite electrodes (PIGEs) in contact with aqueous electrolytes exhibit two well-defined one-electron couples. Prussian blue crystals possess a cubic structure, with carbon-coordinated Fe + ions and nitrogen-coordinated Fe + ions, in which potassium ions, and eventually some Fe + ions, are placed in the holes of the cubes as interstitial ions. The redox couple at more positive potentials can be described as a solid-state process involving the oxidation of Fe + ions. Charge conservation requires the parallel expulsion of K+ ions [77] ... 

A variety of techniques has been employed to investigate aliovalent impurity-cation vacancy pairs and other point defects in ionic solids. Dielectric relaxation, optical absorption and emission spectroscopy, and ionic thermocurrent measurements have been most valuable ESR studies of Mn " in NaCl have shown the presence of impurity-vacancy pairs of at least five different symmetries. The techniques that have provided a wealth of information on the energies of migration, formation and other defect energies in ionic solids are diffusion and electrical conductivity measurements. Electrical conductivity in ionic solids occurs by the motion of ions through vacancies or of interstitial ions. In the case of motion through vacancies, the conductivity, a, is given by... 

Fig. 12.1. The structure of La2Ni04 (65917) showing how the structure is composed of (LaO)2 and Ni02 layers. The + sign marks the position of the interstitial ion. The large open circles represent 0 , the small open circles Ni and the filled circles La +.

The addition of 0.18 interstitial ions to the formula unit of La2Ni04 requires that the oxidation state of Ni be increased to +2.36. Given that the equatorial Ni-O bonds have a length of 194 pm and therefore a bond valence of 0.46 vu, this increase in the oxidation state of Ni allows the axial bond valences to be increased from 0.08 to 0.26 vu reducing the length of the Ni-Oa iai bonds from 259 pm to the more acceptable value of 215 pm. This in turn reduces the valence required for the axial La-O bond by 0.18 vu which, together with the extra valence contributed by the interstitial 0 , reduces the distortion around La " " to an acceptable level. It is difficult to calculate the BSI and GII for this compound since one needs to know how the interstitial 0 ions are ordered within the LaO double layer, but clearly the BSI will be considerably reduced from the value 0.29 vu that it had before the introduction of the defect and subsequent electronic relaxation. This form of the structure is stable and is the form normally found when the material is prepared in air. 

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