Defect cation interstitial

Frenkel defects (Cation vacancy plus same cation as interstitial)... 

Similarly, in thermal equilibrium, some ionic crystals at a temperature above absolute zero enclose a certain number of Frenkel pair defects, that is, anion and cation interstitials in the structure. Since the concentration of Frenkel pair defects at equilibrium at an absolute temperature, T, obeys the mass action law, then [16]... 

Note in Fig. 4 that there are four primary ionic defect species which we must consider, namely cation interstitials, cation vacancies, anion interstitials, and anion vacancies (denoted by the superscripts ci, cv, ai and av, respectively). In the case of non-simultaneous place exchange, referred to... 

Fig. 7. Schematic representation of charged cation interstitial (ci) and anion interstitial (ai) bulk concentration profiles within the oxide, leading to defect currents of cation and anion interstitials and subsequent chemical reaction leading to a continual increase in oxide layer thickness, L, with time, t.

As an example, it is conceivable that in some metal—oxide systems, the cation interstitials entering the oxide at the metal—oxide interface (x = 0) rise to appreciable bulk concentration values C(cl) (0). These defects can then migrate through the oxide layer. Chemical reaction of any such interstitials which happen to reach the oxide—oxygen interface (x = L) will serve to deplete the number at that interface. Thus the bulk concentration C,(d)(L) will be much lower than the number Ccation interstitials from x = 0 to x = L can be expressed as a particle current density J(C1). This particle current density proceeding from the source interface (x = 0) to the sink interface (x — L) can be essentially uniform (i.e. J(ci) independent of position x) if there is no build-up or depletion of the bulk density C(ci) between source and sink. On the other hand, any build-up or depletion of the bulk density C(x) at a position x within the layer will require the current to decrease or increase, respectively, at that position x in order to supply or take away, as the case may be, the requisite number of such defects. 

Fig. 9. Electric field polarities produced by the easy diffusion of various charged defect species, (a) Cation interstitials (b) anion interstitials (c) cation vacancies ...

Intrinsic point defects are deviations from the ideal structure caused by displacement or removal of lattice atoms [106,107], Possible intrinsic defects are vacancies, interstitials, and antisites. In ZnO these are denoted as Vzn and Vo, Zn and 0 , and as Zno and Ozn, respectively. There are also combinations of defects like neutral Schottky (cation and anion vacancy) and Frenkel (cation vacancy and cation interstitial) pairs, which are abundant in ionic compounds like alkali-metal halides [106,107], As a rule of thumb, the energy to create a defect depends on the difference in charge between the defect and the lattice site occupied by the defect, e.g., in ZnO a vacancy or an interstitial can carry a charge of 2 while an antisite can have a charge of 4. This makes vacancies and interstitials more likely in polar compounds and antisite defects less important [108-110]. On the contrary, antisite defects are more important in more covalently bonded compounds like the III-V semiconductors (see e.g., [Ill] and references therein). 

If the semiconductor is an ionic solid, then electrical conduction can be electronic and ionic, the latter being due to the existence of defects within the crystal that can undergo movement, especially Frenkel defects (an ion vacancy balanced by an interstitial ion of the same type) and Schottky defects (cation and anion vacancies with ion migration to the surface). This will be discussed further in Chapter 13, as ionic crystals are the sensing components of an important class of ion selective electrodes. 

Figure 5 Defect cluster structures in U02+j - The parent structure is built from cubes with an oxygen ion at each comer (a), alternating with similar cubes containing a metal cation. Interstitial oxygen ions (dark circles), occupy empty cubes, but are displaced from the center along (111) (b), or (110) (c). The 2 2 2 cluster consists of four such cubes, two of which contain (111) interstitial oxygen ions and two of which contain (110) interstitial oxygen ions. The oxygen vacancies are marked V (d)...

Cation interstitials provide an alternative way to create positively charged defects. These interstitials may be formed by transfer of cerium cations located on the surface to an interstitial position and by the removal of two anions to the gas phase for each cerium interstitial formed. The process can be simply represented in the following equation ... 

Figure 6.3 Formation of cation interstitial in a Frenkel defect.

Figure 12.8 Binary ionic crystal showing defects that can lead to lattice diffusion, (a) Frenkel defect vacancy-interstitial pair), (b) Schottky defect (anion-cation vacancy). (After Kingery ct a .. 1976.)...

The electrochemical growth of a continuous and homogeneous film can be well thought-out as a result of the migration of metallic cations by means of one of the above-mentioned transport processes. It is worthy to note that the dislocation of ions breeds defects (vacancies, interstitial ions, etc.). Hence, either the dislocation of defects or the migration of atoms can be used to describe the transport process [1], Mathematical details are found in the cited references and also in the reviews [2,3]. 

In accordance with these regularities the electrical conductivity of oxide interlanthanoids is determined by interaction of stoichiometric and impurity defects. Variation of P02 in gas environment which is in equilibrium with the oxide alters the kind of compensation of charge mismatch of impurity defects - from compensation only by ionic defects - cationic and anionic vacancies, interstitial ions up to compensation only by electrons or by holes contributing to the relative component of electroconductivity. 

Similar to H+ implantation, optimized MgO thin films have been implanted with 1.5 MeV Li+ ions for various fluences (lO MO ions cm ). Irradiation of crystalline MgO with energetic metal ions produces stable vacancies and interstitials in the anion sublattice. Elastic collisions with energetic particles also produce cation vacancies, but these defects do not survive because the cation interstitials quickly recombine with the vacancies. Optical absorption bands can monitor these defects induced by ion implantation. Similar observations have been already done on MgO crystals after neutron irradiation (Kappers et al. 1970, Monge et al. 2000). In crystalline MgO material irradiated with Li+ ions, the well-known defects are (1) oxygen vacancies (primarily the one-electron F center), (2) oxygen divacancies Fj, (3) V and V centers (cation vacancies that have trapped one or two holes, respectively) produced by the capture of holes by existing vacancies, and (4) an unidentified defect that absorbs at 2.16 eV (572 nm) (Gonzales et al. 1991). 

An n-type cation interstitial metal-excess oxide semiconductor has an excess of interstitial cations or oxygen anion vacancies in the crystal lattice. An excess of electrons maintains the neutrality and electrical conductivity. The oxygen vacancies are created from single-point defects. The nonstoichiometric oxide MO2-X with large cations is oxygen deficient and creates anion vacancies. 

The Oxide as a Semiconductor. In addition to direct reduction of RX via pits or other defects in the oxide layer, it is possible that reduction occurs indirectly, by transport of charge through the oxide from Fe to RX. Possible charge carriers within the oxide include (i) anion and cation vacancies (lattice sites where ions are absent), (ii) anion and cation interstitials (sites where ions are imbedded between lattice sites), and (iii) electrons and holes (52). In this section, we will focus on electrons as the charge carriers and examine the transport of electrons through the oxide layer, treating the oxide as a semiconductor. 

Anti-Schottky defects Cations on interstitials are compensated by anions on interstitials (Fignre 1.16d). 

Figure 1.16 (a) Frenkel defect, vacancies in the cation lattice are compensated by cations on interstitials (b) anti-Frenkel defect, vacancies in the anion lattice are compensated by anions on interstitials (c) Schottky defect, vacancies in the cation lattice are compensated by vacancies in the anion lattice and (d) anti-Schottky defect, cations on interstitials are compensated by anions on interstitials. 

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