Point defect: also intrinsic

Chapter 2) and cannot be eliminated from the solid. They are called intrinsic point defects. This residual population is also temperature dependent, and, as treated later (Chapter 2), heating at progressively higher temperature increases the number of defects present. 

Movement through the body of a solid is called volume, lattice, or bulk diffusion. In a gas or liquid, bulk diffusion is usually the same in all directions and the material is described as isotropic. This is also true in amorphous or glassy solids and in cubic crystals. In all other crystals, the rate of bulk diffusion depends upon the direction taken and is anisotropic. Bulk diffusion through a perfect single crystal is dominated by point defects, with both impurity and intrinsic defect populations playing a part. 

Let us first discuss intrinsic disorder types where the number of moles of the components is almost constant and independent of the component activities. Thus, the majority point defect concentrations are also (almost) independent of the component. activities. It follows that only two types of (intrinsic) defect formation reactions are allowed... 

Point defects are an important part of the work in this paper. There are many reasons for the formation of point defects in minerals and their presence can exert important perturbations on the properties of the material (4). Point defects are formed because of the thermally driven intrinsic disorder in a lattice, the addition of aliovalent impurities or dopants, the presence of metal-nonmetal nonstoichiometry, and the creation of nonideal cation ratios. The first three source of defects are well-known from binary compounds but the last is unique to ternary compounds. Ternary compounds are much more complex than the binary compounds but they also have gained a great deal of attention because of the variety of important behavior they exhibit including now the presence of superconductivity at high temperatures. The point defects can be measured by introducing probe ions into the lattice. 

There are many factors which contribute to dielectric loss and in the case of the complex ceramic compounds discussed above, to achieve a satisfying understanding of the relative magnitudes of the various loss mechanisms is challenging. There will be contributions to loss intrinsic to the idealized structural chemistry of the material and it is now clear that this is complicated by a domain structure. There will also be contributions of an extrinsic nature, particularly those associated with departures from the ideal structure, point defects and... 

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). 

Balanced populations of point defects do not alter the anion to cation ratio, or overall stoichiometry of the crystal. In addition, the numbers of such defects must be such as to maintain charge neutrahty. Calculations similar to those for monatomic solids show that the free energy of crystals containing such balanced populations of point defects is lower than that for a defect-free crystal, and these defects are also intrinsic defects, occurring in pure crystals under equilibrium conditions at all temperatures above OK. Two important examples of such balanced point defect populations have been described. 

The given effect is reversible, i.e. after the external loading was removed the equalization of concentration of point defects over the specimen s volume takes place. Later on it was shown by Kosevich [7] that the intrinsic defects of a crystal (vacancies and interstitials) can also diffuse in an inhomogeneous field of external stress, and the forces that act on them are equal accordingly to... 

Fig. 2 illustrates (he room-temperature photoluminescence (PL) spectra recorded from the as-prepared ZnO colloidal solution and the ZnO nanostructure formed after deposition of the colloid on the silicon substrate. An UV band at 385 nm was detected from all ZnO products. In addition, a broad orange-red photoluminescence band centered at around 620 nm could be also observed in some materials. The UV photoluminescence peak at 385 nm is well known to be related to the exciton emission, ihe mechanism of visible emission is suggested mainly due to the present of various point defects, either extrinsic or intrinsic, which can easily form recombination centers. Photoluminescence measurements show that the deposited ZnO nanostructures have the stronger UV emission than the ZnO nanoparticles in the colloidal solutions. The better UV emission characteristic of deposited ZnO is suggested to be due to the lower defect density and oxygen vacancies in ZnO nanocrystals in the first case. Similar results have also been reported previously [8]. In addition, the aqueous surrounding can change the surface states of ZnO nanocrystals. It is well known that surface states may... 

Fig. 3.3. Illustration of the main proton transfer mechanisms (a) defect mechanism in a densely packed structure (b) loosely packed structure with a high concentration of mobile species (c) quasi-liquid state with a proton jump contribution In (a) the conductivity is favoured by intrinsic (interstitial rabbits) or extrinsic (impurity elephant) point defects. An orientation defect (hippopotamus in the wrong orientation) can also favour disorder of rabbits (Oj for Zr02 CaO, H for KHSO4) (b) the tree sublattice is a perfectly stable loosely packed structure and a high rabbit disorder can exist without affecting the host lattice (e.g. NH4 in p-AljOj) (c) only the mobile species sublattice is considered here these entities are moving with different speeds in different directions and some are hopping such may be the image of a quasi liquid or surface liquid (V205.nH20, HUP).

In additimi to foreign cation and anion dopants, native point defects are also usually present in the material. Examples are vacancies, interstitials, or substituents. These defects can have a similar influence on the optical, electrical, and catalytic properties as dopants. They are formed by intrinsic defect-chemical reactions, or by a change in the lattice stoichiometry due to exchange of, e.g., oxygen with the gas phase. Since virtually every defect affects the performance of the material in some way, the ability to understand and predict the relatiruiship between dopants and defect concentrations is of paramoimt importance for designing efficient photoelectrodes. 

The point defects, in turn, are classified as native (intrinsic) and substitution defects. The intrinsic point defects appear as a vacancy (the absence of an atom in a crystal lattice position) or as an interstitial defect (the presence of the host crystal atom in an interstitial position). The host crystal atoms can be substituted for another atom of a different chemical species at a regular lattice site or at the interstitial position (impurity center or substitution defect). The point defects can also be classified as neutral and charged relative to the host crystal lattice. The perturbation of a solid by... 

Point defect is the major subject and key problem for the studies of solid chemistry. It is also the most interesting subject in catal dic researches. In solid chemistry, we would discuss mainly the formation and equilibrium of point defect, the changes of current carriers (electron and cavity) and the influences of it on solid properties caused by the existence of point defect and how to control the types and concentrations of it in solid. Point defects can be divided into intrinsic defects and impurity defects. 

The intrinsic defects fall into two main categories, i.e., Schottky disorder and Frenkel disorder. As these point defects do not change the overah composition, they are also referred to as stoichiometric defects. Their thermal generation will be exemphfied for a metal oxide MO using the Kroger-Vink notation, and assuming that activities of point defects are equal to their concentrations. Hence, the law of mass action is apphcable to these equilibria... 

We have previously established (i) that, at finite temperatures, ionic and electronic point defects are required as local chemical excitations at equiUbrium, (ii) that we can write ideal mass action laws in all cases for low concentrations of defects, and (iii) that we know what parameters influence our mass action constants. We can now turn to a specific consideration of defect chemistry. Let us consider first internal defect reactions and pure single crystals By internal defect reactions in pure substances we mean processes that occur as a consequence of nonzero temperature in the otherwise perfect crystal without neighbouring phases being involved. (For two of these reaction tjrpes, however, we will need surfaces as sinks or sources of monomeric units, i.e. of lattice molecules.) In binary systems such processes leave the composition within the sohd undhanged. If we refer to the Dalton composition , we also speak of the intrinsic case. 

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