Interstitial oxygen

Shallow donors (or acceptors) add new electrons to tire CB (or new holes to tire VB), resulting in a net increase in tire number of a particular type of charge carrier. The implantation of shallow donors or acceptors is perfonned for tliis purjDose. But tliis process can also occur unintentionally. For example, tire precipitation around 450°C of interstitial oxygen in Si generates a series of shallow double donors called tliennal donors. As-grown GaN crystal are always heavily n type, because of some intrinsic shallow-level defect. The presence and type of new charge carriers can be detected by Flail effect measurements. 

Another example of the effects of non-stoichiometry on diffusion is to be found in U02+. Here the defect consists of interstitial oxygen ions of high... 

Observed Calculated for Interstitial Oxygen Calculated for Iron Vacancies... 

Assume that the iron atoms in the crystal are in a perfect array, identical to the metal atoms in the sodium chloride structure, and that the 0.058 excess of oxygen is due to interstitial oxygen atoms being present, over and above those on the normal anion positions. The unit cell of the structure now contains 4 Fe and (4 x 1.058) O. The density is calculated to be 6076 kg m-3. 

If the interstitial oxygen atoms are transformed to oxide ions, electrons must be provided, generating two holes ... 

Figure 4.8 Structure of a Willis 2 2 2 structure (a) an empty Og cube (b) the stacking of four Og cubes in the UO2 structure (c) an Og cube containing a < 111 > interstitial oxygen ion (1d) an Og cube containing a < 110> interstitial oxygen ion and (e) the 2 2 2 cluster.

Both the oxides (3- and (3"-alumina show extremely high Na+ ion conductivity. As the structure suggests, the conductivity is anisotropic, and rapid sodium ion transport is limited to the two-dimensional conduction plane. There is almost unimpeded motion in the Na+ layers, especially in (3"-alumina, which lacks interstitial oxygen ion defects in the conduction plane, and the conductivity is of the same order of magnitude as in a strong solution of a sodium salt in water. The conductivity is a... 

The material would be expected to be a hole (p-type) semiconductor. However, in this compound the interstitial oxygen ions can diffuse fairly quickly, and the oxygen diffusion coefficient is higher than normal, so that the compound shows both high oxygen diffusivity and electronic conductivity, a situation referred to as mixed conductivity (Section 8.7). 

The closely related phase La2Cu04 (Section 4.3.3) is similar. In both of these materials there is strong evidence to suggest that the interstitial oxygen atoms are not distributed at random but are partly or completely ordered, depending upon preparation conditions. This has significance for high-temperature superconductors (Section 8.6). 

Wan J, Goodenough JB, and Zhu JH. Nd2 xLaxNi04+5, a mixed ionic/electronic conductor with interstitial oxygen, as a cathode material. Solid State Ionics 2007 178 281-286. 

Intrinsic Frenkel disorder, in which some of the oxygens are displaced into normally unoccupied sites, is responsible for the oxide ion conduction in, for example, Zr2Gd207, Fig. 2.11. The interstitial oxygen concentration is rather low, however, and is responsible for the low value of the preexponential factor and for the rather low (by -Bi203 standards ) conductivity. 

T.Q. Duong, C. ladecola, S.G. Kim, Effect of hyperoxia, hypercapnia, and hypoxia on cerebral interstitial oxygen tension and cerebral blood flow, Magn. Reson. Med. 45 (2001) 61-70. 

Figure 5.16 Willis clusters in UOj+y Open circles, oxygens small filled circles, uranium atoms in the UO2 structure. A 2 2 2 defect cluster consists of two interstitial oxygen atoms (large filled circles) and two normal oxygen atoms displaced from their ideal positions (open squares) to new interstitial positions (hatched circles). A 2 1 2 defect cluster would consist of one interstitial oxygen and two normal oxygens displaced from their ideal positions. (After Anderson, 19726.)...

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