Interstitial pair conduction

The quotient of electrical conductivity a and thermal conductivity X is inversely proportional to the temperature. The ionic conductivity of solids depends on the lattice type and the type of the defects. The conductivity increases with temperature. This property is used to distinguish the ion conductor from the electron conductor. For vacancies and interstitials in the ion lattice, conductivity depends on the formation enthalpy for vacancy-interstitial pairs, Afo v. 

Fig. 4.3 Schematic representation of interstitial cationic pair formation (a) and migration from one non bridging oxygen to another in a cation conducting glass (b).

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... 

Oxides. The only certain lower oxides are NbO and Nb02. TaOx compositions from x = 2 to 2.5 comprise a Ta2Os phase with interstitial Ta atoms, and not discrete phases or compounds. Nb02 has a rutile-type structure with pairs of fairly close (2.80 A) Nb atoms, presumably singly bonded to each other. NbO, which has only a narrow range of homogeneity, has metallic luster and excellent electrical conductivity of the metallic type.14... 

Figure 1 is a schematic representation of Frenkel s notion an atom or ion can get dislodged from its normal site to form etn interstitial-vacancy pair. He further proposed that they do not always recombine but instead may dissociate and thus contribute to diffusional transport and electrical conduction. They were free to Wcuider about in a "random walk" mcuiner essentially equivalent to that of Brownian motion. . . this meant they should exhibit a net drift in an applied field. 

Electrons and holes can be produced by thermal motion or the absorption of light. Excitons are electron/hole pairs. Excitons are produced when an electron takes up energy, but not a sufficient amount to escape the hole produced. Consequently, the electronic charge of an exciton is zero. The exciton can transport energy but cannot conduct an electric current. Empty lattice sites are called site defects or vacancies. Atoms residing in sites between lattice ponts are called interstitial defects. 

When a positron enters a solid, it thermalizes in a time much shorter (few picoseconds) than its lifetime (few hundred of picoseconds). At the time of annihilation, the positron has diffused over a certain distance (few hundreds of nm). This diffusion process is critical to determine the state of the electron-positron pair at the time of annihilation the positron migrates in a region energetically favorable, namely the interstitial region of the lattice. The positron might also be trapped if it reaches a defect and experiences a longer lifetime. Otherwise, it annihilates with an electron in the valence or conduction band. Observing the annihilation radiation provides information on the electronic structure in momentum space. 

---