Hopping type

The simplest model involves ion hopping between sites on a lattice (not fixed as in a solid electrolyte such as Agl) with the ions obeying a hopping type equation... 

Compared with the momentum of impinging atoms or ions, we may safely neglect the momentum transferred by the absorbed photons and thus we can neglect direct knock-on effects in photochemistry. The strong interaction between photons and the electronic system of the crystal leads to an excitation of the electrons by photon absorption as the primary effect. This excitation causes either the formation of a localized exciton or an (e +h ) defect pair. Non-localized electron defects can be described by planar waves which may be scattered, trapped, etc. Their behavior has been explained with the electron theory of solids [A.H. Wilson (1953)]. Electrons which are trapped by their interaction with impurities or which are self-trapped by interaction with phonons may be localized for a long time (in terms of the reciprocal Debye frequency) before they leave their potential minimum in a hopping type of process activated by thermal fluctuations. 

As already demonstrated, in the given structures at x[Pg.624]

Soviet scientists have been particularly interested in impurity effects in ID metals. It was shown (INV 8) that impurities in a half-filled band give a singular enhancement in the density of states at the Fermi surface. This may be another manifestation of the well known impurity localization of states in ID. This latter implies (INV 13) that at T=0, o(to)-K) as u>-K). With increasing temperature, phonons allow a hopping type transport from one localized site to another, with increasing conductivity. At still higher temperatures, phonons scatter the electrons with a corresponding decrease in a(co). The theory developed fits quantitatively with experiments on TCNQ salts with structural disorder. 

In contrast, the electrical conductivity of Cc203 shows a clear break at approximately 950°C in the relationship of logarithnm of conductivity vs. 1/T. The conduction mechanism seems to be similar to that for mixed valence oxides by a kind of hopping-type conduction in the lower temperature region. On the contrary, the conduction in Ce203 resembles to other stable rare earth oxides in the higher temperature range. 

Electrical properties of thin rare earth oxide film have also been studied. The conductivity of praseodymium thin film oxide was measured as a function of temperature and oxygen pressure [9]. The oxide film was found to act as a p-type conduction at temperatures high than 630°C and was a n-type semiconductor at the temperatures of 400-630°C. Thermally evaporated EU2O3 thin film on a glass substrate is also obtained in an amorphous state. From the measurement of frequency dependence of the ac conductance, the predominant mechanism could be ascribed to the result of a hopping type. The ac conductivity measurements were also carried out for thin film of SC2O3 at temperatures between 4 and 295 K [10]. The conductivity was found to obey the relationship of ai(ffl)=Aco which depends on frequency and s is dependent on temperature and is a little lower than unity. By using a classical hop mechanism between randomly distributed localized states, a model was proposed and applied to scandium oxide with the assumption that the localized states are caused by lattice vacancies. The model is expected to be... 

Gileadi et al. showed that it was possible to obtain aluminum ion-conducting nonpolar aprotic solutions [54-56]. Aromatic solvents such as toluene and ethylbenzene can dissolve AlBrj or AljBrg and LiBr or KBr, thus forming clusters of M+ AIBr4 or M+AljBr," (M+ is the alkali metal ion) which conduct ions by a hopping-type mechanism (in contrast to the ion transport via frictional motion in polar systems). In such solutions it is possible to plate aluminum and thus form a surface film s free active metal electrode in aprotic media (provided that the aprotic media are sufficiently pure and do not contain atmospheric and/or protic contaminants) [57],... 

Organometallic and inorganic polymers have frequently been investigated as possible conducting polymers. An early trick was to look for extended conjugation and then introduce mixed valence states. This most often led to charge-hopping type semiconductors. Thus, both poly(ferrocenylacetylene) and poly(vinyl-... 

Such high concentrations of gap states attached to the valence band essentially affect the electronic charge transport in particular, they are responsible for the p-type character and the very low electrical conductivity. Aside from the electric conductivity in extended band states, a hopping-type conduction must be expected in localized gap states. The electronic properties of boron carbide can be consistently described by a band scheme, which highlights deep energy levels in the band gap (2.09 eV) at 0.065, 0.18, 0.47, 0.77, 0.92 and 1.2 eV (values based on optical measurements), related to the valence band edge. This allows the largely consistent description of all reliable experimental results [537]. 

---