Solid partial electrical conductivity

The diversity of EEP reactions on a solid surface can be illustrated by the survey if interaction between excited atoms of mercury and zinc oxide [186]. When atoms of Hg get to an oxidized surface of ZnO at room temperature, an increase in the semiconductor electrical conductivity take place (Fig. 5.3, curve 2). The electrical conductivity change signal is irreversible, and in case of an increase in the temperature, after the Hg flux is disabled, an additional increase in the electrical conductivity (curves 3 and 4) takes place. One can logically suppose that we are dealing here with partial reduction of zinc oxide according to the scheme... 

For the purposes of molecular electronics, the formation of solid-state multiple linear stacks, causing electron delocalization, favours electrical conductivity. Further, (partial) oxidation of such linear materials can produce non-stoichiometric (or mixed-valent) complexes like K175Pt(CN)4-1.5H20,9 and K1.62Pt(C204)2-2H20,8b which possess higher conductivity than their precursors. 

A metallic band structure is realized when the CT solids have a partial CT state and molecules form uniform segregated columns or layers. Figure 1 shows electrical conductivity data for 1 1 low-dimensional TTF TCNQ system, as a function of redox potentials [82]. The two lines a and b are related to the equation expressing the relationship between 7d, Ea, and the Madelung energy M 5) (5 = degree of CT) between partially charged component molecules (eq. 2) [83], where and Ea are... 

The electronic and optical properties of a solid may be influenced considerably by the state of the surface. The effects of the surface on optical properties will be discussed further in a later section at this point we will attempt only to demonstrate that the electrical conductivity may be partially controlled by the state of the surface. 

Cyanide complexes of platinum occur most commonly in the divalent state, although there has been increasing interest in the complexes formed with platinum in a higher oxidation state. Among the complexes most recently studied have been the mixed valent complexes where platinum cyanides in the divalent state are partially oxidized. These complexes form one-dimensional stacks with Pt-Pt interactions. In the solid state these materials show interesting electrical conductivity properties, and these compounds are discussed by Underhill in Chapter 60. In this section the preparative procedures and spectroscopy of the complexes will be covered, but for solid state properties the reader is referred to Chapter 60. 

The 1 1 solid adduct of tetrathiafulvalene (TTF) and tetracyanoquin-odimethane (TCNQ) is the first-discovered molecular metal, which consists of alternate stacks each composed of molecules of the same type (Fig. 4.1.3). A charge transfer of 0.69 electron per molecule from the HOMO (mainly S atom s lone pair in character) of TTF to the LUMO of TCNQ results in two partially filled bands, which account for the electrical conductivity of TTF TCNQ. 

A partially filled band or a band of vacant energy levels just higher in energy than a filled band a band within which, or into which, electrons must be promoted to allow electrical conduction to occur in a solid. 

A series of AgF+ salts (AgFMF4 with M = B, Au and AgFMF6 with M = As, Au, Ir, Ru, Sb, Bi) have been prepared. The first was AgFAsF6 [9] made by the interaction of AgF2 with AsF5 in aHF. They all have the weak, temperature independent paramagnetism indicative of a partially filled band and suggestive of metallic character [12]. In no case however has electrical conductivity of metallic type been demonstrated in any one of these solids. A wide variety of synthetic routes have been demonstrated for the preparation of (Ag—F)]]+ salts. 

Fig. 7. The electrical conductivities of binary rare-earth oxide fluorides, Ln-Ln 203F6 measured at 650 C under an oxygen partial pressure of 1.33 x 10 Pa. , more than 1 Sm 1 3 0.1-1 Sm Q, less than 0.1 S m (reproduced with permission from Solid State Ionics, 23 (1989) 99 [19]).

In the above radical-cation salts, the crystal contains partially oxidized donors, while the electroneutrality is achieved by the presence of closed shell anions. The structural requirements necessary for electrical conductivity in solid salts can also be met upon mixing of donors and acceptors in the resulting charge-transfer (CT) complexes both the donor and acceptor exist in a partially oxidized and reduced state, respectively. Famous examples are the conducting CT complexes formed upon mixing of perylene (112) [323. 324] and iodine or of tetrathiafulvalene (TTF, 119) as donor and 7,7,8,8-tetracyanoquinodimethane (TCNQ, 120) as acceptor [325-327] the crucial structural finding for the... 

These calculated electrolyte interface electrical potentials are then used as Dirichlet boundary conditions to determine the electrical potential field throughout the solid elements (solid interface elements and solid only elements) via the Laplace equation. For solid interface elements (which may be partially electrolyte) the electrical conductivity (Sm"1) is calculated via... 

As an example, we briefly describe here the observations made for Laj. Cax-Fe03. In this solid solution system, a single perovskite-brownmillerite intergrowth structure ( = 3) is foimd at composition x = % [224]. The intergrowth structure exhibits ideal stoichiometry, i.e. LaCa2Fe30g, at reduced oxygen partial pressures near the minimum observed in the electrical conductivity [199]. For any other composition, a disordered intergrowth is observed. 

The electrical conductivity of electrons in a solid depends on the ability of an electron to move to a higher energy level when accelerated by an electric field. The energy change is very small, so that only partially filled bands can conduct. In semiconductors thermal energy will promote a few valence-band electrons into the conduction band. These electrons can now move in the field. So can the electrons in the valence band whose energies are just below the levels of the promoted electrons. 

---