Band theory distortion

Recombination luminiscence is used, of course, to a limited extent, in luminiscence analysis of inorganic substances to detect trace amounts of a large number of elements. Recombination crystallophosphors, for example sulfides, are ionic crystals consisting of matrix (CaS, SrS, ZnS, CdS and others) whose crystalline lattice may easily suffer distortion on the introduction of minute amounts of metal ions (like Ag+, Cu2+, Mn2+ etc.) called activators. The mechanism of this luminiscence is explained in terms of the band theory of solids54 56). 

The recent experimental confirmation of the existence of one-dimensional metallic systems has led to a rapid increase in the experimental and theoretical study of these conducting systems. The objective of this section is to acquaint the reader with the physical basis of the concepts currently being used to explain the experimental results. Emphasis is given to the development of one electron band theory because of its central importance in the description of metals and understanding the effects of lattice distortion (Peierls transition), electron correlation, disorder potentials, and interruptions in the strands. It... 

Although SrTiOs, which undergoes a spontaneous distortion at low temperature, has received recent attention in band theory calculations (see Section VII. B), few of these interesting systems have yet been thoroughly investigated by many of the techniques described in this article. 

In band theory considerations, electrons are strictly delocalized, however, in many materials, it is better to view electronic defects as located at particular sites. The additional electron from the phosphorus donor atom in silicon, for example, could be viewed as not completely delocalized but located on the Si atom according to p/.-I- Si j. In such cases, the electronic model moves from band-type to polaronic-type model (a polaron is an electronic point defect and is formed when an electron has a bound state in the potential created by a distorted lattice). 

Simply put, a conductive organic polymer, or for that matter any organic polymer, is not a metal, although many of the theories used to explain the electrical nature of ICPs are based on our understanding of conduction mechanisms in metals and semiconductors. Band theory, polarons, bipolarons, thermopower, etc., are but a few examples of the models borrowed from solid-state chemistry and physics to explain the observed electronic behavior of these materials. Lattice distortions that... 

Basically, when analysing the band structures, the equivalent observations apply to typical solid state compounds like thallium halides and lead chalcogenides. In studies on the origin of distortion in a-PbO, it was found that the classical theory of hybridization of the lead 6s and 6p orbitals is incorrect and that the lone pair is the result of the lead-oxygen interaction [44]. It was also noted... 

V-UV Application First Excited State of Linear Polyenes. The first electronic absorption band of perfect linear aromatic polyenes (CH)X, or perfect polyacetylene shifts to the red (to lower energies) as the molecule becomes longer, and the bond length alternation (BLA) would be zero. This was discussed as the free-electron molecular orbital theory (FEMO) in Section 3.3. If this particle-in-a-box analysis were correct, then as x > oo, the energy-level difference between ground and first excited state would go to zero. This does not happen, however first, because BLA V 0, next, because these linear polyenes do not remain linear, but are distorted from planarity and linearity for x > 6. 

---