Band structure halides

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... 

For the alkali metal halides the influence of the anion is practically non-existent as can be seen from the comparison of RbCl, RbBr and Rbl (Table 3). In the more covalent structures both atoms appear to be effected by increase in pressure, as changes in the band structure lead to an increase in conductivity. 

The decomposition products of zirconium halides heated under ammonia arc a blue nitride with the rocksalt structure 6-Zi N (yellow at perfect stoichiometry) and a brown compound of composition Zr,N4 both are diamagnetic, but 6-ZrN is a metallic conductor (superconducting Tc -- 10 K). whereas Zr, N4 is an insulator [286, 287], Band structure calculations suggest ZrN Zr, N4 would form a good Josephson junction [288],... 

An electron in a solid behaves as if its mass [CGS units are used in this review the exception is for the tabulation of effective masses, which are scaled by the mass of an electron (m0), and lattice constants and radii associated with trapped charges, which are expressed in angstroms (1A = 10 8 cm)] were different from that of an electron in free space (m0). This effective mass is determined by the band structure. The concept of an effective mass comes from electrical transport measurements in solids. If an electron s motion is fast compared to the lattice vibrations or relaxation, then the important quantity is the band effective mass (mb[eff]). If the electron moves more slowly (most cases of interest) and carries with it lattice distortions, then the (Frohlich) polaron effective mass (tnp[eff]) is appropriate [11]. The known band effective and polaron effective masses for electrons in the silver halides are listed in Table 1. The polaron and band effective masses are related to a... 

An important distinction between the alkali azides and ionic materials, such as the halides, is the susceptibility of the azides to point-defect production by UV radiation. There has been little research concerned with understanding how UV radiation produces individual point defects in the azides. Clearly the presence of the molecular anion, NJ, is important, and many of the defects are dissociation products of the azide ion. However, just the presence of a molecular anion is not sufficient. Cyanates, for example, do not have defects produced in them by UV light even though the NCO is isoelectronic with N3 [35]. It is important to consider the detailed electronic structure of the azide ion as well as the electronic structure of the lattice. Such factors as the energy of the first excited state with respect to the ground state, the proximity of an unbound state of the NJ to the lower excited state, the electron affinity and the ionization potential, as well as the band structures of individual azides, are important. 

We use a periodic supercell model based on the large unit cell (LUC) method [22] which is free from the limitations of different cluster models applicable mainly to ionic solids, e.g., alkali halides. The main computational equations for calculating the total energy of the crystal within the framework of the LUC have been given in Refs. [22-24]. Here, we shall outline some key elements of the method. The basic idea of the LUC is in computing the electronic structure of the unit cell extended in a special manner at k = 0 in the reduced Brillouin zone (BZ), which is equivalent to a band structure calculation at those BZ k points, which transform to the reduced BZ center on extending the unit cell [22]. The total energy of the crystal is... 

In the second section this electron counting scheme is rationalized in terms of recent band structure calculations. The limitations of the ionic model as well as a variety of structural and electronic relationships for metal-rich halides are also discussed using these theoretical treatments. 

In reality, typical solids of interest occupy a spectrum between these two extremes, from the alkali halides, which are well described by TB theories, to Al, which is well described by NFE theories. (It is not, in fact, straightforward NFE theory, but rather pseudopotential theory, which is successful for Al, but for the moment this distinction will be ignored.) Intermediate between the two cases are, for instance, group IV semiconductors, in which both points of view have proved useful, and transition metals, in which different parts of the band structure conform to NFE and TB descriptions. 

---