Dielectric behaviour and optic mode Gruneisen parameters of the halides of silver,caesium and thallium

The optic mode Gruneisen parameters in silver, caesium and thallium halides are calculated using the Born model for interionic forces and the Szigeti theory of dielectric constants. The strain derivatives of the electronic and static dielectric constants are also evaluated and compared with experimental data. The strain derivative of static dielectric constant reveals the inadequacy of the Born model for the crystals under study. Possible modifications have been suggested to improve the situation. The theoretical values of the optic mode Gruneisen parameters closely agree with recent experimental data. An appropriate process has been adopted to evaluate the average values for the Gruneisen parameter.     

Electron distribution in silver halides

Solid fast ion conductors such as silver halides and silver chalcogenides have been recently drawing attention, because of their unusually high ionic conductivity in the high-temperature phase at about 200°C. This peculiar property is of particular importance for both research field of codensed matter physics and practical application as solid state sensors. The information of the bonding nature relevant to the electron distribution is desirable, in order to obtain insights into their characteristic property of solid fast ion conductors. An attempt will be made in this paper to review the current information of the bonding nature of silver halides in terms of the electron density distribution. Thus, this paper is primarily concerned with some physical properties of silver halides including the characteristic features of the crystal structure and the phase diagram. The data of alkali halides are also described for comparison. The experimental results of the magnetic susceptibilities of silver halides are summarized from the phenomenological point of view. The magnetic susceptibilities of silver halides were found to be sensitive to the change of temperature and crystal structure. It implies that the valence electrons of silver halides are loosely bound by the ion core. The electron density distribution in Agl and AgBr is also discussed using the results by single crystal X-ray diffractometry and the following points are suggested; in AgBr, the deformation of the charge distribution in bromine ion is much larger than that of silver ion and the radial distribution of valence electrons in bromine ion is enlarged in comparison with that of free ion state. In Agl, charge accumulation is feasible only on the c-directed bond and the difference Fourier map on the (1 1 0) plane indicates the deficiency of electrons around the silver site.     

Dimer centres in variously pretreated thallium doped sodium halides

A detailed examination of the luminescent characteristics of variously pretreated heavily doped NaBr:Tl phosphors has been carried out. A prominent emission band at 420 nm exhibited by the phosphor is attributed to the electronic transitions within Tl⁺ ion perturbed by the presence of another Tl⁺ ion occupying nearest neighbour alkali ion site, the centre so formed being known as dimer. It is rather surprising that the dimers are conspicuous by their absence in heavily doped NaCl:Tl phosphors. This aspect of the result is discussed on the basis of the relative sizes of the ions in the phosphors.     

Elastic constants of mixed ammonium halides

In the present communication a method based on the three body forces for the evaluation of second and third order elastic constants of mixed ammonium halides has been described. The Lundqvist three body potential is used along with long range Coulombian and short-range overlap repulsive potentials. The latter is taken of the Born-Mayer type and effective up to first neighbours only. It has been assumed that the charge transfer and short-range parameters depend linearly on the composition of solid solutions. Theoretical results obtained, for the entire range of compositions of mixed NH₄Cl and NH₄Br, have been compared with the recently measured experimental values and the theoretical results of other workers, whenever these are available.The authors are thankful to Dr. J. Shanker, Department of Physics, Agra University, Agra, for helpful discussions.     

Higher order elastic constants of alkali halides

The Coulomb, van der Waals and repulsive lattice sums occurring in the higher order elastic constants up to sixth order have been calculated for the rocksalt and cesium chloride structures. Numerical values of the static elastic constants up to sixth order based on a rigid ion model with van der Waals and Born-Mayer type central force interaction between first and second nearest neighbors are calculated for several alkali halides representing both structure types. Fair agreement with the available experimental third and fourth order elastic constant data is found.     

Optical properties of nanocrystalline silver halides

A review of quantum confinement effects in nanocrystals of silver bromide (AgBr) and silver iodide (AgI) is presented. AgBr is an indirect gap semiconductor while AgI has a direct band-to-band lowest energy transition. An examination of the low-temperature optical properties of quantum confined AgBr grown using a variety of synthetic techniques will be made. The dynamics of some of the involved excitonic processes will be measured and discussed in reference to a possible breakdown in the momentum selection rules as the nanocrystals are made smaller. Other explanations for this behavior such as impurity exclusion and surface effects will also be considered, as will the dynamics associated with the trapping of excitons at intrinsic iodide impurities in AgBr. Absorption measurements on AgI nanocrystals will be discussed and compared with the exciton photophysics in AgBr. Both AgBr and AgI display an increasing blue shift of their luminescence, arising from the recombination of excitons, as the crystallite size decreases. The luminescence intensity arising from this process increases with decreasing size in AgBr but it disappears in small crystals of AgI. This leads to the conclusion that in the latter material nonradiative decay channels are opening up as the size decreases.     

Electron-lattice interaction in the absorption spectra of thallium doped alkali halides

The electron-lattice interaction of NaCl:Tl⁺, KCl:Tl⁺, KBr:Tl⁺, and KI:Tl⁺ is discussed using the moments of the absorption bands. The discussion is based on a theory ofToyozawa andInoue andHonma. Consistency of the data is found for the absorption measurements. An analysis of the band shift under applied stress shows for theA-band in KCl and KBr and for theA- andB-band in KI that the electronlattice coupling constants derived from these data assuming next neighbour interaction differ considerably from those derived from the second moments of the bands.     

High pressure studies of sodium and silver halides

Abstract

We have investigated the structural phase transitions in sodium and silver halides theoretically under high compressions by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital (FPLMTO) method. Our results confirm the recent high pressure experimental observations of crystallographic phase transformations in sodium halides (Leger et al. (1998) J. Phys.: Condens. Matter, 10, 4201) and silver halides (Hull and Keen (1999) Phys. Rev., B59, 750. The calculated transition pressures agree with the experimental data.     

Surface space charge potential in thin silver halides

An analysis is made of the surface space charge layer in thin silver halides. The proposed model takes into account that the surface sites have a binding energy different from the normal lattice sites. A well defined number of surface sites is introduced in the thin film space charge potential calculations. Numerical computations have been performed for pure films and films doped with divalent ions which are homogeneous distributed on normal lattice sites.     

Phase stability and electronic properties of silver halides

In this work, we study the phase stability and electronic properties of silver halides ( AgBr, AgCl and AgI) using the full-potential linearized augmented plane wave method within the density functional theory. In this approach, the Wu–Cohen generalized gradient approximation was used for the exchange–correlation potential. Moreover, the modified Becke–Johnson approximation was also used for band-structure calculations. Various structural phase transitions were considered here in order to confirm the most stable structure and to predict the phase transition under hydrostatic pressure. In addition, we have studied the band structures of the stable phases of these compounds which reveal that the three compounds exhibit semiconducting behavior. The results obtained are compared with other calculations and experimental measurements.     

Theoretical investigations on structural, elastic and electronic properties of thallium halides

Theoretical investigations on structural, elastic and electronic properties, viz. ground state lattice parameter, elastic moduli and density of states, of thallium halides (viz. TlCl and TlBr) have been made using the full potential linearized augmented plane wave method within the generalized gradient approximation (GGA). The ground state lattice parameter and bulk modulus and its pressure derivative have been obtained using optimization method. Young's modulus, shear modulus, Poisson ratio, sound velocities for longitudinal and shear waves, Debye average velocity, Debye temperature and Grüneisen parameter have also been calculated for these compounds. Calculated structural, elastic and other parameters are in good agreement with the available data.     

Second-order elastic constants of copper and silver

The second order elastic constants of copper and silver have been calculated by the method of homogeneous deformation from a total energy expression. The overlap energy has been considered by Moriarty potential. A good agreement between computed and experimental values is found except C12 of silver.     

Localized modes of Li and Na impurities in silver halides

The absorption bands due to the infrared-active localized modes of Li and Na substitutional impurity in AgBr and of Li in AgCl are studied at 2 K. The isotope effects of the localized modes are observed in AgBr: Li and AgCl: Li systems. The high frequency sidebands of the Li⁶ localized mode in AgBr are also measured.     

The photoelectron spectra of gaseous silver halides

Ionization potentials have been determined for gaseous AgCl, AgBr and AgI using He(I) photoelectron spectroscopy. The solids were vaporized directly into the photon beam from a heated substrate. Molecular orbitals were assigned to observed spectral bands based upon simple molecular orbital arguments, band intensity relationships and observed spin—orbital splitting. The spectra of the gaseous silver halides are more complex than those of other diatomic halides as a result of Ag(d)-X(π) interactions which occur in these molecules.     

The formation volume of Frenkel defect in silver halides

The formation volume ν_f of a (cation) Frenkel Defect is calculated in AgCl and AgBr. The proposed formula for ν_f has no adjustable parameters and contains the formation enthalpy (of a Frenkel defect), the compressibility of the solid at absolute zero and the pressure derivative of the compressibility. The calculated values are in good agreement with the experimental results.