Ionic Covalent Crystals Research Articles

Spectrally controlled atom-by-atom photoassembly of silver clusters on the surface of ionic-covalent crystals

The fundamental possibility of implementing light-controlled atom-by-atom assembly of silver clusters beginning with dimers and trimers via photostimulated diffusion of individual atoms initially adsorbed on the surface of AgCl and ZnS single crystals by mass-spectrometric ion deposition is shown. Possible mechanisms of this process are discussed, including the charge exchange in adatoms via alternating capture of holes and electrons accompanied by the transition to neighboring, energetically more favorable adsorption sites; hopping displacements of adatoms over the surface by gaining kinetic energy upon nonradiative recombination of photocarriers from their levels; and low-barrier diffusion of short-lived negatively charged adatoms with a captured electron.

Low-threshold power limitation of optical radiation in crystals with sensitized anti-Stokes luminescence

This paper shows that it is possible in principle to implement low-threshold power limitation in ionic-covalent crystals with adsorbed metal-cluster-dye-molecule structures that possess sensitized anti-Stokes luminescence. The power-limitation effect for radiation with λ=660nm has been detected and investigated at a temperature of 77K for AgCl(I) nanocrystals 40-70nm in size with adsorbed dye molecules and few-atom clusters of silver, dispersed in a gelatine matrix. The effect was observed for radiation with a density of 1017-1019quant/cm2sec at a pulse width of 1ms. The Z-scanning technique was used to establish that the main limitation mechanism is self-focusing, which results from variation of the refractive index of the medium when the electron concentration on traps in the band gap of the nanocrystals varies.

Special features of the phonon spectrum and non-radiative transitions in the Cr 3+ -doped ionic–covalent crystals

A new model which describes the peculiarities of the phonon spectra of homo- and heterodesmic crystals is developed in the present paper. A salient feature of the model is that the specific properties of the heterodesmic compounds (existence of large covalent groups) are considered explicitly by inclusion of the corresponding terms into the dynamic equations. Numerical calculations of the phonon spectrum for the model heterodesmic crystal show a large gap (more than two orders of magnitude) between the optical and acoustic branches. Large variations of the non-radiative transition probabilities in doped homo- and heterodesmic crystals are explained by the effective lowering of the phonon state density in the latter case. Application of the model developed to the comparative studies of CeSc 3(BO 3) 4:Cr 3+ and LiSrAlF 6:Cr 3+ crystals resulted in good agreement with available literature data.

Photostimulated formation of anti-stokes luminescence centers in ionic covalent crystals

Photostimulated formation of anti-stokes luminescence centers in ionic covalent crystals

A Method for Determining the Ionization Spectra of Monodispersed Clusters of Noble Metals Adsorbed on the Surfaces of Ionic–Covalent Crystals

The method described is based on the vacuum deposition of separate monodispersed molecular metal ions from a mass-separated beam on the surfaces of ionic–covalent crystals and the detection of ultralow concentrations (108 cm–2) of adsorbed states using the method of photostimulated luminescence flash.

Electronic Structure of Interfaces and Extended Defects in Ionic Covalent Crystals

AbstractA consistent description is given in terms of the Green function method of electronic states associated with interfaces and extended defects in ionic covalent crystals. Closed expressions are derived for calculating the density and dispersion of impurity states. These expressions do not require the knowledge of wave functions of electrons, rather they operate with a limited number of Fourier transforms of Green functions of an ideal crystal. Formulas are given for calculating the electrostatic part of the potential at the defect and on the surface allowing an effective summation of lattice sums. The theory is illustrated by the example of an ideal chain of oxygen vacancies in AMO3‐type oxides.

Stationary and photostimulated luminescence of ionic-covalent crystals with adsorbed low-atomic silver and copper clusters

Stationary and photostimulated luminescence of ionic-covalent crystals with adsorbed low-atomic silver and copper clusters

Microscopic theories of order-disorder structural phase transitions in crystals

The status of the microscopic theories is reviewed for the following types of order-disorder structural phase transitions in crystals: a) phase transitions in crystals with hydrogen bonds; b) orientational phase transitions in ionic-covalent crystals; c) orientational phase transitions in molecular crystals; d) order-disorder phase transitions in ordered interstitial and substitutional alloys. Attention is focused primarily on the static properties, i.e., the nature of the ordering, the phasetransition thermodynamics, and the distinctive features of these phase transitions in comparison with other phase transitions, particularly magnetic ones. The methods employed in the theoretical description of order-disorder structural phase transitions are examined—the Hamiltonians used, approximate methods for calculating the statistical properties (with allowance for correlation effects), exactly solvable models, etc. The existing theories of the phase transitions in various real crystals, including problems and results, are critically discussed for phase transitions of types a) and b).

Binding forces in crystals having the sphalerite lattice

A check is made of the applicability of an interpolation equation proposed for the binding energy in mixed ionic-covalent crystals having the sphalerite structure. It is shown that the parameters of the equation can be adjusted for most such crystals to match the physical properties. The equation reveals that the covalent interaction makes a large contribution to the total lattice energy.