AgCl Crystals Research Articles

Molecular Orbitals of the Self-Trapped Holes and the Zero-Field Splitting of the Self-Trapped Excitons in AgCl Crystals

The admixture parameters of the ligand chlorine orbitals in the molecular orbitals of the ground state and excited states of the self-trapped holes in AgCl crystals are estimated from the hyperfine constants and g values of ESR experiments. Using the result of the analysis, the zero-field splitting, D , of the self-trapped exciton (STE) is calculated to be negative. Comparison with the experimental value shows the exchange energy between the hole and the electron in the STE to be ∼5 cm -1 .

The Quasimolecular Approach to the Electronic Structure Calculations for Silver and Copper Halides

AbstractThe quasimolecular large unit cell (QLUC) approach in the framework of complete neglect of differential overlap (CNDO) approximation is used for the one‐electron energy level and wave function calculations in symmetry points of the Brillouin zone of silver and copper halides AgX (X = F, Cl, Br), and CuX (X = Cl, Br). The parametrization scheme of the CNDO approximation is discussed. The results obtained are in agreement with the results of other band structure calculations and with the experimental data. The calculations of the impurity ion Cu+ in the silver site are also carried out in the framework of QLUC‐CNDO approach for AgCl and AgBr crystals. The impurity levels have t2g and eg symmetry and are situated near the top of the valence band. The centres of impurity bands, caused by the periodical distribution of defects in this model, are shown to converge with the increasing quasimolecule size to the energy levels of a single defect.

Interactions between self- and solute trapping of photocarriers in Pd-doped AgCl

EPR studies on ultraviolet-irradiated crystals of AgCl: Pd have confirmed earlier results on AgCl: Cu for the existence of an energy barrier in the self-trapping of the photohole. The height of this barrier is near 1.8 meV. Migration of the self-trapped hole was found to be athermal for temperatures below 30 K; above 35 K the self-trapped hole hops, with a diffusivity given by $D=7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\mathrm{exp}(\ensuremath{-}61 \frac{\mathrm{meV}}{\mathrm{kT}})$ ${\mathrm{cm}}^{2}$/sec. This suggests that the value of the electron transfer integral is about 1% of the energy of the phonons involved, that the bandwidth for the self-trapped hole is of the order of 2 meV, and that the binding energy of the hole is approximately 0.1 eV. The dependence on temperature of the efficiency of photoproduction of various trapped-carrier palladium centers was determined, and was correlated with the migration of the self-trapped hole. The presence of a small amount of ${\mathrm{Fe}}^{2+}$ "tracer" served to indicate those decay processes that were due to thermal release of trapped holes.

Dielectric and anharmonic behaviour of silver halides, thallous halides, alkaline earth fluorides, and lead fluoride crystals

AbstractAn analysis is made of the dielectric behaviour and the anharmonic contribution to the dielectric constants of AgCl, AgBr, TlCl, TlBr, CaF2, SrF2, BaF2, and PbF2 crystals. The volume and temperature derivatives of static and electronic dielectric constants, and of the energy gap between bonding and antibonding states are derived from the experimental data using Phillips‐Van Vechten dielectric theory. The temperature derivatives of dielectric constants at constant volumes are used to estimate the magnitudes of anharmonic contributions. The effect of anharmonicity is discussed in terms of different theories. The volume derivatives of static dielectric constant are calculated using the Born‐Meyer theory of interionic potentials. The calculated quantities are compared with available experimental data. The applicability of the Mott‐Littleton theory of dielectric polarization is also discussed for the crystals under study.

Analysis of Crystal Binding and Grüneisen Parameter in Cesium Halides, silver Halides, and Alkaline Earth Oxides

AbstractAn analysis of the crystal binding energies and the Grüneisen parameter γ is performed in CsCl, CsBr, CsI, AgCl, AgBr, MgO, CaO, SrO, and BaO crystals. Values of cohesive energy, the Grüneisen parameter, and its volume derivative are calculated for all the crystals under study. Different theories of γ are used and compared with each other. Various potential functions for the repulsive energy are used in the calculations. The van der Waals interactions are also taken into account. The potential parameters for the repulsive energy are determined using input data on interionic separation and bulk modulus. The Born‐Landé inverse power and the Born‐Mayer exponential forms are adopted for repulsive interactions upto second neighbours. The results obtained are compared with available experimental data.

Formation of Wavy Slip Bands in AgCl Crystals

Formation of Wavy Slip Bands in AgCl Crystals

Structure and annealing of cation Frenkel defects in silver halides after irradiation with thermal neutrons

Polarised 110Ag (I=1, T12/=24.6 s) nuclei were produced in the reaction 109Ag (npol, gamma )110Ag in AgCl and AgBr single crystals. The hyperfine interaction of the 110Ag nuclei with radiation-induced point defects has been studied. Positions and shapes of nuclear magnetic resonance curves could be explained assuming a random distribution of positively and negatively charged point defects. From this analysis and from additionally measured migration enthalpies, derived from annealing stages, the defects could be identified as cation Frenkel pairs. The nuclear quadrupole moment of 110Ag was determined as mod Q mod =0.24*10-24 cm2 by measuring the spin-lattice relaxation time of the 110Ag nuclei.

Compressibility coefficients and Moelwyn-Hughes parameters of solids under pressure

Expanding energy as a function of volume in a Taylor series and taking the Padé approximant, different equations of state have been obtained, which are first tested with Born-Mayer and logarithmic model solids. These equations of state are applied to compute pressure, compressibility and the Moelwyn-Hughes parameter as functions of volume for NaCl, AgCl and CsCl crystals. The results agree fairly well with the observed data.

Load relaxation studies of AgCl

Room-temperature load relaxation experiments were performed on single crystals of nominally pure, UV-illuminated, γ-irradiated, and Cd 2+-doped single crystals of AgCl. The resulting log σ-log ϵ curves, all of which are concave downward, were analyzed in terms of Hart's two branch rheological model. The constant hardness log σ-log ϵ curves obtained at several prestrain levels on nominally pure crystals can be translated into a single master curve demonstrating that the plastic deformation behavior of nominally pure AgCl at room temperature can be described by a mechanical equation of state. The log σ-log ϵ curves for UV-illuminated and Cd 2+-doped crystals deviate at low strain rates from those for nominally pure crystals suggesting that dislocation-precipitate and dislocation-point defect interactions are important at low strain rates in the UV-illuminated and Cd 2+-doped samples. At a given stress level, the log σ-log ϵ curves of the γ-irradiated crystals are more steeply curved than those of the nominally pure crystals and can be superimposed onto the curves for the nominally pure crystal by translation to higher stress and strain rate levels. These observations indicate that the high-temperature branch in Hart's model contributes more to the flow stress in the γ-irradiated samples than it does in the nominally pure crystals. In addition, the deformation behavior of crystals deformed along the [100] and [111] axes were studied in order to compare flow on the {110}〈110〉 and {001}〈11̄0〉 slip systems. The log σ-log ϵ curves for the [100] crystal are more steeply concave downward than those for the [111] crystals, indicating that the high-temperature mechanism in Hart's model is more important for flow on {110}〈110〉 than on {001}〈110〉.

Unified study of the lattice mechanical properties of the AgCl crystal

The silver halide crystals have many peculiarities in their physical properties which are uncommon among the simple ionic solids. The different theoretical models used to calculate the properties of this group of solids have concentrated on explaining one or the other of them. But these calculations have one major difficulty common to all. It has not been possible to give a consistent description of the dielectric properties and the dispersion of phonons in the framework of a single model. We have shown from a microscopic analysis of the energy expression of a system of ions occupying arbitrary positions that there exist two types of short-range polarization mechanisms, one arising out of the overlap of the unperturbed wave functions and another due to the perturbation of the wave function. These two effects taken together resolve the incompatibility mentioned above. Based on this analysis we have proposed a model in terms of which we have correlated the cohesion, the stability, the phase-transition properties, the elastic and the dielectric properties, and the phonon dispersion of the AgCl crystal. It is seen that the present model broadly reproduces all the characteristic features of this crystal. Moreover, we have tried to assess in a rough way the relative importance of the different interactions envisaged and their relation to the typicalities of this solid. Finally, some of the limitations of the present calculation are also pointed out.

An optically detected magnetic resonance study of manganese-doped silver chloride

ODMR spectra have been observed in manganese-doped AgCl crystals and precipitates at low temperatures. During crystal growth from the melt or during annealing some of the Mn 2+ is converted to Mn 3+, which is an electron trap. The low-temperature luminescence is interpretated as recombination between self-trapped holes and [Mn 2+nV Ag] donors.

Compression phenomena and Mach shock waves in fast nucleus-nucleus collisions

Central collisions between He, C and O projectiles in the energy range 0.2 GeV/nucleon to 4.2 GeV/nucleon and Ag and Br targets in 4 mu nuclear track detectors (AgCl crystals and nuclear emulsion) have been investigated. The measurements were directed towards effects at the target site where Mach shock phenomena were expected to be seen. Angular and energy spectra of target fragments partitioned into two groups with Z=1 and Z>or=2 have been measured up to energies of 30 MeV/nucleon. The angular distributions of the He group from events with a high number of slow secondaries show characteristic peaks varying with the projectile energy in a way consistent with the Mach shock model. The corresponding energy spectra extend to higher values in the angular region of the peaks, as expected from the model.

Interionic forces in AgF, AgCl and AgBr crystals

An interionic force model has been used in which the short range nearest neighbour and the next nearest neighbour repulsive interactions and the van der Waals’ interactions are balanced by the long range electrostatic forces. The nearest neighbour and the next nearest neighbour interactions are derived from the overlap integrals for outer shell electrons. The van der Waals’ interactions are estimated from the Slater-Kirkwood variational method. The cohesive energy, the bulk modulus and its pressure dependence for AgF, AgCl and AgBr crystals have been calculated and compared with experimental data.

Ruthenium (III) -doped silver bromide and silver chloride: An EPR study

Ru3+ and Ru2+ ions are substitutionally incorporated into melt-grown crystals of AgBr and AgCl. The d5 trivalent ion predominates in samples prepared in oxidizing environments, such as Br2 and Cl2, and it has been the subject of a detailed magnetic resonance study. It is distributed between four structurally inequivalent lattice sites in both salts. These sites are distinguished by the arrangement of charge-compensating silver-ion vacancies in the two closest cation subshells. Reasonable dopant-vacancy configurations have been deduced from the EPR data.

Gruneisen parameters of some cubic crystals using third-order elastic constants

The third-order elastic constants of 26 cubic crystals and two mixed crystals belonging to octahedral class have been evaluated from the available literature data on the hydrostatic pressure variation of the corresponding second-order constants using the method elaborated [O. H. Prasad and M. Suryanarayana Acustica 38, 192–193 (1977)] wherein the third-order constants can be evaluated without any reference to uniaxial stress measurements. The evaluated third-order constants for each of the crystals are used in the well-known equations [W. P. Mason and T. B. Bateman, J. Acoust. Soc. Am. 36, 644–652 (1964)] to obtain the 39 mode gammas all of which add up to give the Gruneisen parameter γ of the crystal in question. The correctness of the evaluated parameters is verified by evaluating the thermal expansivity α of each crystal using the familiar equation connecting α with γ The crystals studied are NaCl, NaBr, NaF, LiF, KCl, KBr, KI, KF, RbCl, RbBr, RbI, AgCl, AgBr, MgO, CsBr, Nb, Fe-α, Ag, Cu, Au, Al, Ge, Si, ZnSe, ZnTe, RbAg4I5, and mixed crystals AgCl-AgBr and KCl-KBr near room temperature and in a few cases at different temperatures and pressures.

Evaluation of the exchange charge polarization in AgCl and AgBr crystals

We have evaluated the exchange charge polarization in AgCl and AgBr crystals using the overlap integrals for outer shell electrons. An analysis of the dielectric behaviour of these crystals is then presented within the framework of the exchange charge shell model. It has been possible to find a qualitative and quantitative description of the dielectric properties of AgCl and AgBr.

Thermoluminescence in Ag(Cl, I) mixed crystals

The distribution of traps in AgCl + 1% mol AgI mixed crystals was obtained by thermoluminescent fractional glow technique. The distribution reveals a prominent maximum at 240 meV. These traps are characteristic of mixed crystals. Shallow traps also exist, but they appear to be connected with host AgCl crystal. The nature of the deep trap in mixed Ag(Cl, I) crystals is discussed on the basis of the fundamental optical properties of pure AgI and AgCl crystals.

Direct observation of the path of ultrasound wave propagation in AgCl crystals by the "print-out effect"

The effect of ultrasound waves on the "print-out effect" in AgCl cystals was studied at room temperature. When a longitudinal stationary ultrasound wave existed in crystals subjected to electric field and light pulses, black planes caused by the print-out effect were produced. These planes were located perpendicular to the direction of the wave propagation. The distance between the darkened planes roughly equaled a half-wavelength of the ultrasound. They were arranged in nodal planes. It is concluded that these planes are the result of a print-out effect due to fresh dislocation moved by the shear stress of stationary longitudinal waves. The trace of the ultrasound wave propagation was directly observed by the darkening, which is the first direct observation of the wave propagation trace in solids. The effect of a traveling wave on the print-out effect was not observed.

Kinetics of laser excited luminescence and photovoltage in AgCl crystals

Kinetics of laser excited luminescence and photovoltage in AgCl crystals

Optically-detected EPR of excitons in AgCl, AgCl:Br and AgBr

Under irradiation with X-rays at 4K AgCl emits a broad band peaking at 2.52 eV. This luminescence shows magnetic circular polarization with a resonance in low magnetic field. EPR has been detected in the excited state giving rise to the emission. This indicates that the self-trapped exciton is the source of the emission and that self-trapping involves the 4d wavefunction of the silver ion. In AgCl crystals containing about 2 mole per cent of bromine an additional centre emits in the 2.52 eV region and excited-state EPR shows that this is due to electron-hole recombination at silver sites perturbed by bromine impurities. AgBr emits luminescence at 2.14 eV at 4K and EPR of the excited state has been measured.