AgBr Crystals Research Articles

Spectroscopic studies of Pr^3+ ions in silver halide crystals

The luminescence of AgBr and AgBrCl crystals doped with Pr(3+) rare-earth ions was investigated in the visible and infrared spectral ranges. We measured the excitation, emission, and absorption spectra as well as the kinetic parameters over a broad temperature range. The strong influence of the silver halide composition on Pr(3+) ions spectroscopic properties was revealed. We calculated useful optical parameters for Pr:AgBr crystals by using the Judd-Ofelt approximation. Good agreement between the theoretical and the measured parameters was obtained.

Rare earth ion diffusion in AgBr crystals

The luminescence of silver bromide crystals doped with rare earth ions (Nd 3+ and Er 3+) was investigated in the visible and near infrared spectral ranges. Crystal doping was produced by growing in the melt and by a diffusion method. The diffusion profile of rare earth ions in AgBr crystals was measured by the luminescence method. The luminescence parameters of AgBr crystals doped by the diffusion method were compared with those for crystals doped by adding Nd 3+ and Er 3+ to the melt. Two processes in rare earth ion transport were observed and explained by two different diffusion mechanisms. High values of the diffusion coefficient for rare earth ions in AgBr crystals makes it possible to prepare doped crystals of high optical quality. These would be suitable for investigating possible laser action in these materials.

Morphology and growth mechanism of multiply twinned AgBr and AgCl needle crystals

In this study the morphology and growth mechanism of AgX (X=Cl, Br) needle crystals will be revealed. The silver halide needles are grown by two different techniques. AgBr needles are grown with an extractive crystallization in a dimethylsulfoxide–water mixture and AgCl needles are grown from the vapor phase. All needles contain nonparallel twin planes. The morphology of the AgBr needles grown from dimethylsulfoxide is the same as the needles grown in the industrial precipitation process in water. The side faces of these solution grown needles are built up of relatively slowly growing {1 1 1} faces. Both needle tops are composed of three relatively fast growing {1 0 0} faces making a ridge structure. The vapor-grown needles have two different morphologies. For both types the side faces consist of four {1 1 1} and two {1 0 0} faces. All {1 1 1} faces are linked via twin planes to a fast growing {1 0 0} face. The preferential unidirectional growth of all needles is caused by cross-twinning. The preferential growth occurs along the intersection line of the twin planes. Between the twin planes a rough growing face appears on the needle top. This face is capable of increasing the growth rate of the other top faces owing to the substep mechanism.

Luminescence method for the study of Nd3+ ions diffusion in AgBr crystals

This study of diffusion of Nd in AgBr is motivated by our interest in the preparation of Nd-doped AgBr crystals that show lasing effects in the near and midinfrared. The doping can be done in several ways. We propose that monitoring the infrared luminescence along the direction of diffusion of activator ions in crystals of AgBr is a sensitive and minimally invasive method of determining the diffusion coefficient, the diffusion enthalpy, and the temperature dependence of these parameters for ions of interest. We have used this technique to study the diffusion of Nd3+ in AgBr crystals. To establish the concentration range in which the luminescence method is useful, the dependence of the luminescence kinetics on the Nd-ion concentration was investigated. The temperature dependence of the diffusion coefficient (D0=1.2×10−5 cm2/s at room temperature), and the diffusion enthalpy (H=0.51 eV) were determined. We found that high values of the diffusion coefficient and low values of the diffusion enthalpy facilitate preparation of high optical quality Nd-doped AgBr crystals. Such crystals are candidates for lasing action.

Femtosecond transient absorption and luminescence decay studies of spectrally sensitized photographic emulsions

Fast electronic processes taking place immediately after excitation of sensitized silver halide emulsions were studied with picosecond time-correlated single-photon counting (SPC) and femtosecond transient absorption (TA). The fluorescence decays of the J aggregate obtained by SPC measurements were also analyzed as a sum of three exponentials. For the J aggregates of a benchmark dye, a dimethyl-μ-ethyl-thiacarbocyanine, a component with a 20 ps decay time, which was absent in samples containing no AgBr crystals, was attributed to J aggregates of the sensitizer adsorbed on the AgBr microcrystals. The fluorescence decay results were compared with photographic quantum yields in an attempt to show the relevance of the spectroscopic measurements. The second component (150 ps) is a combination of fluorescence decays of the unaggregated sensitizer molecules not adsorbed on the AgBr microcrystals and monomers of the dye on AgBr surface. The fluorescence decay was found to be independent of the crystal coverage. Prolonged illumination bleached the fluorescence of the fast component, which is consistent with depletion of the J aggregates by electron injection. The decay time of this fast component shows strong dependence on both the structure of the dye and the composition of the silver halide. Excitation in the narrow absorption band of the J aggregates with a femtosecond pump pulse resulted in ground state bleaching followed by a recovery that was analyzed as a sum of three exponentials. The fast (170 fs and 2 ps) components of the recovery are related to the relaxation of bi- or multiexciton states and to singlet-singlet annihilation processes. The remaining component (1 ns) can be attributed to the radiative relaxation of dye molecules not adsorbed on the silver halide crystals. The observed bleaching recovery demonstrated a complex dependence on both the excitation flux and the total illumination dose.

Role of the excited electron in the diffusion of interstitials in AgCl and AgBr

The shallow electrons associated with certain impurities or interstitial silver in AgBr and AgCl are believed to play key roles in some of the photoinduced processes. We have performed theoretical calculations on these systems using a method developed earlier for excited electron centers in insulators. In both AgCl and AgBr crystals the Frenkel-pair energies are found to vary monotonously as a function of the vacancy-interstitial separation, which shows the energetical tendency of the interstitial diffusion toward the vacancy. To obtain the very small activation energy of the interstitial silver diffusion, a simple model of quadrupolar deformation of the ${\mathrm{Ag}}^{+}$ ion is employed. We found that the barrier for diffusion from the second cell to the vacancy is substantial (e.g., 0.4 eV) compared with a much smaller value far from the vacancy, $\ensuremath{\approx}0.07 \mathrm{eV}.$ The role of the excited electron in silver diffusion is studied. Our calculation shows that the electron can be bound to the silver vacancy in a diffuse orbital when an interstitial silver ion is present nearby. The diffuse electron is found to further reduce the activation energy at moderate distance from the vacancy. Moreover, the large barrier in the second cell from the vacancy was significantly reduced by the diffuse electron. Possible mechanisms for the recently observed anomalous heat generation in photoexcited silver halides are presented.

A Study on the Mechanism of Nucleation and Growth of Twin Tabular AgBr Crystals

On the mechanism of nucleation and growth of (111) tabular silver halide crystals, it is proposed that twinning (nucleation) occurs by stacking fault and that the anisotropic lateral growth is brought about by the ridge trough structure on tabular grain side faces. Although the “coalescence” model for nucleation and the “rough-smooth” side face structure for anisotopic growth were proposed recently, they have not yet been supported by direct experimental evidence. We analyzed the nucleation process by a combination of light scattering and X-ray diffraction and examined the coalescence of untwinned fine grains after decomposition of gelatin. These experimental results provided evidence to support the coalescence model for twinnning. We observed by TEM imaging of ultramicrotomed sections of side faces of tabular grains that have relatively thick spacings between parallel twin planes and confirmed that on the side faces of tabular grains, both rough-smooth and ridge-trough structures exist.

The Mechanism of Cyanine Sensitizing Dye Adsorption on AgBr Crystals

Time evolution of the adsorption of cyanine sensitizing dyes onto silver halide crystals was investigated. Observing the adsorption process in this method was found to be useful in determining dye adsorption mechanisms and their rate determining steps, in revealing how the N-substituents of cyanine dyes change the adsorption, and in exploring the relationship between rate determining steps and the states of the dyes adsorbed.

Extended x-ray absorption fine structure and molecular dynamics simulation studies on the structure of CuX- and AgX-based glasses (X = Cl, Br and I)

Extended x-ray absorption fine structure (EXAFS) spectra at the Cu K-edge and Ag K-edge were measured for copper(I) halide-based and silver halide-based glasses, respectively. Molecular dynamics (MD) simulation was also performed for the glasses with the same compositions. The bond distances, A-X, in the copper(I) halide-based and the AgI-based glasses, which were obtained from the EXAFS analyses, were almost the same as those in the CuX (X = Cl, Br, I) and AgI crystals. The coordination numbers of halide ions around the monovalent metal ions were nearly four as well as those in the crystals. These results were consistent with those of the MD simulation. On the other hand, the EXAFS analyses showed that the bond distances in the AgCl-based and the AgBr-based glasses were 0.1 Å shorter than those in the AgCl and AgBr crystals. These values are in the range of those of the halogenoargentate crystals having three- or fourfold coordination. However, the coordination numbers obtained from the EXAFS analyses were larger than four. The MD simulation suggested that the coordination shells in the AgCl-based and the AgBr-based glasses are highly disordered.

Theoretical Studies of EPR Parameters for Cr3+ Ion at an Orthorhombic Defect Site in AgBr Crystal

In this paper, the high-order perturbation formulas of EPR zero-field splittings D, E and g-factors gi (i = x, z) for d3 ions in orthorhombic symmetry are established by using a two-spin–orbit coupling parameter model, in which both contributions from the spin–orbit coupling of the central d3 ion and that of the ligands are included. Based on these formulas, the EPR parameters for the Cr3+ ion at an orthorhombic defect site in the AgBr crystal are calculated by considering the suitable defect-induced lattice distortion. The calculated parameters D and E show good agreement with the observed values. The reasons for the disparity of the gi-shift Δgi = (gi — gs) between the calculation and the experiment and the importance of the contributions to the EPR parameters from the spin–orbit coupling of ligands are discussed.

Theoretical Studies of EPR Parameters for Cr3+ Ion at an Orthorhombic Defect Site in AgBr Crystal

In this paper, the high-order perturbation formulas of EPR zero-field splittings D, E and g-factors gi (i = x, z) for d3 ions in orthorhombic symmetry are established by using a two-spin–orbit coupling parameter model, in which both contributions from the spin–orbit coupling of the central d3 ion and that of the ligands are included. Based on these formulas, the EPR parameters for the Cr3+ ion at an orthorhombic defect site in the AgBr crystal are calculated by considering the suitable defect-induced lattice distortion. The calculated parameters D and E show good agreement with the observed values. The reasons for the disparity of the gi-shift Δgi = (gi — gs) between the calculation and the experiment and the importance of the contributions to the EPR parameters from the spin–orbit coupling of ligands are discussed.

The effective mass of a slow-moving surface polaron in a semi-infinite crystal

There is weak bulk but strong surface coupling between electrons and phonons in a semi-infinite crystal. Here some properties of the surface polaron in a semi-infinite crystal, weak coupling with bulk longitudinal optical (BO) phonons and strong coupling with surface longitudinal optical (SO) phonons, are studied. The effective Hamiltonian of a slow-moving surface polaron in a semi-infinite crystal is derived using an improved linear combination operator and perturbation method. If we consider the interaction between phonons of different wave vectors in the recoil process, the influence of this interaction on the effective Hamiltonian, the induced potential and the effective mass of the surface polaron are discussed. Numerical calculations for an AgBr crystal, as an example, are performed, and some properties of the surface polaron in a semi-infinite crystal are discussed.

Electron microscopical investigation of tetrahedral-shaped AgBr microcrystals

Tetrahedral AgBr crystals present in emulsions precipitated at high pAg values are investigated by conventional electron microscopy and electron diffraction. The defect structure of these crystals is determined to consist of two non-parallel (111)-type twin planes enclosing an angle of 70.5°. A growth model is presented explaining the morphology of both needle crystals and tetrahedral crystals in terms of the influence of twin planes on the growth behaviour of the crystals.

Absorption spectrum of silver bromide crystals and fibers in the 9–11 μm wavelength range

The absorption spectrum in the 9–11 μm range of both undeformed and plastically deformed AgBr crystals and polycrystalline optical fibers was investigated by tunable laser calorimetry. The observed spectral features are discussed in terms of the crystal defect structure.

Role of Dislocations in Silver Halide Photography

Dislocations and donor centers have important functions in the optimization of the performance of silver halide emulsion grains. The basic properties of dislocations relevant to photographic sensitivity together with the experimental observations which established these properties are reviewed in this paper. Internal latent image and internal particles of photolytic silver are formed during exposure of AgCl and AgBr crystals, sensitized with Ag2O and Ag2 donor centers, by the separation of Ag atoms along the dislocation lines. The surface sites of termination of dislocations have enhanced reactivity compared with low-energy surfaces. Dissolution, chemical sensitization, and the initiation of surface chemical development occur at higher rates at these sites. These properties led to the concept of the dislocation sensitivity center. Development centers are formed by the combination of Ag atoms with Ag2 latent image growth nucleus precursors at the surfaces of dislocation sensitivity centers. The formation of Ag2 molecules along the subsurface dislocation of the center has to be minimized. Microcrystals of AgCl and AgBr with dislocations introduced during nucleation and initial growth pass into solution at a high rate during a short period of ripening leaving dislocation-free growth nuclei. For stable distributions of dislocations in microcrystals, concentration gradients of halide ions have to be established in heterogeneous crystals. Controlled densities of dislocations are introduced to reduce the elastic strains associated with steep concentration gradients. In double-structure grains with a higher iodide shell and in triple-structure grains with a higher iodide narrow zone, the concentration gradients can be established by the addition of a fine dispersion of Ag(Br,I) particles or of an iodide ion releasing compound. The resulting dislocation distributions are stable because of the immobility of the iodide ions in the crystal. The mechanisms involved in the creation of dislocations in structured tabular microcrystals are discussed.

Effects of Thermal and Mechanical Treatment on the Luminescence of AgBr Crystals

Effects of Thermal and Mechanical Treatment on the Luminescence of AgBr Crystals

Scanning Tunnelling Microscopy Studies of AgBr Crystals on HOPG

Silver bromide crystals deposited on graphite (HOPG) were investigated with a scanning tunnelling microscope under ambient conditions. Different shapes and sizes of microcrystals, including octahedral forms and tabular grains, were observed and examined. With higher magnification atomically resolved images of the surfaces were obtained and the lattice constants determined. In addition, the AgBr crystals were examined by a scanning electron microscope.

Investigations of the Reactions Involved in Formation of the Light-Sensitive Phases in Thermally Developed Photomaterials.

We have carried out electrochemical, sensitometric, and electron microscope investigations of the reactions of the photosensitive phase of a thermally developable photomaterial. We have found that formation of icrocrystals of silver bromide by metathetical reaction of silver stearate with bromide in aqueous solution leads to precipitation of silver bromide as a separate phase, accompanied by dissolution of silver stearate microcrystals. The size of the AgBr microcrystals increases with increasing concentration of the brominating agent, but does not exceed 0.07 μm. Latent image centers formed by the AgBr microcrystals are capable of catalyzing thermal development if the microcrystals have been formed in the presence of the carboxylate. The process of thermal amplification of the latent image centers may, however, be achieved on AgBr crystals having no physical contact with silver stearate. We propose that thermal development of latent image centers on AgBr crystals proceeds by diffusion of silver stearate-derived “droplets” to the development centers. Fog formation, on the other hand, nucleates independently of the AgBr microcrystals.

Optical and Physical Properties of Silver Colour Centres in Microcrystals in Silver Bromide Films

AbstractResults are presented of investigations of silver colour centres of molecular and colloidal types arising from the deep photolysis of AgBr microcrystals in ultra-fine-grain emulsions. Using experimental data connecting the position of the absorption maximum of first, secondary and “limited” colour centres, their dimensions and the sizes of the AgBr crystals, values of solubility, partial degree of solubility and surface tension ofAg in AgBr are calculated. Dependence of these values on the dimensions of the AgBr crystals are studied. It is shown that the surface tension increases ivilh the number of atoms in the colloidal particles. The solubility of Ag changes by 6 orders oj magnitude as the colloidal particles decrease from massive to atomic size. Good agreement is shown between the experimental results and the predictions of thermodynamic theory for the nucleation and growth of a new phase. The possibility of a photostimulated phase transition from molecular centres to colloidal ones is considered.

Effect of ionic polarizability on impurity-vacancy association in silver halides

Abstract Ionic conductivity measurements were used to determine the association enthalpies of divalent impurity ions and silver vacancies in silver chloride and silver bromide. The impurities were chosen, cognizant of previous work, to elucidate the effects of the ionic radius, the electronic structure, and host material. Conductance as a function of temperature was successfully measured for single crystals of AgCl doped with V 2+ , Cr 2+ , Fe 2+ , and Co 2+ , and on single crystals of AgBr doped with Mg 2+ , Ca 2+ , Sr 2+ , Ni 2+ and Cd 2+ . One main result from this investigation is the apparent independence of association enthalpies in AgCl from the size and electronic shell configuration of the first row transition metal ions. As expected, association enthalpy values were found to be smaller in AgBr than in AgCl. Also, a striking relationship between the ionic polarizability and the association enthalpy is demonstrated for AgBr.