Optical absorption along the three crystallographic axes of (NH4)2ZnCl4 single crystals is reported over a temperature range from 276 K to 350 K. Our results clearly confirm the antiferroelectric to commensurate phase transition at 319 K. Analysis reveals that at the absorption edge the type of transition is the indirect allowed one. The indirect band gaps at various temperatures are determined and their temperature dependence is estimated in the antiferroelectric and commensurate phases. Optical parameters related to the temperature dependence of the energy gap are evaluated. The single-effective oscillator model was used to describe the imaginary part of the dielectric constant, which facilitate calculation of the dipole moment parameters. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap and the result is compared with that calculated graphically. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach characteristic parameters are determined and their temperature dependence is investigated. Correlation between different parameters is considered. Most of the results confirm the optical isotropic nature of AZC crystals. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical transmittance measurements were carried out near the absorption edge of (NH4)2ZnCl4:xSr2+ single crystals, where x = 0.00, 0.020, 0.039, 0.087 or 0.144 wt%. The dependence of the absorption coefficient (α) on photon energy in the range 3.4-6.4 eV and on temperature in the range 276-350 K was examined. With increasing temperature the absorption edge was shifted to the lower energy side; this shift was attributed to the temperature dependence of electron-lattice interactions. Due to the presence of Sr2+ in (NH4)2ZnCl4:xSr2+ a change in the absorption band centred at 5.30 eV was observed. In the case of higher strontium doping, the optical absorption coefficient showed a pre-transition stage just before the onset of the band-to-band transition. Exponential dependence of α on h according to the Urbach rule was observed and the Urbach parameters were determined. The steepness parameter was strongly affected by doping; its behaviour at 319 K evidently showed the antiferroelectric to commensurate phase transition. The temperature dependence of Eg was also estimated using data for the steepness parameter, and hence Eg at different temperatures and Sr2+ concentrations was evaluated

Optical transmittance measurements were carried out near the absorption edge of (NH4)2ZnCl4:xSr2+ single crystals, where x = 0.00, 0.020, 0.039, 0.087 or 0.144 wt%. The dependence of the absorption coefficient (α) on photon energy in the range 3.4-6.4 eV and on temperature in the range 276-350 K was examined. With increasing temperature the absorption edge was shifted to the lower energy side; this shift was attributed to the temperature dependence of electron-lattice interactions. Due to the presence of Sr2+ in (NH4)2ZnCl4:xSr2+ a change in the absorption band centred at 5.30 eV was observed. In the case of higher strontium doping, the optical absorption coefficient showed a pre-transition stage just before the onset of the band-to-band transition. Exponential dependence of α on h according to the Urbach rule was observed and the Urbach parameters were determined. The steepness parameter was strongly affected by doping; its behaviour at 319 K evidently showed the antiferroelectric to commensurate phase transition. The temperature dependence of Eg was also estimated using data for the steepness parameter, and hence Eg at different temperatures and Sr2+ concentrations was evaluated

Optical transmittance measurements near the absorption edge in the energy range 3.4–6.4 eV have been carried out on (NH4)2ZnCl4:xSr2+ single crystals with x=0.000%, 0.020%, 0.039%, 0.087% or 0.144%. The introduction of Sr2+ in low concentration, x=0.02% or 0.039%, little bleach the intense charge transfer band observed for the undoped sample with a maximum at 5.30 eV. Doping with Sr2+ in a little bit higher concentration, x=0.087% or 0.144%, led to the disappearance of this band. While doping with Sr2+ has no effect on the indirect allowed type of optical transition in AZC, the optical energy gap decreased with increasing Sr2+ concentration. Measuring reflectance of AZC permitted the calculation of the refractive index n, the extinction coefficient K and both the real r and imaginary i components of the dielectric permittivity as functions of photon energy. The validity of Cauchy–Sellimaier equation was checked in the wavelength range 5.45–5.8 eV and its parameters were calculated. Applying the single-effective-oscillator model, the moments of (E) could be estimated. Electric susceptibility, as a measure of intraband transition, was calculated and its dependence on photon energy was considered

Optical transmittance measurements near the absorption edge in the energy range 3.4–6.4 eV have been carried out on (NH4)2ZnCl4:xSr2+ single crystals with x=0.000%, 0.020%, 0.039%, 0.087% or 0.144%. The introduction of Sr2+ in low concentration, x=0.02% or 0.039%, little bleach the intense charge transfer band observed for the undoped sample with a maximum at 5.30 eV. Doping with Sr2+ in a little bit higher concentration, x=0.087% or 0.144%, led to the disappearance of this band. While doping with Sr2+ has no effect on the indirect allowed type of optical transition in AZC, the optical energy gap decreased with increasing Sr2+ concentration. Measuring reflectance of AZC permitted the calculation of the refractive index n, the extinction coefficient K and both the real r and imaginary i components of the dielectric permittivity as functions of photon energy. The validity of Cauchy–Sellimaier equation was checked in the wavelength range 5.45–5.8 eV and its parameters were calculated. Applying the single-effective-oscillator model, the moments of (E) could be estimated. Electric susceptibility, as a measure of intraband transition, was calculated and its dependence on photon energy was considered

Considerable changes were observed in the X-ray diffraction pattern of ammonium zinc chloride after doping with Sr2+ in different concentrations and after irradiating with γ rays at different doses. The effect of γ-radiation and Sr2+ content on optical parameters of (NH4)2ZnCl4:x Sr2+ single crystals (x = 0.00, 0.020, 0.039, 0.087 or 0.144 wt.%) has been investigated. The transmittance near the absorption edge of unirradiated crystals and crystals irradiated with different γ-doses has been measured, hence the absorption coefficient (α) was calculated. The values of α at room temperature increased under the influence of γ-irradiation. The rate by which α increases with photon energy just before the absorption edge is strongly inhibited by higher γ-doses. The type of intraband transition in (NH4)2ZnCl4: x Sr2+ single crystals was found to be of the allowed indirect transition, and γ-irradiation had no effect on the type of transition. The values of the optical energy gap (Eg) were calculated as a function of γ-dose. The effect of γ-irradiation was found to be more pronounced on samples doped with x = 0.087 or 0.144 wt.% Sr2+. The results can be discussed on the basis of γ-irradiation-induced defects and Sr2+ concentration. A diagram representing probable transitions to the created bands due to irradiation could be constructed

Considerable changes were observed in the X-ray diffraction pattern of ammonium zinc chloride after doping with Sr2+ in different concentrations and after irradiating with γ rays at different doses. The effect of γ-radiation and Sr2+ content on optical parameters of (NH4)2ZnCl4:x Sr2+ single crystals (x = 0.00, 0.020, 0.039, 0.087 or 0.144 wt.%) has been investigated. The transmittance near the absorption edge of unirradiated crystals and crystals irradiated with different γ-doses has been measured, hence the absorption coefficient (α) was calculated. The values of α at room temperature increased under the influence of γ-irradiation. The rate by which α increases with photon energy just before the absorption edge is strongly inhibited by higher γ-doses. The type of intraband transition in (NH4)2ZnCl4: x Sr2+ single crystals was found to be of the allowed indirect transition, and γ-irradiation had no effect on the type of transition. The values of the optical energy gap (Eg) were calculated as a function of γ-dose. The effect of γ-irradiation was found to be more pronounced on samples doped with x = 0.087 or 0.144 wt.% Sr2+. The results can be discussed on the basis of γ-irradiation-induced defects and Sr2+ concentration. A diagram representing probable transitions to the created bands due to irradiation could be constructed

The dielectric permittivity (∈) and dielectric losses (tan δ) of ferrocene crystallites are measured with different frequencies in the temperature range 140-375 K. The results confirm the existence of the phase transition at 163.9 K in addition to anomalous behaviour while heating indicating the presence of a metastable structure in the temperature range 250-290 K. The d.c. electrical conductivity (a) is measured in heating-cooling cycles between 120 and 300 K. The behaviour while cooling is completely different from that while heating and the phase transition becomes less pronounced in the second cooling-heating cycle. The σ-T relationship reveals an activation energy of conduction of 0.89 eV in the high temperature range while cooling. The effect of irradiating samples of ferrocene with γ-rays in different doses on both the dielectric permittivity and the electrical conductivity is also discussed. The current density (J) is measured as a function of the applied electric field (E) at three different temperatures; below the phase transition point, at room temperature and at 345 K in order to estimate the type of conduction dominates and the effect of E upon the conductivity measurements of this material. The effect of γ-irradiation with different doses on the J-E characteristics of ferrocene is also considered