The dielectric constant, , and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH4)2ZnCl4 (AZC) crystal in the 300–450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl2 were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie–Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J–E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole–Frenkel and Richardson–Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The ln σ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.

The dielectric constant, , and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH4)2ZnCl4 (AZC) crystal in the 300–450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl2 were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie–Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J–E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole–Frenkel and Richardson–Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The ln σ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.

The thermal decomposition behaviour of unirradiated and pre-γ-irradiated piperacillin (pipril) as a semi-synthetic penicillin antibiotic has been studied in the temperature range of (273-1072 K). The decomposition was found to proceed through three major steps both for unirradiated and γ-irradiated samples. Neither appearance nor disappearance of new bands in the IR spectrum of piperacillin was recorded as a result of γ-irradiation but only a decrease in the intensity of most bands was observed. A degradation mechanism was suggested to explain the bond rupture and the decrease in the intensities of IR bands of γ-irradiated piperacillin

The thermal decomposition behaviour of unirradiated and pre-γ-irradiated piperacillin (pipril) as a semi-synthetic penicillin antibiotic has been studied in the temperature range of (273-1072 K). The decomposition was found to proceed through three major steps both for unirradiated and γ-irradiated samples. Neither appearance nor disappearance of new bands in the IR spectrum of piperacillin was recorded as a result of γ-irradiation but only a decrease in the intensity of most bands was observed. A degradation mechanism was suggested to explain the bond rupture and the decrease in the intensities of IR bands of γ-irradiated piperacillin

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 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