A. Abu El-Fadl

Effect of Doping and Irradiation on Optical Parameters of Triglycine Sulphate Single Crystals

Crystals of triglycine sulphate (TGS) doped with orthonitroaniline (ONA) irradiated with different doses of gamma-radiation were used to investigate gamma-radiation effect on electrical and optical parameters. The absorption coefficient and the reflectance were measured and hence the extinction coefficient, the optical refractive index and the dielectric constants (er′, er″) of unirradiated and irradiated TGS crystals were calculated. Values of the allowed indirect optical energy gap Egopt. of TGS doped with ONA were calculated as a function of gamma-dose. Value of Egopt. decreases from 4.72 eV to 4.25 eV with increasing gamma-doses from 0 up to 3 Mrad. The values of the static dielectric constant er(0) and the effective electrical conductivity sigma0 at room temperature and their dependence on gamma-dose were also calculated.

Ferroelectric behaviour of triglycine sulphate crystals containing low concentrations of metal ions near the transition temperature

Several ferroelectric properties of triglycine sulphate single crystals both pure and with different doping substitutions in the structure have been studied in the vicinity of the critical point Tc. The parameters engaged in the statistical dipolar theory of ferroelectrics are derived, from dielectric constant and hysteresis loop measurements, allowing a distinction between atomic and dipolar polarisation. The numerical values of the parameters obtained in this way for pure and doped crystals are compared. The internal biasing field is estimated using the data of the pyroelectric measurements. The basic pyroelectric material figure of merit is also calculated and its temperature dependence is reported. The temperature dependence of the coercive field has been measured and found to obey a power law Ec=A(Tc-T)s. The piezoelectric modulus and the electromechanical coupling coefficient have been also measured and their temperature dependence is investigated. The J-E characteristic in both the ferroelectric and the paraelectric phases is examined; variations in the behaviour according to the type of dopant are observed. The effect of polarising pure and doped TGS samples with a DC field of intensity 102 kV m-1 on measured properties is also examined.

Temperature dependence of the optical band gap of nearly perfect K2ZnCl4 single crystals in the ferroelectric phase

Abstract The absorption edge in the main crystallographic directions of K2ZnCl4 single crystal was studied at different temperatures in the ferroelectric phase. The absorption coefficient was computed and the analysis of the data revealed the existence of two optical transition mechanisms. The data on the allowed indirect transition was analysed and interpreted in terms of the two valence bands originated by spin–orbit interaction and crystal-field splitting. The momenta Ep was calculated as the energy difference between Eg1, the first valence and conduction band, and Eg2, for the second valence band at different temperatures. The anomalous behaviour of temperature dependence of the band gap Eg in b-axis is attributed to the relatively weak electrostatic interactions between the cations and the stacks.

Optical studies of K2ZnCl4 single crystals doped with Cs+ in the ferroelectric phase

Abstract The optical band gaps of K2ZnCl4 single crystals doped with Cs+ in the three crystallographic directions at different temperatures were calculated in both direct and indirect transitions. The data on the allowed indirect transition was analyzed and interpreted in terms of valence band initiated by spin-orbit interaction and crystal field splitting. The energy difference Ep between Eg1, the first valence and conduction band and Eg2 for the second valence band was calculated. Analysis of absorption spectra indicates that for a- and c-axes, the indirect Egi and direct Egd energy gaps decreased with increasing temperature. The anomalous behavior of the band gap in the b-axis was attributed to the relatively weak electrostatic interactions between the cations and the stacks. The results are discussed in comparison with our previous work on pure and Mn2+ doped K2ZnCl4 single crystals.

Absorption spectra and optical parameters of lithium–potassium sulphate single crystals

Abstract The optical transmittance and reflectance near the fundamental absorption region along the c- and a-axes of lithium potassium sulphate single crystal (LKS) are measured at room temperature. From the data the absorption coefficient (α) and the optical band gap (Eopt.g) were deduced. The type of transition was determined. The steepness parameter (σ), the temperature dependence of the energy gap and the exciton energy (E0) were also calculated. The extinction coefficient, the refractive index and both the real and imaginary parts of the dielectric permittivity were calculated as functions of the photon energy.

Temperature Dependence of the Absorption Spectra and Optical Parameters in TGS and Cu2+‐Doped TGS Crystals

The effect of temperature on the optical absorption spectra and optical parameters is investigated for pure TGS and TGS doped with Cu 2+ ions. Absorption measurements cover the range from room temperature to about 355 K in the energy range 3-5.5 eV. The temperature dependence of the band gap E g (T) reveals an anomaly at the phase transition temperature for both pure and Cu 2+ -doped TGS crystals. In the region of the absorption edge the absorption coefficient is found to display Urbach-rule behaviour. The characteristic Urbach parameters are determined and their temperature dependence is investigated.

Critical behaviour of dielectric permittivity and spontaneous polarization of triglycine sulphate single crystals doped with organic molecules

Dielectric permittivity, e 22 , spontaneous polarization, P s , and coercive field, E c , of triglycine sulphate (TGS) crystals doped with d-l alanine, orthonitroaniline, paranitroaniline or aniline molecules have been measured from room temperature up to the transition point. A remarkable displacement of the hysteresis loops of polarization along the direction of the field axis and a broadened transition from the polar to the non-polar phase are detected. An increase in the value of P s and a decrease of both e 22 and E c are also observed. The changes in both e 22 and P s attributed to the creation by the dopants of an internal electric field, E b . The internal bias affects the Curie constant and e max in a way similar to that introduced by an external bias. The value of E b in the vicinity of the phase transition and the parameters of the statistical dipolar theory of ferroelectricity have been calculated.

Investigation of the pyroelectric and piezoelectric properties of triglycine sulphate single crystals containing organic molecules

The pyroelectric coefficient and the piezoelectric modulus of triglycine sulphate (TGS) single crystals doped with d-l-alanine, orthonitroaniline, paranitroaniline or aniline molecules has been measured from room temperature up to the transition point. The data are compared with those which the authors have previously obtained on pure TGS. A noticeable increase in the value of the pyroelectric coefficient of the doped samples over the whole temperature range is observed. A significant pyroelectric signal is still detected several degrees above the transition point of pure crystals. The spontaneous polarisation is calculated using data for the measured pyroelectric current. The piezoelectric modulus of the doped crystals reaches its maximum value at a temperature lower than the Curie point of pure TGS before it decreases steadily to zero. A relation between the piezoelectric modulus and the temperature just before the maximum value is deduced. The role of the internal bias created in the crystal after doping is also discussed.

Bulk- and electrode-limited conduction mechanisms in different phases of Mn2+-doped potassium tetrachlorozincate crystal

The current density–electric field intensity relationship of pre-heated potassium tetrachlorozincate (KZC) crystals (undoped and doped with Mn2+ in different concentrations) has been measured along the polar a-axis. The dependence was studied at selected temperatures covering the three high-temperature phases of KZC in order to investigate the type of conduction mechanism dominating in each phase. The original Richardson–Schottky (R–S) equation shows disagreement between calculated and experimental values of the Richardson and optical dielectric constants. The modified R–S equation fits the data well and facilitated the calculation of the barrier height, electronic mobility and high-frequency dielectric constant of KZC. The calculated parameters are in good agreement with the corresponding experimental values. The results indicated that the bulk- and electrode-limited mechanisms contribute to conduction in different phases of KZC. The temperature dependence of the dc conductivity along the polar axis of undoped and Mn2+-doped KZC shows anomalous behaviour in the region of the phase transitions. The dc conductivity and the activation energy of conduction (w) changed significantly due to doping. As indicated by the extremely high w values, superionic conduction is the dominating mechanism in the high-temperature part of the incommensurate (IC) and normal phases of KZC. Suppression of the superionic conduction by Mn2+-doping is observed. The effect of discommensurate pinning by Mn2+ ions and the possibility of dislocation formation on the dc conduction in the IC phase is also discussed.

Differential scanning calorimetric study of Se70Ge20Sb10 chalcogenide glass

Abstract Results of differential scanning calorimetry (DSC) under isothermal conditions on Se70Ge20Sb10 glass are reported and discussed. By using the Avrami equation the activation energy for crystal growth (EG) has been evaluated and the crystallization mechanism has been studied. The results indicate that the crystallization process is a one-dimensional growth. The calculated value of EG is 142.8 kJ mol-1 for Se70Ge20Sb10 chalcogenide glass.