The optical transmittance for potassium ferrocyanide (KFCT) single crystals were measured as a function of the wavelength at different temperatures in the range 275–320 K along the two axes (I 0 ) and (0 1 0) and hence the absorption coefficient (α) and optical band gap Eg were deduced. The type of transition was determined and the validity of Urbach's rule was checked. The steepness parameter (σ) and the exciton energy E0 were calculated at different temperatures. The temperature dependence of the energy gap was estimated and the result was confirmed through the calculation of dEg/dT from information about σ. The refractive index, the extinction coefficient and both the real and imaginary parts of the dielectric permittivity were also calculated as a function of photon energy and the results were discussed. The effect of doping KFCT with 2% of sodium nitroproced, cobalt sulphate or potassium dichromate on the same physical parameters was also discussed

The optical transmittance for potassium ferrocyanide (KFCT) single crystals were measured as a function of the wavelength at different temperatures in the range 275–320 K along the two axes (I 0 ) and (0 1 0) and hence the absorption coefficient (α) and optical band gap Eg were deduced. The type of transition was determined and the validity of Urbach's rule was checked. The steepness parameter (σ) and the exciton energy E0 were calculated at different temperatures. The temperature dependence of the energy gap was estimated and the result was confirmed through the calculation of dEg/dT from information about σ. The refractive index, the extinction coefficient and both the real and imaginary parts of the dielectric permittivity were also calculated as a function of photon energy and the results were discussed. The effect of doping KFCT with 2% of sodium nitroproced, cobalt sulphate or potassium dichromate on the same physical parameters was also discussed

Optical absorption measurements near the absorption edge have been studied in the energy range 3–5.5 eV on pure triglycine suphate (TGS) and TGS crystals doped with Cu2+, Mn2+ or Ni2+ in the temperature range 290–360 K. The temperature dependence of the band gap Eg(T) reveals an anomaly at the phase transition temperature for both pure and doped TGS crystals. Absorption coefficient data in the fundamental region are analysed and interpreted in terms of the allowed indirect transitions. The absorption edge following Urbach's rule exhibited exponential behaviour with temperature. Urbach parameters were calculated at different temperatures and the frequencies of effective phonons and electron–phonon interaction constants were determined for various phases

Optical absorption measurements near the absorption edge have been studied in the energy range 3–5.5 eV on pure triglycine suphate (TGS) and TGS crystals doped with Cu2+, Mn2+ or Ni2+ in the temperature range 290–360 K. The temperature dependence of the band gap Eg(T) reveals an anomaly at the phase transition temperature for both pure and doped TGS crystals. Absorption coefficient data in the fundamental region are analysed and interpreted in terms of the allowed indirect transitions. The absorption edge following Urbach's rule exhibited exponential behaviour with temperature. Urbach parameters were calculated at different temperatures and the frequencies of effective phonons and electron–phonon interaction constants were determined for various phases

The crystal structure of α- and β-LiIO3 at room temperature was examined using X-rays diffraction. Differential thermal analysis (DTA) with different heating rates (5–50 K/min) in the range 300–600 K was performed. The activation energy of phase transition was calculated by two methods based upon DTA. The d.c. electrical conductivity (σ) and dielectric permittivity () of α-LiIO3 crystals were measured along both the a- and c-axes in the temperature range 300–580 K. Anomaly in the σ–T relationship was observed at the α→γ phase transition. The dielectric permittivity increased abruptly at T0=520 K indicating strong structural changes at this temperature. The J–E characteristics along a- and c-axes were measured below and above T0 and hence the type of conduction was estimated. The optical transmittance (Tr) and reflectance (R) at room temperature were also measured in the wavelength range 190–900 nm and hence the absorption coefficient (α), refractive index (n), optical band gap (Egopt) and the type of transition were estimated. The anisotropic character of the measured properties of LiIO3 are discussed

The crystal structure of α- and β-LiIO3 at room temperature was examined using X-rays diffraction. Differential thermal analysis (DTA) with different heating rates (5–50 K/min) in the range 300–600 K was performed. The activation energy of phase transition was calculated by two methods based upon DTA. The d.c. electrical conductivity (σ) and dielectric permittivity () of α-LiIO3 crystals were measured along both the a- and c-axes in the temperature range 300–580 K. Anomaly in the σ–T relationship was observed at the α→γ phase transition. The dielectric permittivity increased abruptly at T0=520 K indicating strong structural changes at this temperature. The J–E characteristics along a- and c-axes were measured below and above T0 and hence the type of conduction was estimated. The optical transmittance (Tr) and reflectance (R) at room temperature were also measured in the wavelength range 190–900 nm and hence the absorption coefficient (α), refractive index (n), optical band gap (Egopt) and the type of transition were estimated. The anisotropic character of the measured properties of LiIO3 are discussed

The dielectric permittivity (ε) and the dc electrical resistivity(ρ)of pure LiNH4SO4 (LAS) single crystals have been measured in the temperature range 230-350 K including the lower phase transition at about 283 K. Anomalous behaviours before and at the transition point were observed. The temperature dependence of ρ yielded AE = 0.11 and 0.91 eV for the energy activating the charge transport mechanisms before and after the transition. A pre-transition phenomena was observed while measuring both ρ and ε. The optical transmittance at different temperatures was also measured and hence the absorption coefficient (α) and optical band gap Eopt,g were deduced. The validity of Urbach's rule was checked and hence the steepness parameter σ and the exciton energy E0 were calculated at different temperatures. ρ, ε, α and Eopt,g for LAS crystals doped with 5% Co2+ or Cu2+ were also measured and they were compared with the results of the pure crystals

The dielectric permittivity (ε) and the dc electrical resistivity(ρ)of pure LiNH4SO4 (LAS) single crystals have been measured in the temperature range 230-350 K including the lower phase transition at about 283 K. Anomalous behaviours before and at the transition point were observed. The temperature dependence of ρ yielded AE = 0.11 and 0.91 eV for the energy activating the charge transport mechanisms before and after the transition. A pre-transition phenomena was observed while measuring both ρ and ε. The optical transmittance at different temperatures was also measured and hence the absorption coefficient (α) and optical band gap Eopt,g were deduced. The validity of Urbach's rule was checked and hence the steepness parameter σ and the exciton energy E0 were calculated at different temperatures. ρ, ε, α and Eopt,g for LAS crystals doped with 5% Co2+ or Cu2+ were also measured and they were compared with the results of the pure crystals

The temperature dependence of the specific heat Cp and the electrical resistivity ρ of pure lithium-ammonium sulphate (LAS) single crystals along the three principal crystallographic directions is investigated in the 300-500 K temperature range. The transition energy ΔQ and the number of both elementary (No) and thermally excited (Ne) dipoles are calculated. It appears that only a small fraction of the total number of dipoles is capable of being thermally excited in the ferroelectric phase. The correlation between the Cp data and the spontaneous polarization Ps is verified. The J-E characteristics indicate the possibility of space charge effects at low measuring fields. Anomalous behaviours before and at the transition point is observed. Thermal annealing is found to be necessary for reproducible results. The temperature dependence of ρ long the polar axis yields the values ΔE = 0.54 and 1.48 eV and ΔE = 1.95 eV for the energy activating the charge transport mechanisms in the ferro- and the paraelectric phases, respectively. A pre-transition phenomenon is observed while measuring both Cp and ρ along the a- and the b-axes. The mechanism of electrical conduction in the measuring range is discussed. Along the polar axis Cp of LAS crystals doped with either Cu2+, Co2+, Ni2+ or Mn2+ is measured in the same temperature range. Above Tc, the temperature range through which log Cp is linearly proportional to (T - Tc) is wider than that predicted theoretically. The transition energy, the fraction Ne/No and Ps are also calculated for doped crystals

The temperature dependence of the specific heat Cp and the electrical resistivity ρ of pure lithium-ammonium sulphate (LAS) single crystals along the three principal crystallographic directions is investigated in the 300-500 K temperature range. The transition energy ΔQ and the number of both elementary (No) and thermally excited (Ne) dipoles are calculated. It appears that only a small fraction of the total number of dipoles is capable of being thermally excited in the ferroelectric phase. The correlation between the Cp data and the spontaneous polarization Ps is verified. The J-E characteristics indicate the possibility of space charge effects at low measuring fields. Anomalous behaviours before and at the transition point is observed. Thermal annealing is found to be necessary for reproducible results. The temperature dependence of ρ long the polar axis yields the values ΔE = 0.54 and 1.48 eV and ΔE = 1.95 eV for the energy activating the charge transport mechanisms in the ferro- and the paraelectric phases, respectively. A pre-transition phenomenon is observed while measuring both Cp and ρ along the a- and the b-axes. The mechanism of electrical conduction in the measuring range is discussed. Along the polar axis Cp of LAS crystals doped with either Cu2+, Co2+, Ni2+ or Mn2+ is measured in the same temperature range. Above Tc, the temperature range through which log Cp is linearly proportional to (T - Tc) is wider than that predicted theoretically. The transition energy, the fraction Ne/No and Ps are also calculated for doped crystals