Search

Influence of TiO2 and ZnO oxides has been investigated elaborately on the conductivity and dielectric properties of unconventional and technologically important 40CuO–40Bi2O3–20X (where X=TiO2 and ZnO) glasses. The dc electrical conductivity of ZnO containing glass is found to be higher than that of TiO2 containing one at all temperatures (300–600 K). A change from adiabatic to nonadiabatic small polaron hopping conduction is observed due to the influence of TiO2 and ZnO oxides. It has been perceived that the ZnO doped glass shows relatively more ac conductivity compared to that of the TiO2 doped one, consistent with the dc conductivity data. Random-free-energy barrier model of Dyre [J. Appl. Phys. 64, 2456 (1988)] is suited to be the most appropriate one for explaining the frequency dependence of ac conductivity data and frequency exponent. The large dielectric constant in both the glasses can be explained by the response of Bi3+ ions to ac. [ABSTRACT FROM AUTHOR]

DC conductivity of electron doped antiferromagnetic Ca1-xNaxMnO3 (0 ≤ x ≤ 0.3) perovskites has been studied elaborately. Na doping initially increases the dc conductivity and decreases the activation energy of CaMnO3. Above 5% doping, it starts decreasing the conductivity. The Debye temperature (θD) also decreases from 419 to 322 K with the increase of x. The dc conduction in undoped sample takes place by small polaron hopping (SPH) conduction in the non-adiabatic regime. There may be a tendency of large polaron formation in 10-30% Na doped samples due to larger ionic radius of Na1+ compared to that of Mn4+ and Ca2+. Strong e-ph interaction is observed in undoped sample followed by a weaker interaction with the increase of Na concentration. Mott's variable range hopping (VRH) model can explain the dc conductivity at very low temperatures (T < θD/4). It has been detected that single-phonon assisted hopping is responsible for the dc conduction in the Na doped CaMnO3 manganites. [ABSTRACT FROM AUTHOR]

Abstract: DC electrical conductivity (σ dc) of electron-doped antiferromagnetic CaMn1− x Cr x O3 (0⩽x⩽0.3) has been discussed elaborately in the light of polaron hopping conduction. The increase in Cr doping concentration increases the conductivity and decreases the activation energy. Non-adiabatic polaron hopping conduction is observed in all the manganites at high temperatures. The analysis of σ dc data shows that small polarons are formed at lower concentrations (⩽5%) of Cr doping and undoped samples. However, large polarons are materialized at higher doping (⩾10%) concentrations. This is consistent with the fact that doped Cr3+ has larger ionic size compared to that of Mn4+. Again, strong electron–phonon (e–ph) interaction is perceived in undoped and 5% Cr-doped samples but not in manganites with larger doping concentration. This also confirms the formation of larger polarons with the increase of x. Mott''s variable range hopping (VRH) model can elucidate the dc conductivity at very low temperatures. It has been detected that single phonon-assisted hopping is responsible for the dc conduction in the Cr-doped CaMnO3 manganites. [Copyright &y& Elsevier]

The dc conductivity of VN-PbO-TeO2 glasses with different mole percentages of VN, PbO and TeO2 has been measured in the temperature range 125-450 K. The conductivity of the glasses increases with increasing VN content for a fixed mole percentage of PbO. Neither Mott's variable-range hopping (VRH) model at low temperatures (T < ΘD/4, where ΘD is the Debye temperature) nor Greaves' VRH model at intermediate temperatures (ΘD/4< T < ΘD/2) describe the dc conductivity data for these glasses. Multiphonon tunnelling transport of strongly coupled electrons is also unable to account for the carrier transport. However, at high temperatures (T > ΘD/2), conduction is shown to be due to small-polaron hopping in the non-adiabatic regime. Alteration of the VN content causes a change in the model parameters achieved from best-fitting curves for the glasses. Modulated differential scanning calorimetry analysis shows that the glass transition temperatures Tg in this system vary from 269 to 302°C. [ABSTRACT FROM AUTHOR]