Electric field-induced phase transition from the glasslike to paraelectric phase and dielectric spectra hardening in PMN single crystal

One of the key points in the physics of the relaxors is their response to the applied DC field.Many studies of this topic were made,in particular on the influence of the field on the dielectric properties.However,practically,in all the cases,the measurements were performed at a fixed frequency and usually with the change in the temperature at the fixed field strength.In this paper,we report the evolution of the dielectric spectra at low frequencies(0.1 Hz<ω<1 kHz)at fixed temperature 246 K on changing the DC electric field applied in(111)from 1 kV to 7 kV.Cole-Cole function was used to describe the spectra and the field dependences of the mean relaxation timeτ,the oscillation strengthΔεand the width parameterαwere determined.The obtainedτ(E)andΔε(E)provide evidence of the field-induced transition from the nonpolar glass-like phase to the nonpolar paraelectric phase at around 1.5 kV/cm.In the paraelectric phase,very fast hardening of the spectra was observed withτchanging from 10 s to about 10−4 s.The performed analysis demonstrated that the earlier reported positive C-V effect is completely determined by the spectra harden­ing,whileΔεdoes not show any change in the glass-like phase and monotonously decreases with a field increase in the paraelectric state.For complete understanding of the microscopic origin of the observed phenomena,a detailed study on the short-and long-range structures at the same condition is necessary.

Effect of interface carbonization on dielectric properties of potassium nitrate nanocomposite based on porous glasses

Dielectric properties and structure of pure and carbon-modified nanocomposites on the base of porous glasses with an average pore diameter of 6 nm(PG6)with embedded KNO3 have been studied at the temperature diapason of 300-430 K and at fre-quencies of 0.1-3×10^(6) Hz on cooling.X-ray diffraction studies of these samples have shown,that in modified and unmodified composites there is a mixture of the low-temperature paraelectric phase(α-phase)and the ferroelectricγ-phase.In modified composites,a decrease in permittivity and conductivity is observed.Dielectric response has been analyzed in the framework of modern theoretical models.Two relaxation processes have been identified and their origin has been determined.It has been found that the main contribution to the dielectric response of nanocomposite material PG6+KNO3 is provided by charge polarization on interfaces,which can be governed by modifying the inner pore surfaces.DC-conductivity of both composites has been estimated and the activation energies have been determined.Activation energy change observed in a vicinity of 360 K is attributed to the phase transformation and the appearance of KNO3α-phase.