R. Sobiestianskas

Electrical conductivity dispersion in Co-doped NASICON samples

Impedance spectra of Co-doped NASICON polycrystalline samples (x = 2.0) were measured in the frequency range 0.5 MHz-1.2 GHz and analyzed in terms of the conductivity dispersion. The dispersion of bulk electrical conductivity obeys the power law frequency dependence σ(ω) = σ 0 + Kω n . It is observed from room temperature up to 450 K, i.e. up to the temperature of phase transformation of NASICON. The activation energy of the conductivity of grain interiors and the activation energy of the frequency, at which the dispersion sets on, have the same value of 0.313 eV. The activation energy value of the frequency dependent part of conductivity (0.077 eV) constitutes the proper fraction of the activation energy of the dc conductivity. Similar dispersion is found at lower frequencies for the grain-boundary conductivity. It extends in temperature up to 550 K, well over the temperature of phase transformation of NASICON. The phase transformation, taking place in the crystal grains of the material, does not influence the electrical behavior of grain boundaries. The activation energies for the grain boundary conductivity and for the grain-boundary onset frequency also have equal values (0.440 eV).

Dielectric dispersion in ferroelectric (CH3)2NH2Ga(SO4)2.7H2O crystal

Abstract This paper presents the results of the investigation of the spontaneous polarization, dielectric anisotropy and dispersion in the new ferroelectric, dimethylammonium gallium sulphate. On cooling, the crystal undergoes two first-order phase transitions at T c1 = 136 K and T c2 = 106 K with the spontaneous polarization along the x-axis between T c1 and T c2. In the vicinity of T c1 the crystal shows a critical slowing down of polarization fluctuations. In the far paraelectric phase, ferroelectric dispersion of the Debye-type is caused by a single relaxational soft mode related to the flipping motion of the dimethyl-ammonium groups in a double well potential with an activation energy 0.09 eV = 7.6 kT c1 in the paraelectric phase. Near T c1 and in the ferroelectric phase the ferroelectric dispersion overlaps with the domain wall dispersion and causes a complicated dielectric spectrum.

Microwave dielectric dispersion in ferroelectric telluric acid ammonium phosphate

Abstract The paper presents the results of the investigation of dielectric dispersion in monoclinic telluric acid ammonium phosphate crystal over the frequency range 1 MHz to 77 GHz. It is shown that ferroelectric dispersion of the Debye type along the direction perpendicular to the plane (101) occurs in the frequency range 108 to 1011 Hz and is caused by a single relaxational soft mode. The relaxational soft mode is caused by the flipping motion of protons between two potential minima in a hydrogen bond. The frequency of the flipping proton mode in the paraelectric phase varies according to vs = 0.94 (T – To ) GHz and decreases to 1.75 GHz at Tc = 317.3 K. The relaxational mode gives the main contribution to the high static dielectric permittivity of this crystal which fits the Curie-Weiss law. The results confirm the order-disorder nature of a proper ferroelectric phase transition of second order.

Microwave dielectric dispersion in diglycine nitrate

Abstract The paper presents the results of the investigation of the dielectric dispersion in diglycine nitrate crystals over the frequency range 1 MHz to 54 GHz. It is shown that the ferroelectric dispersion of the Debye type along the polar [101] direction is in the frequency range 108 to 1011 Hz and it is caused by the single relaxational soft mode. The frequency of the “flipping” proton mode in the paraelectric phase on approaching Tc varies according to γs = 0.34 (T - Tc ) GHz and makes the main contribution to the static permittivity which fits the Curie-Weiss law with the Curie constant C = 845 K. The magnitude of the activation energy of dipole reversal is found to be δU = 1.6 kTc . The results confirm the order-disorder type of ferroelectric phase transition.

Effect of Confinement on the Dynamics of Methanol

In this paper the results of dielectric spectroscopy of MCM-41 mesoporous material with incorporated MEOH are presented. Three dispersion regions of the confined MEOH have been found. The dielectric dispersion in the low temperature region is caused by MEOH molecules forming an interfacial layer with the inner surface of the mesoporous support. MEOH in the middles of the pores shows typical saddle like behaviour of the relaxation time. With incorporation of Al atoms in the framework of MCM-41 molecular sieves the saddle process becomes much weaker and vanishes at Si/Al = 64. Also, this process is followed by increase of conductivity.

Dielectric properties of (NH4)3H(SO4)2 crystals in room-and high-temperature phases

The dielectric/impedance spectra of (NH4)3H(SO4)2 crystal has been studied in room- and high-temperature phases at frequencies up to 3 GHz. The results are analyzed by calculating the distribution function g(τ) of the relaxation times. The splitting of the spectrum into two relaxators on approaching phase transition temperature indicates possible two paths for protons contributing to conductivity in high-temperature phase. Results support idea of involvement in superionic transport of ammonium N(1)H4 protons along with protons in H-bonds between the apical oxygen atoms of sulfate tetrahedra.

Microwave spectroscopy of new organic-Inorganic ferroelectrics

Abstract The paper presents the results of the dielectric investigations of the fundamental ferroelectric dispersion in the new or less studied order-disorder type organic-inorganic ferroelectrics.

Dielectric Properties and Conductivity of Iron Oxide-Barium Titanate Composites

Sintered composites of iron oxide plus barium titanate in form of core-shell structures are investigated by impedance spectroscopy. The measurements were performed in the frequency and temperature ranges 20 Hz to 1 MHz and 150 K to 510 K, respectively. The composites exhibit strong low-frequency dispersion of electrical modulus and dielectric permittivity which is caused by different mechanisms of electron transfer at low and high temperatures. A hopping transport process with activation energy of 0.22 eV for exchange between Fe2+ and Fe3+ was tentatively assigned to reign below room temperature. It gradually changes to a conducting mechanism with activation energy of 0.45 eV above 345 K.

Microwave and Ultrasonic Studies of NH4HSeO4 Crystals

The paper presents the results of dielectric studies of the sequence of four phase transitions in AHSe crystals in the frequency range 100 Hz to 77 GHz, and ultrasonic studies in the range 10 to 70 MHz.

Ferroelectric dynamic of some H-bonded crystals

Abstract The results of study on dynamic dielectric properties of some H-bonded crystals are presented. It is shown that relaxational soft modes are responsible for the ferroelectric phase transition in RbD2PO4, PbHPO4 and NH4HSeO4 and their contribution accounts for the static permittivity. Glass-like dielectric relaxation with temperature depended relaxation time τ was observed in the ferroelastic phase of Cs5D3(SO4)4. Non-Debye dielectric relaxations characteristic to ionic conductors were observed in the high-temperature phases of Cs5D3(SO4)4 and NH4HSeO4.