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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.

The dielectric behaviour of ferroelectric hydrogen-bonded deuterated betaine phosphite crystals is investigated in the frequency range up to 4 GHz. The dielectric dynamics of Debye type shows a critical slowing-down of the polarization fluctuations. The frequency of the relaxational soft mode in the para-electric phase varies according to v s = 0.2(T - 295.8) GHz and decreases to 0.55 GHz at T C = 298 K.

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.

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.

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.

La- and Mn-doped multiferroic BiFeO 3 thin films were fabricated and characterized at radio- and microwave frequencies using the parallel-plate capacitor structures at a room temperature. A single phase epitaxial-quality thin films were grown by RF sputtering on SrTiO 3 (100) substrate with highly conductive SrRuO 3 and Ti/Au layers used as bottom and top electrodes. In the frequency range 40 MHz – 30 GHz the Bi 0.9 La 0.1 Fe 0.9 Mn 0.1 O 3 (BLFMO) films show relatively low losses and dielectric constant value close to 90. Analysis of the frequency dependences of loss tangent shows power law 1/3 with estimated value of microwave loss tangent less than 0.06 at frequencies below 20 GHz. We assume that the oxygen vacancies in the thin film act as charged defects and cause extrinsic microwave losses.

This paper presents the results of the investigation of dielectric dispersion and ultrasonic velocity in the ferroelectric (CH3)2NH2Al(SO4)2 · 6H2O crystal. The crystal shows a critical slowing down process of polarization with an extremely long relaxation time of the dipole system (τ = 1.6 · 10−7s at the phase transition point). The dielectric response over the frequency range up to 56 GHz in the paraelectric phase can be well described in terms of a monodispersive Debye-type formula. The activation energy of dipoles in the paraelectric phase is 0.11 eV = 8.5 kTc . The results show that the proper ferroelectric phase transition is nearly critical and of the order-disorder type.

We present the dielectric dispersion in a Pb(Fe1/2Nb1/2)O3 (PFN) thin film grown on (001) SrTiO3 substrate from 20 MHz to 20 GHz in the temperature range of 270 to 340 K. In the radio frequency region, the hopping charge transport and associated polar nanoregion and/or domain-wall motion contribute to the spectrum. In the microwave region, relaxational dispersion was observed with a dielectric contribution of Δe ≈ 600 at room temperature having a characteristic frequency (fm)0.4 = 9.8 · (T−T0) Hz, where T0 is 660 K. It is associated with possible mode-softening behavior, related to the onset of polar nanoregions at T0. The dielectric permittivity shows similarities with 1-dimensional Ising model behaviour.

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.