J. Grigas

Dielectric dispersion of the relaxor PLZT ceramics in the frequency range 20 Hz-100 THz

The dielectric dispersion of the transparent relaxor ferroelectric ceramics PLZT 8/65/35 and 9.5/65/35 was determined in a wide frequency range including the microwave and infrared range. The number of observed polar phonons in infrared spectra gives evidence about the locally broken cubic symmetry and the presence of polar nanoclusters in the whole investigated temperature range up to 530 K. A single broad and symmetric dispersion that occurs below the polar phonon frequencies was fitted with the Cole-Cole formula and a uniform distribution of Debye relaxations. On decreasing temperature, the distribution of relaxation times becomes extremely broad which indicates increasing correlation among the clusters. The mean relaxation time diverges according to the Vogel-Fulcher law with the same freezing temperature 230±5 K for both ceramics, but different activation energies 1370 K and 1040 K for the 8/65/35 and 9.5/65/35 sample, respectively. The shortest relaxation time is about 10-12 s and remains almost temperature independent. Below room temperature, the loss spectra become essentially frequency independent and the permittivity increases linearly with decreasing logarithm of frequency. The slope of this dependence is proportional to T 4 in the investigated temperature range (above 210 K) which indicates appreciable anharmonicity of the potential for polarization fluctuations.

High-frequency dielectric spectra of AgTaO3-AgNbO3 mixed ceramics

This paper presents the results of dielectric susceptibility investigations performed in the radiowave, microwave, submillimetre wave and infrared (IR) regions on the AgTaO3-AgNbO3 solid solution with Nb contents of 0.3, 0.4 and 0.6. An intense dipolar relaxation, which is responsible for the temperature dependence of the static permittivity and which freezes out at low temperatures, has been evidenced in the submillimetre wave region.

Dielectric Relaxation in Ba2NaNb5(1-x)Ta5xO15 Single Crystrals.

This paper presents the results of dielectric dispersion in Ba 2 NaNb 5(1- x ) Ta 5 x O 15 (BNNT) single crystals over the frequency range 1 kHz to 4 GHz. The fundamental dielectric dispersion occurs above 10 7 Hz and is caused by a soft relaxational mode. This mode gives the main contribution to the high static permittivity which fits the Curie-Weiss law. The results indicate the anharmonic motion of the Nb(Ta) atoms and the order-disorder nature of the proper ferroelectric phase transition in BNNT.

Soft mode and its electronic potential in SbSI-type mixed crystals

The paper presents the results of ferroelectric dispersion at microwaves and of calculation of electronic potential dependence upon normal coordinates of B1u mode in paraelectric and ferroelectric phases in SbSI-type mixed crystals. Obtained large anharmonicity of the electronic potential at Sb (or Bi) site confirms the splitting of the soft B1a mode, the main component of which causes the ferroelectric dispersion at microwaves. Dependence of the electronic potential on composition revealed all factors which change the temperature of phase transition and the soft mode dynamics. It also explains why the pure SbSeI, BiSI and BiSel are not ferroelectrics.

Splitting of the XPS in Ferroelectric SbSI Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of the ferroelectric SbSI single crystal. Results revealed the new phenomenon--splitting of the core levels at the phase transition temperature T cl . The XPS are measured with monochromatized Al K f radiation in the energy range 0-1400 eV and the temperature range 130-330 K. Experimentally obtained energies are compared with the results of theoretical ab initio calculations. This is the first observation of the splitting of the XPS in a ferroelectric crystal. The mechanism of the XPS splitting in SbSI crystals is discussed.

Microwave Dielectric Spectroscopy of Ferroelectrics

This paper is a review article that emphasizes that dynamical processes of electromagnetic wave interaction with ferroelectrics proceed in such a way that characteristic frequencies of various excitations related to polarization lie in microwave range: i) a soft mode frequency in a number of displacive ferroelectrics on approaching the Curie temperature drops into the millimeter wave range or splits to components, the lower of which lies at microwaves and explains the static permittivity; ii) a frequency of relaxation soft mode in a number of order-disorder ferroelectrics lies also in the microwave range, and iii) relaxation of permittivity in relaxors and dipolar glasses also occurs at microwaves. Results of the complete dielectric spectra of the fundamental dielectric dispersion in the frequency range 1 MHz to 1012 Hz and its microscopic description in the most characteristic materials are presented and discussed.

Origin of ferroelectricity in SbSI

Abstract There are three phases in the phase diagram of SbSI: ferroelectric (T 410 K). In the antiferroelectric phase the arrangement of the polar chains becomes antiparallel (up and down). It causes a disorder of the Sb atoms. Ferroelectricity and phase transitions in SbSI are closely related to its electronic structure and phonon interaction. The disorder results in an asymmetric double-well soft-mode potential. It leads to threefold splitting of the soft mode frequency. They all show large absorption peak and deep dielectric dispersion in millimeter and far IR region.

X-ray Photoelectron Spectroscopy of Sb2S3 Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of semiconductor ferroelectric Sb2S3 single crystals, which show weak phase transitions and anomalies of various physical properties. The XPS were measured with monochromatized Al K α radiation in the energy range 0-1450 eV and the temperature range 160-450 K. The valence band is located 0.8-7.5 eV below the Fermi level. Experimental results of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of Sb2S3 crystal. The chemical shifts in Sb2S3 crystal for the Sb and S states are obtained. Results revealed that the small structural rearrangements at the phase transition T c1 = 300 K shift the Fermi level and all electronic spectrum. Also, temperature dependence of a spontaneous polarisation shifts the electronic spectra of the valence band and core levels. Specific temperature-dependent excitations in Sb 3d core levels are also revealed.

Microwave study of the soft ferroelectric mode in Sn2P2S6 crystals

Abstract This paper presents the results of the investigation of dielectric dispersion in semiconductive ferroelectric Sn2P2S6 crystals over the frequency range 1 kHz to 78.5 GHz. The main dielectric dispersion in Sn2P2S6 is caused by the soft ferroelectric Bu mode and in the vicinity of the Curie point it occurs in the millimeter region. The frequency of the soft mode in the paraelectric phase varies according to vs = 35 (T-Tc )½ GHz on approaching the Curie point. The soft mode is strongly overdamped. Close to Tc the relative damping is y/vs = 14. The dielectric contribution of the soft mode is equal to the static dielectric permittivity and it explains its whole temperature-dependence.

Dielectric relaxation in ferroelectric (CH3)2NH2AL(so4)2 · 6H2O crystal

Abstract 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.