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Organized by
the Institute of Experimental Physics, University of Bialystok
Jonas Grigas, Lithuania
Microwave dielectric spectroscopy of ferroelectrics
Jonas Grigas
Vilnius University, Lithuania
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The aim of dielectric spectroscopy is to determine the complex permittivity of various excitations related to polarization. Dynamic processes in ferroelectrics proceed in such a way so that the characteristic frequencies lie in the microwave region. By microwaves we mean the frequency range 0.3 to 3000 GHz. A soft mode frequency in a number of displacive ferroelectrics on approaching the Curie temperature drops into the microwave range. The relaxational soft mode frequency in order-disorder ferroelectrics lies entirely in the microwave range. Spectra of the fundamental ferroelectric dispersion and losses give information on the microscopic dynamic mechanism of phase transitions and show the frequency regions were ferroelectrics are useful for applications for active and passive devices. Possibilities of light and neutron scattering and infrared reflectivity techniques are limited by 10 cm-1 or 300 GHz. We have developed broad-band methods do study excitations at lower frequencies.
Frequency Domain Dielectric Spectroscopy operates from low frequencies to 900 GHz. Although there is now complete overlap and coverage of the radio-frequencies to the infrared band, the experimental methods bases on coaxial, waveguide and free-space techniques is still divided. Resonant methods are not conventional for microwave dielectric spectroscopy (MDS) when dielectric losses are high. The discrete frequency and narrow-band methods also have not had their day and will not be discussed.
The coaxial technique is the most convenient in the meter and decimeter wavelength range. The specimen is placed in the end of the coaxial line between the inner conductor and the short piston and forms the capacitor. When the conditions of the quasi-stationary electric field distribution in the sample are satisfied, the permittivity and losses can be calculated from the formulas of the static capacitor. When the permittivity of the sample is high, a dynamic capacitor, which takes into account the homogeneous distribution of the electric field in the sample, should be used.
In the centimeter and millimeter wave range, the waveguide methods based on the measurements of wave parameters are used. The most general case is the plane-parallel dielectric plate filling the cross-section of the rectangular waveguide. Dielectric spectra are obtained from the measured complex amplitude reflection and/or transmission coefficients. These methods are useful for ceramic materials. However, a great number of ferroelectric anisotropic single crystals can not be grown of sufficient dimensions to totally fill a cross-section of a waveguide. The rigorous solution for the reflection and transmission coefficients of the main TE10-wave for the thin dielectric cylinder and the cylinder of any radius were obtained and enabled us to perform MDS of thin ferroelectric rods of needle-shaped single crystals in the centimeter and millimeter wave range. Such the spectrometers, developed at Vilnius University, provides the temperature and frequency dependences of practically any value of permittivity and losses of solids.
Microwave helicons and magnetoplasma resonance are used for the determination of the dielectric parameters of semiconductors and narrow-gap semiconducting ferroelectrics.
The results of the solved electrodynamical problems and all the methods mentioned above, together with the most interesting results revealed by the MDS, will be discussed in the lectures which attempt to survey new approaches to MDS of ferroelectrics, semiconductors, superionics and other solids in almost the initial state of intensive development of this spectroscopy [1].
MDS of semiconductors revealed anomalous dielectric properties, soft mode and friable crystal lattices, and various phase transitions in the family of low-dimensional chalcogenides. A number of crystals of this family are either ferroelectrics or incipient ones.
The soft mode and phase transition problems of many ferroelectrics have been discussed for three decades to a great extent because of the soft mode frequency near the Curie temperature lies beyond the limits of conventional spectroscopical techniques. MDS of primarily displacive semiconductive SbSI-type, Sn2P2S6-type and other ferroelectrics have shown that in the vicinity of the Curie temperature the frequency of the phonon soft mode is strongly anharmonic and drops to the microwave range. The soft mode behavior shows that displacive-like and order-disorder-like behaviors are in fact regimes dominant at different temperatures. In the vicinity of the Curie temperature, the soft mode splits into several components different of which are seen by different spectroscopies. It seems that this feature is rather common in ferroelectrics.
MDS revealed also amplitudon and phason excitations in semiconductive ferroelectrics TlGaSe2 and Tl In S2 with an incommensurate structure modulation.
The complete dielectric spectra of ferroelectric dispersion in quasi-one-dimensional hydrogen-bonded ferroelectrics and in a number of new or less-studied other order-disorder ferroelectrics and relaxor ceramics will be discussed in the frequency range 106 to 1011 Hz. Based on these studies the dynamical models of the ferroelectric phase transitions were developed.
In toto, manifestations of various excitations in ferroelectrics and other solids which lie beyond the limits of optical and neutron spectroscopy can be successfully studied by means of contemporary MDS.
[1] J.Grigas. Microwave Dielectric Spectroscopy of Ferroelectrics and Related Materials (OPA Gordon & Breach Science Publ., Amsterdam).
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