J. Petzelt

Polar optic phonons in SbSI-type single crystals

Far infrared reflectivity measurements of vapour-grown single crystals SbSI, SbSeI, BiSI, and BiSeI have been performed in the region of 50-400 cm-1 at 25 and -170°C. The results are analysed by Kramers-Kronig relations to obtain the dielectric response functions. From the results the eigenvectors of all B1u normal modes have been calculated. For the soft mode in SbSI it is obtained that all atoms vibrate along the ferroelectric c axis. The Sb displacements are 1.3 times larger and of opposite direction to the S and I displacements.

Far infrared and submillimetre studies of the ferroelectric phase transition in (VF2-F3E) copolymers

Abstract Polarized transmission spectra of oriented (VF2-F3E) copolymer film were measured in the 8–500 cm−1 range for the VF2/F3E composition of 100/0, 80/20, 70/30 and 60/40 as a function of temperature in both ferroelectric and paraelectric phases. The spectra were evaluated to obtain the dielectric response function and absorption coefficient. In comparison with pure PVF2, the lowest-frequency mode at 73cm−1 splits in the copolymer samples into two components at ∼51–61 and ∼ 115 cm−1 which are assigned to Brillouin zone centre and boundary (Z-point) librational modes around the polymer chain axis with respect to the Brillouin zone of the ideal PVF2 crystal. The lower mode of this pair disappears in the paraelectric phase in agreement with selection rules for regular 50/50 copolymer. Higher-frequency modes do not strictly obey the selection rules because of compositional and dynamical disorder. In the submillimetre range, 8–30 cm−1, additional relaxation (over-damped oscillation) was found which is ass...

Thermodynamic theories of phase transitions in betaine compounds

Abstract We propose phenomenological description of various structural phase transitions in betaine compounds.

Far-infrared spectroscopy of modulated and ferroelectric betaine calcium chloride dihydrate

Far-infrared and near-millimetre transmission and reflectivity measurements of betaine calcium chloride dihydrate were carried out between 300 and 15 K. All modes below 300 cm-1 were classified according to symmetry and their wavevectors q using selection rules for mode activation in the modulated phases. Almost all activated modes with q not=0 can be assigned assuming condensation of the single order parameter of Lambda 3 symmetry (Py(k) wave). In the low-temperature homogeneous ferroelectric phase, traces of modulated phases are observed depending on the sample clamping. On the basis of the authors assignment the dispersion curves of the eight lowest phonon branches along the modulation direction were constructed for the four main modulated phases (incommensurate and commensurate with delta =1/4, 1/5, 1/6). The behaviour of amplitudons and pseudophasons is discussed. The E//y mode below 10 cm-1 in the incommensurate phase has been assigned to the inhomogeneous Lambda 2(c*-2k) mode (c* is the reciprocal-lattice vector and k the modulation wavevector).

Dynamic critical behaviour in TSCC

Abstract Paraelectric soft mode behaviour in TSCC was reinvestigated using near-millimetre dielectric spectroscopy and compared with low-frequency dielectric data. Farther above Tc the classical displacive mean-field behaviour was observed with the extrapolated Curie temperature ≌ 2.6 K above Tc. Closer to Tc deviations from Cochran law as well as appearence of the central mode are correlated with deviations from the Curie-Weiss law for the low-frequency permittivity. The central mode is assigned to critical cluster dynamics.

Soft modes in betaines

Abstract Betainea are a new family of crystals including ferroelectric, antiferroelectric and incommensurate crystals /1–3/. We have used submillimeter backward wave oscillator spectroscopy /4/ to investigate the dynamical properties of betainearsenate (BA), deuterated BA (DBA), betainephosphate (BP) and betainecalciumchloride (BCC) at the frequencies 3–30cm−1.

Far infrared spectroscopy of the phase-transition sequence in BCCD

Abstract Polarized far infrared (8-650 cm−1) transmission and reflectivity measurements of BCCD have been carried out from 300 K to 10 K using monochromatic backward-wave-oscillator and Fourier transform spectroscopy. The spectra were evaluated to obtain the complex dielectric function. Using the recent thermodynamic theory of the phase transition sequence, derived selection rules for the activity of phonon modes and published Raman data we were able to assign all the rich spectra below ∼40 cm−1. It enabled us to construct, in a semiquantitative way, the dispersion of 10 lowest phonon branches in the k=(0,0, k;) direction in all main phases and to explain the complex soft-mode behaviour in this compound.

Infrared properties of (NH4)3H(SeO4)2 and Rb3H(SeO4)2

Abstract Infrared reflectivity (30 – 4000 cm−1, E°C polarization) of (NH4)3H(SeO4)2 and Rb3H(SeO4)2 has been measured as a function of temperature in the normal and superionic phases. The dielectric function was evaluated by means of Kramers-Kronig analysis. The broad absorption due to the proton disorder recently observed in Cs3H(SeO4)2 seems to be also present but needs further investigation.

Submillimeter spectra of the Rb2ZnCl4 and Rb2CoCl4 crystals at low temperatures

Abstract The present paper reports the observation in the spectra of Rb2ZnC14 and Rb2CoC14 crystals of lattice absorption wide line which transforms into a set of low-frequency phonon modes at low temperatures.

Multirelaxation response of modulated phases in structural order-disorder systems

We study properties of multirelaxational phenomena originating due to interactions of modes in the incommensurate dielectrics of the order-disorder type described by the Ising-like model and the Bloch equations. Results found theoretically are discussed in relation to observed properties of some dielectric materials including NaNO2.