G. Völkel

Broadband dielectric spectroscopy of water confined in MCM-41 molecular sieve materials : low-temperature freezing phenomena

Dielectric properties of water adsorbed in pure siliceous and aluminium containing mesoporous MCM-41 materials have been investigated in the frequency range from 20 Hz to 1 MHz. The dielectric spectra revealed three dispersion regions, liquid-like free water in the centre of the mesopores, an intermediate water layer with reduced mobility, and an interfacial water layer at the inner surface of the mesopores. The analysis of the relaxation time distribution by means of a double-well potential indicates a strong dependence of the barrier height of reorienting water molecules dipoles in the interfacial layer on the Si/Al ratio in the framework.

Electron spin-lattice relaxation in Mn2+-doped ferroelectric TSCC

Abstract The spin-lattice relaxation of ferroelectric TSCC: Mn 2+ has been investigated by means of the electron spin echo method in the range between 4.2 and 160 K. In the vicinity of the phase transition an anomalous increase of T 1 has been detected deviating from the spin-lattice relaxation in the remaining temperature range.

Estimation of critical exponents for the ferroelectric transition in Mn2+-doped tris-sarcosine calcium chloride (TSCC) by means of EPR

Abstract The static rotation angle of the EPR fine-structure tensor of Mn 2+ centers of ferroelectric TSCC correlates with the order parameter. Its temperature dependence shows two distinct regions. In the vicinity of the phase transition temperature the critical exponent is β = 0.322 ± 0.012, below this critical region its temperature dependence points at a phase transition of weak first order.

Critical fluctuations in trissarcosine calcium chloride (TSCC) observed by the electron spin echo (ESE) method

Abstract In the close vicinity of the second order phase transition in manganese doped ferroelectric TSCC a critical T 2 effect of the paramagnetic Mn 2+ center has been detected by means of the electron spin echo method. The strong decrease of T 2 required the highest time resolution and sensitivity of the spectrometer. The theoretical explanation of the observed effect shows, that TSCC is a ferroelectric material of relaxational type with anisotropic interaction.

Structural phase transitions in partially deuterated betaine phosphite crystals studied by dielectric and electron paramagnetic resonance methods

Abstract Partially deuterated betaine phosphite (DBPI) crystals were investigated by measurements of the dielectric constant and by means of electron paramagnetic resonance (EPR). Two phase transitions could be detected in the temperature range from 100 K to 380 K: a high-temperature phase transition at Tc1 = (355 ± 3) K and a ferroelectric phase transition at Tc2 = (297 ± 3) K. The EPR studies show that the ferroelectric phase transition is related to the simultaneous ordering of the protons and deuterons in the O-H/D… O bonds.

Observation of potassium hyperfine interactions in X-irradiated KH2AsO4 through the method of electron spin echo envelope modulation

Abstract Hyperfine interaction frequencies of 1H and 39K nuclei near the AsO4-4 radical in X-ray irradiated KH2AsO4 (KDA) have been observed through the method of electron spin echo envelope modulation (ESEEM). This method enabled us to record nuclear hyperfine interaction (ENDOR-like) spectra around the ferroelectric phase transition of KDA for the first time. The ESEEM spectrum of 39K exhibits a clear change when passing the ferroelectric phase transition temperature, but that of close protons does not. The result for close protons is in agreement with the symmetry breaking of the AsO4-4 site as observed via the EPR spectrum [5]. Finally, at 4.2 K the hyperfine interaction parameters of a 39K nucleus near the AsO4-4 unit could be determined through the ESEEM method.

EPR, ENDOR and ESE investigations on chromium modified PbTiO3 ceramics

Abstract EPR and ENDOR studies show that chromium is incorporated in mono crystalline regions of the grains in PbTiO 3 ceramics as Cr 5+ ions at Ti 4+ sites. The local symmetry of the Cr 5+ impurity is lower than of the host Ti 4+ site in the crystallite. ESE measurements of the spin-lattice relaxation of the Cr 5+ centre point to a structural transition at 10 K.

Epr investigations of the dynamics of ferroelectric tris-sarcosine calcium chloride

Abstract The strong decrease of the spin-lattice relaxation rate and the characteristic line asymmetry obtained for paramagnetic resonance experiments on TSCC near the transition temperature are explained by the influence of lattice defects on the ferroelectric properties. Experiments near room temperature point to a softening of the lattice prior to the hypothetical ferroelastic transition.

Esr–investigations on γ–irradiated ferroelectric betaine compounds

Abstract Part of this work was supported by the BMFT. Valuable support by J. Huttermann and J. Albers, Universitat des Saarlandes, are gratefully acknowledged.

EPR investigations of critical order parameter fluctuations in ferroelectric tris-sarcosine calcium chloride (TSCC)

Abstract The EPR spectra of Mn 2+ -doped TSCC show in the immediate vicinity of the ferroelectric transition a critical line broadening. It is angular-dependent and may be related to a critical slowing down of the polarization fluctuations at the phase transition point. The observed line asymmetry points at a strong influence of the real structure of the crystal on the effects investigated.