Abstract. The structural phase transition in titanite near 500 K (averaged symmetries A2/a-~P21/a) and a second anomaly around 900 K have been studied using infrared spectroscopy on single crystals aqnd powder samples, measurements of the dielectric properties and the specific heat. The same synthetic single crystal was used in all experiments.The phase transition near 500 K is associated with a break in the temperature evolution of phonon frequencies and absorption intensities. Some phonon signals decrease rapidly under further heating and their extrapolated intensities disappear at ca. 850 K. The most dominant temperature effect relates to Ti-O phonons with amplitudes along the crystallographic a axis. These phonons show large LO-To splitting and continue to soften under heating even at temperatures above the transitions point (ca. 500 K).The softening of these modes correlates directly with the increase of the real part of the dielectric constant with a well-pronounced anomaly at 500 K. The dielectric losses also increse with increasing temperature. Measurements under strong field do not show antiferroelectricity. The transition at 500 K generates a small but sharp )~-anomaly in the excess specific heat. A second, weaker anomaly was found near 850 K. The results are discussed in terms of thermodynamic models.

General aspects of high performance Brillouin spectroscopy in polymers using special scattering geometries such as 90A-scattering geometry are discussed. Technical improvements are reported resulting in absolute accuracies up to 0.05 % for sound velocity determination. A method of data analysis is presented delivering simultaneously the complete set of elastic stiffness constants. The influence of birefringence on the Brillouin line shifts in anisotropic polymeric systems is estimated and techniques to reduce this influence are proposed. The determination of the principal refractive indices by Brillouin spectroscopy is discussed. Furthermore, a quantity D x, which is sensitive to hypersonic relaxation processes, is introduced.

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The crystal structure of DMABA has been determined at 293 and 180 K. It crystallizes in the monoclinic P21/c space group. The structure was refined to R1 = 0.0466 for 1269 independent reflections at 293 K and R1 = 0.038 for 1728 reflections at 180 K. DMABA is isomorphous with the chlorine analogue, DMACA. Its anionic sublattice forms corrugated two-dimensional layers in the bc plane. There are two crystallographically independent, disordered dimethylammonium cations in the crystal structure. One of them occupies cavities inside the polyanionic layers; the other is located between the layers. The temperature dependence of lattice parameters has been determined between 100 and 290 K. The presence of the ferro-paraelectric phase transition at 164 K has been confirmed. The complex dielectric permittivity of DMABA single crystals in the frequency range 100 Hz-10 GHz between 10 K and 300 K has been measured. The dynamic dielectric behaviour of DMABA is found to be determined by the properties of two independent relaxators: a low frequency one (~1 MHz at Tc) with a critical slowing down of its dynamics and a high frequency one (~5 GHz), which is thermally activated and contributes to the electric permittivity at least from about 80 K.

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