PHASE TRANSITION(S) IN TITANITE CATISIO5 - AN INFRARED SPECTROSCOPIC,DIELECTRIC RESPONSE AND HEAT-CAPACITY STUDY

The structural phase transition in titanite near 500 K (averaged symme tries A2/a --> P2(1)/a) and a second anomaly around 900 K have been st udied using infrared spectroscopy on single crystals aqnd powder sampl es, measurements of the dielectric properties and the specific heat. T he same synthetic single crystal was used in all experiments. The phas e transition near 500 K is associated with a break in the temperature evolution of phonon frequencies and absorption intensities. Some phono n signals decrease rapidly under further heating and their extrapolate d intensities disappear at ca. 850 K. The most dominant temperature ef fect relates to Ti - O phonons with amplitudes along the crystallograp hic 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 th e increase of the real part of the dielectric constant with a well-pro nounced anomaly at 500 K. The dielectric losses also increse with incr easing temperature. Measurements under strong field do not show antife rroelectricity. The transition at 500 K generates a small but sharp la mbda-anomaly in the excess specific heat. A second, weaker anomaly was found near 850 K. The results are discussed in terms of thermodynamic models.