RESUMO
Strontium stannate is under study as an ultra-stable dielectric material for microelectronic applications at low temperatures. It is known to have a remarkably temperature-independent dielectric constant from 27 K to room temperature. However, we report anomalies in the Raman spectra, dielectric response, and differential thermal analysis of strontium stannate compatible with a structural phase transition at 160 K. Further anomalies are seen in calorimetric and Raman data (but not dielectric data) that suggest another phase transition at 270 K. A preliminary x-ray powder diffraction study confirms a small change in the pseudo-cubic lattice constant a(T) at the lower transition.
RESUMO
The broadband conductive spectroscopy covering the range 10(-2) to 10(6) Hz has been employed to examine both the ionic and the segmental motions in polymer salt complexes (PSCs), consisting of polyethylene oxide (PEO), LiClO4, or LiCF3SO3. The temperature dependence of dc conductivity has been analyzed using Vogel-Tamman-Fulcher (VTF) equation and the results suggest that the temperature dependence of the dc conductivity can be well described in terms of the free volume changes with temperature. The ac conductivity at low temperatures (below 223 K) remains unchanged with the temperature, exhibiting nearly linear frequency dependence. This observation, not previously reported in polymers, indicates clearly that nearly constant loss (NCL) phenomenon, usually observed in glassy and ceramic ion conductors, is also operative in polymer electrolytes. Furthermore, a crossover from a NCL to cooperative ion hopping at higher temperatures in the frequency dependence of the conductivity has been observed. The analysis of the temperature dependence of both dc and ac conductivities demonstrates that there exists a direct correlation between the ionic conductivity and the segmental relaxation processes.
