REPOSITORY > RESULTS

Abstract

KTaO3 belongs to the group of incipient ferroelectrics, which are materials important from the fundamental, as well as from the applications, point of view. It is well known in its single-crystal form; however, the processing of technologically interesting ceramics and thin films is a challenging task and knowledge of their dielectric properties is still at a basic level.
In the frame of the thesis we have prepared KTaO3 powder from mechanochemically activated K2CO3-Ta2O5 powder mixtures by heating at 800°C. The powders were found to be heterogeneous at the microscopic level; therefore, additional heating at the same temperature was performed. Single-phase KTaO3 ceramics with relative densities above 95 % were obtained by hot pressing the powder compacts at 1250°C for 2 h. The ceramics have a bimodal grain size distribution in the μm range. The dielectric permittivity of the hot pressed ceramic, prepared from the double-calcined powder, measured at 5 K and 1 kHz is 4080, which is almost twice the value obtained for the ceramic prepared from the single-calcined powder. In fact it is comparable to the value reported for single crystals, i.e., 4500.
The lattice dynamics of the KTaO3 ceramics with enhanced dielectric properties were evaluated for the first time. Three polar modes expected for the cubic structure were observed. As in single crystals, the lowest-frequency TO1 mode (soft mode) strongly softens on cooling, while the TO2 and TO4 mode frequencies do not change with temperature. The permittivity does not show any significant dispersion below the soft-mode frequency and its value in the kHz and GHz ranges is mainly given by the intrinsic polar lattice mode contribution. The soft-mode frequency agrees with the values found in single crystals; this indicates a negligible influence of the grain boundaries on the dielectric response in KTaO3, unlike in other ferroelectric or incipient ferroelectric perovskite ceramics.
Furthermore, we prepared solution-derived polycrystalline KTaO3 thin films on polycrystalline alumina and (0001) sapphire substrates. The 200-nm-thick phase-pure perovskite films were obtained after heating at 900°C by adding 30 at.% excess of potassium to the sols.
Broadband dielectric spectroscopy was employed to evaluate the properties of the films. In contrast to the single crystals and ceramics, a peak was observed in the temperature-dependent dielectric permittivity measured in a radio-frequency and microwave ranges. The low-temperature ferroelectric state was fingerprinted by the quasi-static polarization measurements. Analyses of the lattice dynamics of the KTaO3 films on sapphire revealed that the soft mode softens on cooling, with a minimum frequency at ~60 K. It was found to be linearly coupled to a central mode, which is silent in the paraelectric phase, and becomes coupled to the polarization below ~60 K. The result
gives strong evidence for the paraelectric-ferroelectric phase transition, which was not observed before in polycrystalline KTaO3 thin films.
We also studied the processing and properties of the solution-derived KTa0.6Nb0.4O3. It is a composition in the solid solution of KTaO3 with the ferroelectric KNbO3 (K(Ta,Nb)O3), and is a candidate material for microwave applications.
We have employed extended X-ray-absorption fine-structure spectroscopy to follow the structural evolution of the potassium acetate and transition-metal alkoxide-based sols upon refluxing. While the monomeric Ta-species were found to be rather inert, the dimeric Nb-alkoxide started to form oligomers upon prolonged refluxing. The K-O-transition-metal correlations were detected in all the sols and the number of K neighbors around Nb increased upon refluxing, saturating at 24 h.
The formation of bimetallic species between K and both transition metals strongly affects the crystallization behavior of the films on polycrystalline alumina substrates. We obtained single-phase perovskite films after heating at 900°C only from the 24-h-refluxed solutions, while the films prepared from the 1-h-refluxed solutions had a multi-phase composition and heterogeneous microstructures. The dielectric properties were strongly enhanced as well. The room-temperature tunability, measured at 1 MHz and an applied field of 92 kV/cm, has the value of 3.8.
The room-temperature permittivity decreases from 2430 at 10 kHz to 590 at 14.5 GHz and the relaxor-like behavior was confirmed by a fit to the Vogel-Fulcher law. Even though this frequency dependence and the rather high dielectric losses represent a challenge for the applicability of the solution-derived K(Ta,Nb)O3 thin films in microwave devices, the observed formation of nanoscale heterogeneities in the sols provides a hint for further improvements to the functional properties.