Dynamika sieci krystalicznej a pojawianie się ferroelektryczności w wolframianie bizmutu Bi 2WO6 i antyferroelektryczności w niobianie srebra AgNbO3 i roztworach stałych na jego bazie

Abstract

Three groups of materials were studied in this thesis. First studied group was Bi2WO6 (BWO). Two types of BWO single crystals were examined. BWO is an uniaxial ferroelectric material studied mainly due to its high spontaneous polarization, high Curie temperature as well as its ion conductivity, and excellent photocatalytic properties. BWO crystals were studied by X-ray diffraction, microscopic methods (SEM), dielectric and Raman experiments. Moreover, pyroelectric effect was examined. Dielectric studies confirmed existence of dielectric anisotropy. Raman experiments evidenced that symmetry distortion slowly decreases with increasing temperature. Moreover, the Raman spectra show no signs of phase transition in studied temperature range. At least five phonon modes show significant softening. Frequency of these modes does not decrease to zero in the vicinity of P21ab-B2cb transition. In higher temperatures, above P21ab-B2cb transition, the temperature dependence of frequency of 57 cm-1 mode is expected to change and decrease to zero in vicinity of ferroelectric phase transition. This mode is responsible for appearance of ferroelectric state in Bi2WO6. The second studied group of materials were nonstoichiometric AgxNbO2.5+x/2 ceramics. This research was performed to evaluate whether deficient of silver in crystal lattice is responsible for appearance of wide maximum of dielectric permittivity. Ceramics were obtained for silver concentrations x equal to 0.95, 0.98, 1.00, 1.02, and 1.05. Obtained samples were studied by scanning electron microscopy, X-ray diffraction, dielectric and thermal (DSC) experiments. SEM and XRD experiments showed that samples contained mainly perovskite phase of AgNbO3. Only for x=0.95 a small quantities of secondary phase were observed. For all compositions metallic silver precipitations appear and their quantity increase with the increase of x. Moreover, SEM and XRD studies show increase of homogeneity with increase of silver to niobium ratio. Finally, it was concluded that 2% excess of Ag2O in initial composition significantly increases the quality of ceramics. Dielectric and thermal measurements (DSC) were carried out in wide temperature range. They revealed that the phase transitions become less diffused and shifted towards higher temperatures with the increase of silver concentration. These temperature shifts are more significant for the low temperature phase transitions, which are connected with appearance of the polar ferroelectric and antiferroelectric states, than for high temperature transitions related to oxygen octahedra tilts. This happens because displacements of Ag ions create electric dipoles, which form polar phases in AgNbO3 with dipoles created by Nb ion displacements. Increase of silver concentration leads to increase of both quantity and value of interacting dipoles and thus increase temperatures of phase transitions. Last group of studied materials were silver lithium niobate ceramics Ag1-xLixNbO3 (ALN). ALN were studied in vicinity of morphotrophic phase transition (x≤0.1). These materials were studied mainly because of their high spontaneous polarization and their promising, as for lead-free materials, values of piezoelectric coefficients. Samples were examined by SEM method, XRD diffraction, mass spectroscopy SIMS, dielectric, thermal DSC and Raman experiments. Characterization of samples by SEM, XRD and SIMS point to their high quality. They consist mainly of perovskite phase and small amounts of secondary phase. Grains show high homogeneity. Only small amounts of secondary phase appear in spaces between grains as well in vicinity of holes. For concentration of lithium between 0.05 ÷ 0.06 morphotrophic phase transition from rhombic M1 phase and rhombohedral R phase occurs. Temperature evolution of dielectric permittivity for ceramics with x≤0.05 are very similar to those observed for pure AgNbO3. For x=0.06 maximum of ε’(T) related to M1-M2 transition vanish and for x>0.06 only two wide maxima of ε’(T) are observed. First maxima is related with transition from ferroelectric R phase to antiferroelectric M phase. Second is related with transition from M phase to paraelectric O phase. With increase of lithium concentration, character of all observed transition becomes more diffusive. This indicates increase of structural disorder. Thermal studies confirm these results. Basing on these studies phase diagram for Ag1-xLixNbO3 was proposed. Analysis of Raman spectra points to relationship between crystal lattice dynamics and appearance of antiferroelectric state and its transformation to ferrielectric state for x≤0.05, as well as appearance of ferroelectric order for solutions with lithium concentration higher than 0.06. Analysis of central peak shows that temperature evolution of relaxation time for Ag0.98Li0.02NbO3 and Ag0.96Li0.04NbO3, related with dynamics of displacements of Nb ions changes in partial freezing temperature Tf and in temperature of ferrielectric transition M1-M2. Moreover, maximum of intensity of central peak observed in vicinity of M2-M3 transition corresponds well with maximum of low frequency dielectric permittivity. This suggests that Nb relaxation is responsible for appearance of wide maximum of ε’(T). Analysis of low frequency Raman spectra for ferroelectric Ag0.9Li0.1NbO3 ceramic showed that strong central peak appears in the vicinity of R-M transition, while low frequency phonon mode shows significant softening. Therefore, this mode is responsible for appearance of ferroelectric state.