B-cation ordering and dielectric properties of relaxor ferroelectric Pb(Sc1/2Nb1/2)O3
Molten salt synthesis and solid state synthesis were used to make relaxor ferroelectric Pb(Sc1/2Nb1/2)O3,abbreviated PSN in most literature. Many studies have previously been conducted on this material to obtain so called B- cation ordering of the scandium and niobium atoms. The reason for the interest in this is the ordering effect on the dielectric properties. To obtain cation ordering, this material is typically annealed for longer times in a temperature interval which is quite high (approx. 940°C-1200°C). In this thesis we studied the molten salt synthesis of PSN using a 0.5NaCl-0.5KCl salt combination and its effect on ordering when varying different parameters. The goal was to synthesize cation ordered PSN at lower temperatures than before. However, we were able to synthesize phase pure PSN, but no cation ordering could be observed. The syntheses were carried out using two different routes, both with and without a ScNbO4 precursor, the first referred to as the Wolframite method. Although no B- cation ordering could be observed, there were other interesting observations. When using the precursor, the purities were consequently higher. At least one unknown secondary phase was present in almost all syntheses without the precursor. In addition, different temperatures, times, and heating- and cooling rates had a similar influence on the particle sizes. The particle sizes increased with increasing time and synthesis temperature, both with and without the precursor. Fast and slow heating rates resulted in smaller and bigger PSN particles respectively. The same tendency was observed for the measured crystallite sizes. From the results of both crystallite and particle sizes, the importance of particle sizes on reaction rates were revealed as the Wolframite method showed a slower reaction rate. This is thought to be related to the large ScNbO4 precursor particles acting as PSN nucleation sites. The calculated lattice constants were in the lower range of PSN literature values but not as low as for cation ordered PSN. It was also found that the PSN powder with the smallest calculated crystallite sizes also showed the smallest lattice constants, but no significant correlation was found between them. The ordering effect on the crystal lattice is visualized and used to explain both the smaller lattice constant of ordered PSN, and the ordering effect on the dielectric properties.
To perform dielectric measurements, solid state synthesis was first used to make dense PSN pellets. Several test samples were made until the final PSN pellets reached 92.5% to 94.5% of theoretical density. The purity of the PSN pellets was established from XRD which were all phase pure after using a 4wt% PbO excess. The excess PbO showed its importance in both maintaining the PSN stoichiometry and as a sintering aid. All samples were additionally annealed at different temperatures and times in an attempt to increase the degree of ordering. However, XRD data showed no signs of cation ordering after annealing but, dielectric spectroscopy showed a clear lowering of the dielectric permittivity maxima (εm) and the permittivity maxima temperature (Tm). According to literature, this can be caused by both ordering and Pb vacancies in the crystal lattice. The frequency dependence was however larger after annealing, which is not expected for ordered PSN. The lowering of Tm and εm, and the increasing frequency dependence is therefore believed to be caused by PbO evaporation. Both experimental results and theory supporting this will be presented, and its effect on the dielectric properties is discussed.
Place, publisher, year, edition, pages
Institutt for materialteknologi , 2013. , 89 p.
IdentifiersURN: urn:nbn:no:ntnu:diva-22544Local ID: ntnudaim:9772OAI: oai:DiVA.org:ntnu-22544DiVA: diva2:649789
Grande, Tor, ProfessorSelbach, Sverre M.Morosov, Maxim