Perovskite-like layered structure A₂B₂O₇ ferroelectrics and solid solutions

• Main Author: Gao, Zhipeng
• Published: Queen Mary, University of London 2013
• Subjects: 666
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.694421

In this project, the ferroelectric materials Pr2Ti2O7, La2Ti2O7, Sr2Nb2O7, La2-xCexTi2O7 (x=0.15, 0.25, 0.35), Nd2-xCexTi2O7 (x=0.05, 0.25, 0.5, 0.75) and Sr2-xBaxNb2O7 (x=0.1, 0.2, 0.3, 0.4, 0.5) were investigated. They have a provskite-like layered structure (PLS), and are well known for their super-high Curie points (>1200 ˚C). Their ceramics were fabricated using Spark Plasma Sintering. For Pr2Ti2O7, single phase, dense and textured ceramics were prepared. The Curie point is greater than 1560 °C which is the highest known Curie Point so far for ferroelectric materials. Pr2Ti2O7 was shown for the first time to be ferroelectric because it showed piezoelectric activity after poling. For the La2-xCexTi2O7 solid solution system, the ferroelectric and dielectric properties of cerium (Ce) substituted La2Ti2O7 (LTO) were investigated. The solubility limit of Ce in La2-xCexTi2O7 was found to be between 0.35 and 0.5 supported by XRD results. The a-, b- and c-axes of the unit cell decrease with increasing Ce substitution. The Curie points (Tc) of La2-xCexTi2O7 (x=0, 0.15, 0.25, 0.35) also decreases. The dielectric constant and loss increase with increasing Ce substitution. Electrical resistivity decreases due to Ce substitution. Cerium can increase the d33 of La2Ti2O7. The highest d33 was 3.9 ± 0.1pC/N for La1.85Ce0.15Ti2O7. In the Nd2-xCexTi2O7 system, the cell volume increases from Nd2Ti2O7 to Nd1.25Ce0.75Ti2O7 and the Curie point (Tc) decreases with Ce increase. For the Sr2-xBaxNb2O7 solid solution system, the effect of Ba substitution on the structure and ferroelectric properties of Sr2-xBaxNb2O7 (x<1.0) was investigated. The a-, b-, c- axes and cell volume increase with Ba addition because Ba2+ is a relatively large ion. A atomic displace move model was developed to explain the spontaneous lattice strain, spontaneous polarization and Curie point change in the orthorhombic phase (Cmc21) with increasing Ba substitution. The critical point of Sr2-xBaxNb2O7 solid solution (x < 0.6) was determined by XRD and was supported by the XPS spectra of Ba 2p and O 1s. Textured ceramics of Sr2-xBaxNb2O7 compounds were prepared using the spark plasma sintering technique and the piezoelectric activity can be improved by Ba substitution, which increases the domain switch mobility. The highest d33 was measured as 3.6±0.1pC/N for Sr1.8Ba0.2Nb2O7. The thermal depoling behaviors of La2Ti2O7, and Sr2Nb2O7 were investigated due to their relatively high d33 piezoelectric constant and high Curie point. Both of them have a high resistance to thermal depoling, especially La2Ti2O7. Ginzburg - Landau theory was used to explain their behavior. The electric resistivity degradation of Sr2Nb2O7 was studied at different temperatures, and it was found to be stable below 800 °C.