Structure property relations in ferroelectric materials

A series of experiments using synchrotron x-ray radiation to study ferroelectrics are performed. Primarily, the ferroelectric relaxor lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃, and related systems are investigated. PMN is an important ferroelectric with a maximum dielectric constant of 30,000 and a broad dielectric transition near room temperature. Diffraction experiments reveal domains of chemical ordering of the Mg and Nb cations. These ordered domains are limited to about 50 Å in size and result in broad and weak superstructure reflections. Crystallographic measurements of superstructure peaks are used to find the structure of the ordered domains.; By doping PMN with lanthanum (La-PMN) to replace the lead, the ordered domain size is increased: up to 900 Å for 10% La content. Again, the structure of the ordered domains is studied and the chemical ordering is seen to change from complete chemical ordering between the Nb and Mg ions (space charge model) to chemical ordering between Nb and (Mg_2/3Nb _1/3)—random layer model. While some of the increase in ordered domain size may be due to the additional charge provided by La3+, an alternative explanation based on internal strain arguments is provided.; The effect of the ordered domain size together with dielectric measurements are described in order to better understand the relaxor behavior. The relaxor behavior can be understood in terms of the interference of the chemically ordered domains with the ferroelectric polar domains. As the chemically ordered domain size increases, the La PMN crystals become more relaxor like. Doping with titanium (PMN-PT) on the Mg/Nb site leads to decreasing ordered domain size and an increase of the ferroelectrically active polar domains. Further evidence of the importance of strain to the degree of chemical ordering is provided by growing thin films of PMN on a substrate with a smaller lattice constant (SrRuO₃). The mismatch in lattice constant produces a compressive strain on the films and the degree of ordering is seen to increase compared to bulk PMN.; A diffraction anomalous fine structure (DAFS) experiment is also performed on PMN to independently confirm the structure found using crystallography. By sitting at a superstructure reflection, only the signal coming from the chemically ordered domains is collected, so the bond lengths from within the ordered domains can be determined.; Finally, experiments on thin films of anti-ferroelectric lead zirconate, PbZrO₃, are performed. PZ undergoes a phase transition from anti-ferroelectric to paraelectric cubic at 230°C. The effect of the thin film PZ samples is to change its phase transition from first to higher order, as measured by the intensity of the superstructure peaks and the orthorhombic order parameter.