Structure-property relationships in strontium barium niobate relaxor ferroelectrics

Studies of the structure-property relationships in strontium barium niobate Sr$\sb{\rm x}\rm Ba\sb{1-x}Nb\sb2O\sb6$ (SBN x/1 $-$ x) have been performed using a combination of dielectric spectroscopy, Sawyer-Tower polarization techniques, and transmission electron microscopy (TEM). The dielectric response was found to be strongly dependent on crystallographic orientation, in distinct comparison to the nearly orientation independent response found for Pb-based perovskite relaxors. The dielectric absorption of SBN was also found to be much stronger along the c-axis than along any other orientation. Investigations of the temperature dependence of the high-field polarization behavior found that a square-to-slim loop hysteresis transition occurred rapidly with increasing temperature.Bright-field imaging of $\langle 100\rangle$-orientated SBN 60/40 and 75/25 crystals has revealed nanopolar domains with a strong shape anisotropy. The morphology of the nanopolar domains was needle-like. SAED patterns revealed the existence of ${1}\over{x}$ (110) incommensurate modulations. Dark-field imaging of the incommensurate reflections found the existence of nanometer-sized ferroelastic domains. Diffuse scattering was observed in the SAED patterns along the $\langle 100\rangle$ direction. This diffuse scattering is believed to arise due to strain gradient interactions between the A-site cation distribution and the incommensuration. The strain gradient interactions are also believed to couple to the polarization. As a consequence, random quadrupolar fields are believed to break the translational invariance of the polarization in the ab plane, resulting in needle-like nanopolar domains.Studies of the effects of annealing and quenching were also performed. The relaxor characteristics of the dielectric response were found to be suppressed after quenching from 800$\sp\circ$C. Subsequent dielectric investigations during thermal cycling below 400$\sp\circ$C revealed thermal hysteresis effects between consecutive heating and cooling measurements. The original relaxor characteristics of the as-grown state were completely recovered by thermal treatment near 400$\sp\circ$C.The evolution of the dielectric permittivity and loss factor under external applied electric field has been studied between $-$170 and 250$\sp\circ$C. It is possible to induce a ferroelectric transition from the glassy phase to a macroscopically polar phase. The poling and depoling temperature depend on the various combinations of thermal treatments and on the applied field strength. The transition between the nanopolar state and the macropolar state is discussed. Some additional anomalies appear in the dielectric response under a bias field.U of I OnlyETDs are only available to UIUC Users without author permissio