Structure and Properties of Bi-containing Mixed A-site Perovskites

This thesis describes a series of studies undertaken to explore the criteria for the stabilization of Bi3+ in a perovskite through the formation of mixed A-site solid solutions. Electronically similar to Pb2+ but with a smaller ionic radius, Bi3+ has the potential to replace and improve upon Pb-based counterparts for dielectric and piezoelectric applications. Building off the discovery that Bi(Zn1/2Ti1/2)O3 increases the tetragonality of PbTiO3, the first part of this work identified a new substituent, Bi(Zn3/4W1/4)O6, that is also able to increase the c/a of PbTiO3. In the process, the crystal chemical criterion for enhancing the tetragonality of PbTiO3 was clarified. From there, these tetragonality-enhancing additives were introduced into lead-free perovskite chemistries in an attempt to stabilize a new generation of environmentally benign ferroelectric materials. The final section returns to lead-containing systems by investigating ternary systems in an attempt to capture the high c/a of the mixed Pb-Bi systems at an MPB. Unexpected dielectric behavior was observed in some compositions, which was shown to be the result of the structure entering a region of 2-phase coexistence. A new crystal-chemical parameter is defined to describe the variance of the B-site displacement factors and used to develop an empirical model for predicting the incidence of this 2-phase coexistence region. An atomistic model similar to those of relaxor ferroelectrics was proposed to explain the behavior. Degree Type Dissertation Degree Name Doctor of Philosophy (PhD) Graduate Group Materials Science & Engineering First Advisor Peter K. Davies