4 days of beam time is requested on HRPD to study the crystal chemistry of the ferroelectric Aurivillius phase Bi4Ti3O12 (Curie Temperature, Tc ~ 675 C, mixed conductor) and a Nb-doped composition (Tc ~ 670 C, dielectric) Bi4Ti2.95Nb0.05O12. The purpose of this experiment is to acquire ND data from 300 to 1000 K to establish accurate crystallographic information related to the changes in polymorphism (including a poorly defined intermediate phase that exists near Tc) and defect chemistry by Nb-doping. Unpublished electrical data and O18 diffusion measurements show Bi4Ti3O12 to have unexpected high levels of oxide-ion conductivity. Understanding the structural origin and conduction mechanism will inform future compositional studies on how to optimise either the oxide-ion conducting (electrolyte) or ferroelectric (insulating) properties of a variety of energy/memory applications.