Enhancement of the ferroelectric properties of multiferroic BiFeO3 are known to occur when small amounts of Bi are replaced by lanthanide ions. Further levels of lanthanide substitution result in structural transitions and the ferroelectric behaviour is eventually lost. From preliminary PXRD measurements we have identified two series of substituted BiFeO3, Bi0.9Sm0.1Fe1-xMnxO3 with 0 < x < 0.5, and 2) Bi1-xTbxFeO3 with 0 < x < 0.2 that show structural transitions as a function of increasing dopant. Here we request 3 days on HRPD to study the evolution of the room temperature crystal structures of these phases as a function of dopant. Once the precise crystal structures are known we will correlate this information with the dielectric and magnetic properties. We will also collect data at 10 K on the end members to check for a transition to ferroelectric R3c symmetry.