The aim of this experiment is to determine accurately the crystal structures of phases in the (1-x)BiScO3-xPbTiO3 (BST-PT) perovskite phase diagram, particularly in the vicinity of the reported morphotropic phase boundary (MPB) between rhombohedral and tetragonal symmetries at x=0.64, where preliminary x-ray powder diffraction shows that we have found a new phase of orthorhombic (or lower) symmetry at ambient temperature.. In so-doing, we aim to improve the understanding of how structural changes at the morphotropic phase boundary occur in a diverse range of systems. We propose to use GEM for this study as the high neutron flux will allow rapid acquisition of data for a large number of compositions and several temperatures. This will yield excellent structural information about this new system and aid in resolving the current controversies about the nature of the MPB.