My understanding is that synthesis of the exactly right crystal structure is very hard (for every 10 lead atoms, 1 - the exact right one - needs to be substitut

My understanding is that synthesis of the exactly right crystal structure is very hard (for every 10 lead atoms, 1 - the exact right one - needs to be substituted with copper), so the samples are small and inhomogeneous. As a result measuring resistivity won't be illuminating until large amounts of perfect material can be produced.