Broadband critical dynamics in disordered lead-based perovskites.

Materials based on the cubic perovskite unit cell continue to provide the basis for technologically important materials with two notable recent examples being lead based relaxor piezoelectrics and lead based organic-inorganic halide photovoltaics. While these materials carry considerable disorder owing to site substitution in relaxors and molecular vibrations in the organic-inorganics, much of the understanding of these materials derives from the initial classic work on lattice vibrations in SrTiO$_{3}$ performed by Prof. R. A. Cowley applying both theory and neutron scattering experiments at Chalk River Laboratories. Neutron scattering continues to play an important role in understanding lattice vibrations in perovskites owing to the cross section and also the appropriate energy resolution achievable with current neutron instrumentation. We discuss the dynamics that drive the phase transitions in the relaxors and organic-inorganic lead halides as probed through neutron scattering and compare the dynamics to those phase transitions that derive from a ``central peak" and also a soft mode. We review some of the past experimental work on these materials and illustrate new data using high resolution time of flight backscattering spectroscopy on organic-inorganic perovskites. We will show that the structural transitions in disordered lead based perovskites are driven by a broad frequency band of excitations.