  Organic-inorganic coupling in the hybrid lead-halide perovskite is a central
issue in rationalizing the outstanding photovoltaic performance of these
emerging materials. Here we compare and contrast the evolution of structure and
dynamics of the hybrid CH3NH3PbBr3 and the inorganic CsPbBr3 lead-halide
perovskites with temperature, using Raman spectroscopy and single-crystal X-ray
diffraction. Results reveal a stark contrast between their order-disorder
transitions, abrupt for the hybrid whereas smooth for the inorganic perovskite.
X-ray diffraction observes an intermediate incommensurate phase between the
ordered and the disordered phases in CH3NH3PbBr3. Low-frequency Raman
scattering captures the appearance of a sharp soft mode in the incommensurate
phase, ascribed to the theoretically predicted amplitudon mode. Our work
highlights the interaction between the structural dynamics of organic cation
CH3NH3+ and the lead-halide framework, and unravels the competition between
tendencies of the organic and inorganic moieties to minimize energy in the
incommensurate phase of the hybrid perovskite structure.
