  Solar cells based on hybrid perovskites have shown high efficiency while
possessing simple processing methods. To gain a fundamental understanding of
their properties on an atomic level, we investigate single crystals of
CH3NH3PbI3 with a narrow transition (~5 K) near 327 K. Temperature dependent
structural measurements reveal a persistent tetragonal structure with smooth
changes in the atomic displacement parameters (ADPs) on crossing T*. We show
that the ADPs for I ions yield extended flat regions in the potential wells
consistent with the measured large thermal expansion parameter. Molecular
dynamics simulations reveal that this material exhibits significant high
asymmetries in the Pb-I pair distribution functions. We also show that the
intrinsically enhanced freedom of motion of the iodine atoms enables large
deformations. This flexibility (softness) of the atomic structure results in
highly localized atomic relaxation about defects and hence accounts for both
the high carrier mobility as well as the structural instability.
