An Unusual Phase Transition Driven by Vibrational Entropy Changes in a Hybrid Organic-Inorganic Perovskite

The driving forces for the phase transitions of ABX(3) hybrid organic-inorganic perovskites have been limited to the octahedral tilting, order-disorder, and displacement. Now, a complex structural phase transition has been explored in a HOIP, [CH3NH3][Mn(N-3)(3)], based on structural characterizations and abinitio lattice dynamics calculations. This unusual first-order phase transition between two ordered phases at about 265K is primarily driven by changes in the collective atomic vibrations of the whole lattice, along with concurrent molecular displacements and an unusual octahedral tilting. A significant entropy difference (4.35JK(-1)mol(-1)) is observed between the low- and high-temperature structures induced by such atomic vibrations, which plays a main role in driving the transition. This finding offers an alternative pathway for designing new ferroic phase transitions and related physical properties in HOIPs and other hybrid crystals.