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Abstract: The driving forces for the phase transitions of ABX3 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(N3)3], based on structural characterizations and ab initio lattice dynamics calculations. This unusual first‐order phase transition between two ordered phases at about 265 K 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 (4.35 J K−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. [ABSTRACT FROM AUTHOR]

Two‐photon‐absorption‐induced photoluminescence (TPL) from nanostructures is generally inefficient since it is a typical third‐order nonlinear optical process. Herein, a hybrid dielectric structure composed of dielectric microspheres (approximately micrometers in diameter) covering a 2D perovskite flake is reported, which provides a straightforward strategy for enhancing the TPL emission. The microspheres in the hybrid dielectric structure not only concentrate the pumping laser but also effectively increase the detection efficiency of the emitted TPL signal. The internal quantum efficiency of the 2D perovskite is also increased in the hybrid dielectric structure due to a reduced nonradiative rate. These effects cooperatively increase the TPL emission by two orders of magnitude in the hybrid dielectric structure. Moreover, the hybrid dielectric structure is proven to be useful for TPL‐based superresolution imaging at a relatively low excitation power of 0.05 mW. This work demonstrates great promise for developing low‐cost, high‐performance nonlinear nanodevices based on hybrid dielectric structures. A hybrid dielectric structure composed of a 2D perovskite flake and dielectric microspheres is used to cooperatively increase two‐photon‐absorption‐induced photoluminescence (TPL) from the 2D perovskite by two orders of magnitude. The hybrid dielectric structure can be utilized for TPL‐based superresolution imaging, requiring significantly less excitation power. [ABSTRACT FROM AUTHOR]