Incipient ferroelectricity in methylammonium lead halide perovskites

Lead halide perovskites are promising materials for optoelectronic applications. Ferroelectricity is often considered as a phenomenon that improves their properties. However, the very existence of ferroelectricity in these materials is still debatable. Here we investigate the dielectric and ferroelectric properties of the single crystals of organic-inorganic hybrid CH3NH3PbX3 (X = I, Br or Cl) and all-inorganic CsPbBr3 perovskites, which are among the most technologically useful materials. We find that MAPbX3 perovskites are incipient ferroelectrics of the order-disorder type and provide experimental evidence. We show that in contrast to SrTiO3 and other known displacive-type incipient ferroelectrics, where the ferroelectric phase is suppressed by low-temperature quantum fluctuations, in the hybrid halide perovskites the order-disorder ferroelectric transition is not attained upon cooling because the first-order octahedral tilting transition to the antiferrodistortive orthorhombic phase occurs at a temperature higher than the expected ferroelectric Curie temperature. The tilting of corner-sharing PbX6 octahedra prevents the ferroelectric ordering of MA dipoles. This insight expands our understanding of the fundamental properties of halide perovskites and suggests strategies for designing ferroelectric organic-inorganic materials. It is widely debated if ferroelectricity exists in lead halide perovskites. Here, the dielectric and ferroelectric properties of CH3NH3PbX3 and CsPbBr3 perovskite single crystals are studied, showing evidence that MAPbX3 perovskites are incipient ferroelectrics of order-disorder type.