Symmetry Breaking and Switching in 2D Halide Perovskites

Detailed crystal structure and symmetry underlie most structure-property correlations in halide perovskites and related semiconductors. For example, symmetry reduction generally may lead to nonlinear optical effects, ferroelectricity, and Rashba/Dresselhaus spin splitting (when accompanied by strong spin orbit coupling). On the other hand, glass-crystal transitions have been used to modulate electrical/optical properties by orders of magnitude and have been used in optical storage disks and phase-change memory. In this talk we will discuss approaches to control ordering and symmetry through organic cation choice and application of external stimuli (e.g., temperature) in 2D metal halide perovskites. Appropriate choice of the organic cation enables reduction of the melting temperature of the hybrids, as well as control over ordering within the melt during cooling, thereby allowing for demonstration of a reversible glass-crystalline transition with distinct kinetics. Further, cation choice can also be used to modulate crystal-to-crystal transition kinetics and can enable stabilizing two different structural types (and crystal symmetries) at the same temperature, depending on thermal cycling. The above approaches to control phase transitions will be discussed and are fundamentally important as well as potentially being of interest for applications connected with degree of crystallinity, symmetry and phase switching.