A 2D Double Perovskite Based on the Chiral Cystaminium Cation Exhibiting Multiple Switches in Quadratic Nonlinear Optical Response.

Over the past decade, double halide perovskites with 2D structural architectures have emerged as a promising alternative to lead-based halide perovskites in the field of semiconductors, motivated by the need to reduce the use of toxic metals. In this work, we present an example of a new 2D lead-free compound, (Cyst)2AgSbBr8, exhibiting switchable nonlinear (SHG, second harmonic generation) optical properties. The use of the cystaminium cation (Cyst2+) which exhibits helical chirality (M and P conformations) helped in inducing phase transitions due to conformational interconversion in the solid state. Four structural phases p1-4 have been identified in the room temperature (RT/20 °C)-120 °C range. The I(SHG) = f(T) curve defines an unprecedented hysteretic behavior with a SHG OFF state at low (T < 4 °C) and high (T > 50 °C) temperature, while in the 4 °C-50 °C temperature range, both SHG(OFF) and (ON) states can occur as a result of the existence of the SHG-inactive p1 phase and the SHG-active p2 phase. The temperature range around RT for which these two phases are stable is also unprecedented. This allows to store hidden information at RT, which can be erased either by heating (T > 50 °C) or cooling (T < 4 °C) leading to all SHG(ON) or SHF(OFF) domains at RT, respectively. Density Functional Theory (DFT) calculations showed a direct band gap for the all phases. Calculations for the relative stability of the different phases confirmed that p1 is the most thermodynamically stable phase, with p2 located slightly higher in energy.