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 <i>p</i>1-4 have been identified in the room temperature (RT/20 °C)-120 °C range. The <i>I</i>(SHG) = <i>f</i>(<i>T</i>) curve defines an unprecedented hysteretic behavior with a SHG OFF state at low (<i>T</i> < 4 °C) and high (<i>T</i> > 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 <i>p</i>1 phase and the SHG-active <i>p</i>2 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 (<i>T</i> > 50 °C) or cooling (<i>T</i> < 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 <i>p</i>1 is the most thermodynamically stable phase, with <i>p</i>2 located slightly higher in energy.