Chain-Melting\nTemperature Depression in the Organic\nLayer of Two-Dimensional Perovskites

Two-dimensional (2D) perovskites\nwith <i>n</i>-alkylammonium\ncations often undergo a solid–solid phase transition upon changes\nin pressure or temperature, during which the organic cations undergo\nan order-to-disorder chain-melting transition. Here, we show that\nblending halides and organic cations of varying lengths decreases\nthe phase transition temperature of Cu- and Mn-based 2D perovskites.\nThe magnitude of this decrease depends on the relative lengths of\nthe two <i>n</i>-alkylammonium cations. For instance, blending <i>n</i>-decylammonium (DA) with <i>n</i>-undecylammonium\n(UDA) results in a 5 °C decrease in the phase transition temperature\ncompared to pure DA<sub>2</sub>CuBr<sub>4</sub>. In contrast, blending\nDA with <i>n</i>-dodecylammonium (DDA) causes a 25 °C\ndecrease. Furthermore, we found that blending Cu<sup>2+</sup> and\nMn<sup>2+</sup> has a minimal effect on the phase transition temperature\ndue to similar lattice spacing between the inorganic layers in DA<sub>2</sub>CuCl<sub>4</sub> and DA<sub>2</sub>MnCl<sub>4</sub>. Our study\nsuggests new design principles for tuning structural phase transitions\nin 2D perovskites through alloying halides, organic cations, and metal\ncations.