Chain-Melting Temperature Depression in the Organic Layer of Two-Dimensional Perovskites

Two-dimensional (2D) perovskites with n-alkylammonium cations often undergo a solid–solid phase transition upon changes in pressure or temperature, during which the organic cations undergo an order-to-disorder chain-melting transition. Here, we show that blending halides and organic cations of varying lengths decreases the phase transition temperature of Cu- and Mn-based 2D perovskites. The magnitude of this decrease depends on the relative lengths of the two n-alkylammonium cations. For instance, blending n-decylammonium (DA) with n-undecylammonium (UDA) results in a 5 °C decrease in the phase transition temperature compared to pure DA₂CuBr₄. In contrast, blending DA with n-dodecylammonium (DDA) causes a 25 °C decrease. Furthermore, we found that blending Cu²⁺ and Mn²⁺ has a minimal effect on the phase transition temperature due to similar lattice spacing between the inorganic layers in DA₂CuCl₄ and DA₂MnCl₄. Our study suggests new design principles for tuning structural phase transitions in 2D perovskites through alloying halides, organic cations, and metal cations.