Effect of Cation Incorporation on the Structural Distortions and Phase Transitions in Mixed Lead Chloride Perovskite Single Crystals

The phase transition behaviors of mixed halide perovskites are highly sensitive to the specific compositions at the A or X sites of the ABX3 perovskite structure. The exact nature of these phase transitions, especially those involving glassy states, remains incompletely understood. To address this issue, we investigated the structural, vibrational, and elastic properties of two MA1–xFAxPbCl3 (x = 0.13 and 0.23) single crystals by using X-ray diffraction, Raman spectroscopy, and Brillouin spectroscopy. Our findings show that the average cubic phase of MA0.77FA0.23PbCl3 is maintained down to 80 K, while MA0.87FA0.13PbCl3 undergoes a structural phase transition to the orthorhombic phase. The vibrational frequencies and elastic constants of MA0.77FA0.23PbCl3 exhibited continuous and mild changes, whereas those of MA0.87FA0.13PbCl3 demonstrated discontinuous and drastic changes at two distinct phase transition temperatures. Additionally, the quasi-elastic central peak in the light scattering spectrum revealed critical differences: a critical slowing behavior for MA0.87FA0.13PbCl3 versus a stretched slowing behavior for MA0.77FA0.23PbCl3, the latter being characteristic of glassy systems. These results indicate the absence of any structural phase transition in MA0.77FA0.23PbCl3, contrasting with the behaviors observed in mixed MAxFA1–xPbI3 and MAxFA1–xPbBr3 systems. The phase boundary between the orthorhombic phase and the glassy state of MA1–xFAxPbCl3 at low temperatures lies between FA content levels of 0.13 and 0.23. The enhanced local heterogeneous environment of MA cations, induced by a more compact lattice structure due to smaller Cl ions, may be the origin of the glassy behavior in MA0.77FA0.23PbCl3.