Effect of Cation\nIncorporation on the Structural Distortions\nand Phase Transitions in Mixed Lead Chloride Perovskite Single Crystals

The phase transition behaviors of\nmixed halide perovskites are\nhighly sensitive to the specific compositions at the A or X sites\nof the ABX<sub>3</sub> perovskite structure. The exact nature of these\nphase transitions, especially those involving glassy states, remains\nincompletely understood. To address this issue, we investigated the\nstructural, vibrational, and elastic properties of two MA<sub>1–<i>x</i></sub>FA<sub><i>x</i></sub>PbCl<sub>3</sub> (<i>x</i> = 0.13 and 0.23) single crystals by using X-ray diffraction,\nRaman spectroscopy, and Brillouin spectroscopy. Our findings show\nthat the average cubic phase of MA<sub>0.77</sub>FA<sub>0.23</sub>PbCl<sub>3</sub> is maintained down to 80 K, while MA<sub>0.87</sub>FA<sub>0.13</sub>PbCl<sub>3</sub> undergoes a structural phase transition\nto the orthorhombic phase. The vibrational frequencies and elastic\nconstants of MA<sub>0.77</sub>FA<sub>0.23</sub>PbCl<sub>3</sub> exhibited\ncontinuous and mild changes, whereas those of MA<sub>0.87</sub>FA<sub>0.13</sub>PbCl<sub>3</sub> demonstrated discontinuous and drastic\nchanges at two distinct phase transition temperatures. Additionally,\nthe quasi-elastic central peak in the light scattering spectrum revealed\ncritical differences: a critical slowing behavior for MA<sub>0.87</sub>FA<sub>0.13</sub>PbCl<sub>3</sub> versus a stretched slowing behavior\nfor MA<sub>0.77</sub>FA<sub>0.23</sub>PbCl<sub>3</sub>, the latter\nbeing characteristic of glassy systems. These results indicate the\nabsence of any structural phase transition in MA<sub>0.77</sub>FA<sub>0.23</sub>PbCl<sub>3</sub>, contrasting with the behaviors observed\nin mixed MA<sub><i>x</i></sub>FA<sub>1–<i>x</i></sub>PbI<sub>3</sub> and MA<sub><i>x</i></sub>FA<sub>1–<i>x</i></sub>PbBr<sub>3</sub> systems. The phase boundary between\nthe orthorhombic phase and the glassy state of MA<sub>1–<i>x</i></sub>FA<sub><i>x</i></sub>PbCl<sub>3</sub> at\nlow temperatures lies between FA content levels of 0.13 and 0.23.\nThe enhanced local heterogeneous environment of MA cations, induced\nby a more compact lattice structure due to smaller Cl ions, may be\nthe origin of the glassy behavior in MA<sub>0.77</sub>FA<sub>0.23</sub>PbCl<sub>3</sub>.