主客体耦合在稳定超容差杂化钙钛矿MHyPbX₃物相中的作用
Role of host/guest coupling in stabilizing the phases of the over-tolerant hybrid perovskite MHyPbX3
AI总结：
该研究从主客体耦合视角解释杂化钙钛矿MHyPbX₃相变序列的组分依赖性，通过第一性原理计算和分子动力学模拟验证能量竞争机制，为预测其他杂化钙钛矿稳定相提供新思路。
摘要：
$MHyPbBr_xCl_{3-x}$ 呈现出有趣的温度诱导相图，从单斜晶系（II相）转变为正交晶系（I相），最终转变为高温无序立方相。然而实验观察表明，在x=0（氯化物）和x=3（溴化物）的组分中，分别不存在立方相和正交相。我们从该系统的主客体框架视角解释了MHyPbX₃（X = Cl, Br）中相变序列的组分依赖性。我们认为，MHyPbX₃的相变序列可基于主体畸变与主客体耦合能之间的竞争来预测。MHyPbCl₃中占主导的耦合作用确保了中间I相的稳定，同时将向立方相的转变推至其分解温度以上。另一方面，MHyPbBr₃中能量尺度的平衡抑制了I相并稳定了立方相。我们的预测得到了基于第一性原理的两种化合物相变温度估算的支持，估算得出的趋势与实验观察一致。此外，从头算分子动力学模拟显示，MHyPbBr₃的立方相源于局部正交结构的无序性，这是能量尺度平衡的明确体现。我们提出，同一概念可用于预测其他杂化钙钛矿中的稳定物相。
英文摘要：
$MHyPbBr_xCl_{3-x}$ exhibits an interesting temperature-induced phase diagram transitioning from monoclinic (Phase-II) to orthorhombic (Phase-I) and eventually to the high-temperature disordered cubic phase. However, experimental observations indicate the absence of either the cubic or orthorhombic phases in compositions with x=0 (chloride) and x=3 (bromide), respectively. We explain the composition dependence of the phase transition sequence in MHyPbX3 (X = Cl, Br) from the perspective of a host/guest framework for the system. We argue that the sequence of phase transitions in MHyPbX3 can be anticipated based on the competition between host distortion and host/guest coupling energies. A dominant coupling in MHyPbCl3 ensures the stabilization of the intermediate Phase-I while pushing the transition to cubic phase beyond its decomposition temperature. On the other hand, a balance of the energy scales in MHyPbBr3 suppresses Phase-I and stabilizes the cubic phase. Our expectations were supported by first-principles based estimates of the transition temperatures in both compounds, which yield trends in agreement with experimental observations. Furthermore, ab initio molecular dynamics simulations revealed that the cubic phase of MHyPbBr3 results from a disorder over locally orthorhombic structures, a clear manifestation of the balance of energy scales. We propose that the same concept can be employed to predict the stable phases in other hybrid perovskites.
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