Switchable K-O bonds unveiling anomalous ferroelastic transitions in a polar hybrid crystal

极性杂化晶体中由可转换K–O配位键引发的异常铁弹转变

Hybrid ferroelastic crystals have garnered considerable interest due to their promising potential as mechanical switches and sensors.The anomalous ferroelastic phase transitions,in which ferroelasticity occurs in the hightemperature phase rather than the low-temperature phase,are of particular interest,but they are sporadically-documented and none of them is involved in breaking of the chemical bonds.Herein,a hydroxyl-containing cation,i.e.,Me_(3)NOH^(+),is employed to construct a three-dimensional hybrid crystal(Me_(3)NOH)_(2)KBiCl_(6)(1).This crystal undergoes distinct twostep structural phase transitions with space-group changes of Pna21–P112_(1)–P63mc,belonging to an anomalous temperaturereversed mm2F2 ferroelastic transition and a normal 6mmF2 ferroelastic transition,respectively.The anomalous ferroelastic transition is entirely driven by switchable K–O coordination bonds involving breaking and reformation.Notably,the dynamic behavior of Me_(3)NOH^(+)cations along with the distortion of inorganic framework enables the manifestation of unusual“high-low-medium”second-harmonic generation-switching behaviors.This study presents the enormous benefits of switchable coordination bonds for inducing anomalous ferroelastic phase transitions,offering valuable insight for the exploration of new multifunctional ferroelastic materials.

近年来,杂化铁弹体由于在机械开关和传感器等领域的应用潜力而引起广泛的研究兴趣.其中,铁弹性出现在高温相的异常铁弹相变尤其受到研究关注,但其实例较少,并且它们几乎都不涉及到化学键的断裂.本文基于含羟基的阳离子,构筑了一例其K–O配位键能发生可逆断裂与重组的极性杂化晶体(Me_(3)NOH)_(2)KBiCl_(6).该晶体经历了两步结构转变,伴随着Pna21–P1121–P63mc的空间群变化,即分别属于温度反转的mm2F2型铁弹相变和正常的6mmF2型铁弹相变.结构分析揭示该反常铁弹转变的来源为K–O配位键的断裂与重建.同时,有机阳离子的动力学变化和无机骨架的变形导致该晶体表现出不寻常的“高-低-中”倍频信号变化.本文展示了动态配位键在诱导异常铁弹相变中的巨大优势,为探索新型多功能铁弹材料提供了重要线索.