Thermal-induced reversible ferroelastic phase transition in a new bromethyl-substituted molecular rotor

A new bromethyl-substituted molecular rotor, [Cu（dabcoCH2Br）（H2O）Br3] （dabcoCH2Br＋=1-（2-bromethyl）-1,4-diazoniabi- cyclo[2.2.2]octane cation）, which belongs to a family of balomethyl-substituted molecular rotors, was synthesized and struc- turally characterized. The reversible phase transition at ca. 250 K was well established for this molecular rotor by thermal analyses, variable-temperature X-ray diffraction, and variable temperature dielectric measurements. The order-disorder trans- formation of the rotator part （dabco moiety） causes ferroelastic phase transition with an Aizu notation of mmmF2/m from high- temperature orthorhombic phase （Pbnm） to low-temperature monoclinic phase （P21/n）. More important, in reference to the density functional theory calculations and structural analyses, the key factors to tune the phase transition behaviors were dis- cussed in detail for this family of halomethyl-substituted molecular rotors.

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

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.

厘米级无铅卤化物双钙钛矿单晶实现铁弹性相变触发介电开关性质

在铅基卤化物钙钛矿突破性发展的推动下,卤化物双钙钛矿因其环境友好、丰富的物理化学性能和结构可调性而成为杰出的多用途材料.尽管卤化物双钙钛矿的研究取得了飞速的进展,但在这个吸引人的钙钛矿家族中对可切换电介质的探索仍然很少.本工作通过溶液冷却法成功地生长了单层卤化物双钙钛矿(3-溴丙胺)_(4)AgBiBr_(8)(1)的大尺寸单晶,其尺寸达20 mm×17 mm×5 mm.它经历了由Aizu符号定义的mmm F2/m的铁弹性相变,通过结构分析可知该铁弹相变是八面体畸变和柔性分子运动协同作用的结果.值得注意的是,它在373 K附近具有理想的铁弹性相变触发的可切换电介质性能,可以在两个稳定的电介质状态之间切换.而且,1展现出间接带隙半导体特性和X射线响应,其中在40 V的偏压下1的灵敏度高达517μC Gy_(air)^(-1)cm^(-2).这项工作为具有智能功能的无铅卤化物双钙钛矿提供了一条有效的应用路径.