Structural Phase Transition and Dielectric Anisotropy Properties of a Lead-free Organic-inorganic Hybrid
Organic-inorganic hybrids have shown great potentials for assembling dielectric phase transition materials, due to their unqiue characteristics of strucutral diversities. Here, we construct a new organic-organic hybrid, (cyclopropylaminium)3BiBr6 (1), which consists of a zero-dimensional inorganic framework of BiBr6 octahedra and ternary cyclic organic cations. It is noteworthy that 1 undergoes a reversible phase transition around Tc = 288 K upon heating, as certified by dielectric and DSC measurements. Variable-temperature structure analyses disclose that the dynamic order-disorder behavior of ternary cyclic organic cation accounts for its phase transition. That is, the frozen ordering of highly-disordered organic cation affords the driving force. Dielectric constants of 1 also exhibit bistable states in the vicinity of Tc associated with its phase transition, corresponding to the high- and low-dielectric states. Particularly, an obvious dielectric anisotropy is observed along its different crystallographic axes directions; in detail, dielectric anomalies along c- axes are larger than that of a- and b-axes. Besides, 1 possesses an optical energy bandgap of ~2.8 eV determined by its inorganic framework. Such dielectric activities induced by order-disorder transformation may highlight a pathway for the design of new electric-ordered materials.