Exceptional second harmonic generation responses, switchable dielectric behaviours, and ferroelectric property in an adduct of hexamethylene-tetramine·bisnopinic acid

Abstract

Most reversible phase transitions undergo transformations from high symmetry with non-active second harmonic generation (SHG) above Tc to low symmetry with active SHG below Tc (Tc refers the phase transition temperature). Here we display a new adduct of hexamethylenetetramine·bisnopinic acid, 1, which exhibits an abnormal phase transition from a low symmetrical structure with strong SHG responses above 235 K to a same symmetrical structure with weak SHG below Tc. X-ray single crystal diffraction research indicates the hydrogen-bond displacements trigger the structural phase transition. A pseudo symmetrical element C/2 exists in the low temperature structure (1-LTP) and vanishes in the room temperature (1-RTP). This abnormal change may be proved that its SHG responses from strong signal 1.04 (above Tc, about one-third of KDP) to weak signal 0.78 a.u. (below Tc). The ferroelectric measurement displays a typical electric hysteresis loop at room temperature with spontaneous polarization (Ps) 0.51 uC/cm2.