Unusual two-step sequential reversible phase transitions with coexisting switchable nonlinear optical and dielectric behaviors in [(CH3)3NCH2Cl]2[ZnCl4]

Abstract

A novel material with coexisting switchable nonlinear optical (NLO) and dielectric properties, [trimethylchloromethyl ammonium]₂[ZnCl₄] [(TMCM)₂-ZnCl₄], which undergoes two reversible solid state phase transitions at 337 K (T_c1) and 258 K (T_c2), respectively, has been successfully synthesized and grown as block crystals. Exceptionally, variable-temperature X-ray single crystal structural analyses of (TMCM)₂-ZnCl₄ reveal that the unusual space group changes from P2₁2₁2₁ at 223 K to P2₁/c between T_c2 and T_c1 to Pmcn above T_c1. As expected, the dielectric value of (TMCM)₂-ZnCl₄ exhibits two remarkable step-like enlargements around T_c1 and T_c2, indicating that it exhibits a low dielectric state, an intermediate dielectric state and a high dielectric state. In particular, the second harmonic generation (SHG) efficiency shows a sharp step-like increase from almost zero to 0.47, and its SHG activities in a high-NLO state can be recovered after several cycles without any attenuation. The transitions of (TMCM)₂-ZnCl₄ could be intrinsically ascribed to the sequential order–disorder transformations of both the cations and anions. All these results demonstrate its potential application as a switchable and tunable molecular dielectric and NLO material.