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

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 (Tc-1) and 258 K (T-c2), respectively, has been successfully synthesized and grown as Nock crystals. Exceptionally, variable-temperature X-ray single crystal structural analyses of (TMCM)(2)-Zncl(4) reveal that the unusual space group changes from P2(1)2(1)2(1) at 223 K to P2(1)/c between 7(c2) and T-c1 to Pmcn above T-c1. As expected, the dielectric value of (TMCM)(2)-ZnO4 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)-ZnO4 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.