Hydrogen Bond-Driven Order–Disorder Phase Transition in the Near-Room-Temperature Nonlinear Optical Switch [Ag(NH3)2]2SO4

Herein, we report a near-room-temperature nonlinear optical (NLO) switch material, [Ag­(NH3)2]2SO4, exhibiting switching performance with strong room-temperature second harmonic generation (SHG) intensity that outperforms the UV–vis spectral region industry standard KH2PO4 (1.4 times stronger). [Ag­(NH3)2]2SO4 undergoes a reversible phase transition (Tc = 356 K) from the noncentrosymmetric room-temperature phase (P4̅21c, RTP) to a centrosymmetric high-temperature phase (I4/mmm, HTP) where both the SO42– anions and [Ag­(NH3)2]+ cations are highly disordered. The weakening of hydrogen bond interactions in the HTP is also evidenced by the lower energy shift of the stretching vibration of the N–H···O bonds revealed by the in situ FT-IR spectra. Such weakening leads to an unusual negative thermal expansion along the c axis (−3%). In addition, both the atomic displacement parameters of the single-crystal diffraction data and the molecular dynamics-simulated mean squared displacements suggest the motions of the O and N atoms. Such a structural disorder not only hinders the phonon propagation and dramatically drops the thermal conductivity to 0.22 W m–1 K–1 at 361 K but also significantly weakens the optical anisotropy and SHG as verified by the DFT theoretical studies.