Hydrogen Bond-Driven Order-Disorder Phase Transition in the Near-Room-Temperature Nonlinear Optical Switch [Ag(NH 3 ) 2 ] 2 SO 4 .

Herein, we report a near-room-temperature nonlinear optical (NLO) switch material, [Ag(NH 3 ) 2 ] 2 SO 4 , exhibiting switching performance with strong room-temperature second harmonic generation (SHG) intensity that outperforms the UV-vis spectral region industry standard KH 2 PO 4 (1.4 times stronger). [Ag(NH 3 ) 2 ] 2 SO 4 undergoes a reversible phase transition ( T c = 356 K) from the noncentrosymmetric room-temperature phase ( P 4̅2 1 c , RTP) to a centrosymmetric high-temperature phase ( I 4/ mmm , HTP) where both the SO 4 2- anions and [Ag(NH 3 ) 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.