Hydrogen Bond-Driven Order-Disorder Phase Transition in the Near-Room-Temperature Nonlinear Optical Switch...

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 (T _c = 356 K) from the noncentrosymmetric room-temperature phase (P4̅2₁ c, RTP) to a centrosymmetric high-temperature phase (I4/mmm, HTP) where both the SO₄ ^2- 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.