Structural Origin of Thermally Induced Second Harmonic Generation Enhancement in RbNaMgP2O7

RbNaMgP2O7 is a newly reported nonlinear optical (NLO) crystal that displays remarkable second harmonic generation (SHG) enhancement upon heating. In this work, we carried out in situ neutron powder diffraction and impedance spectroscopy measurements through its polar-to-polar phase-transition region and confirmed that the dynamic process for the alignment of NLO-active anionic [P2O7]4– groups is composed of thermo-induced incremental rotation of [P2O7]4– groups, which is accompanied by cationic migration enhancement. Furthermore, we elucidated a new mechanism of coupling between anionic group alignment and cationic migration for the first time in NLO materials. This mechanism could offer extra freedom for new NLO material design and anionic group alignment control for a better SHG response via tuning the cationic migration.