Non-Centrosymmetric RbNaMgP₂O₇ with Unprecedented Thermo-Induced Enhancement of Second Harmonic Generation

It is of great difficulty to obtain deep-UV transparent materials with enhanced second harmonic generation (SHG), mainly limited by the theoretically poor transparency of these materials in the deep-UV spectral region. Here we report a new noncentrosymmetric, deep-UV transparent phosphate RbNaMgP₂O₇, which undergoes a thermo-induced reversible phase transition (at a high temperature of 723 K) and correspondingly an evident SHG enhancement up to ∼1.5 times. The phase transition is aroused by the twist of [P₂O₇]⁴⁻ dimers with deviation from the P-O-P equilibrium positions. Theoretical analyses reveal that the enhanced SHG can be ascribed to the thermo-induced collective alignment of SHG-active [P₂O₇]⁴⁻ dimers along the polar axis of high-temperature phase. This work provides an unprecedented physical routine (to SHG-enhanced materials) that is distinguished from the traditional one by chemical design and synthesis.