A-Site Cation Substitution toward Structural Transformation in Phosphates Exhibiting Short UV Transparency and Second...

Noncentrosymmetric compounds have attracted extensive research interest due to their unique functional properties, including second-harmonic generation (SHG), ferroelectricity, and piezoelectricity. In this study, four alkali metal pyrophosphate crystals, Li₂Rb₂P₂O₇, LiK₂RbP₂O₇, LiRb₂NaP₂O₇, and LiRb₃P₂O₇, were identified through A-site cation substitution. By manipulating different alkali metal cations, the former two compounds crystallize in the acentric space groups Pca2₁ (No. 29) and C222₁ (No. 20), while the latter two compounds crystallize in the centrosymmetric space groups P2₁/c (No. 14) and Pnma (No. 62), respectively. The unique structural frameworks of these compounds are composed of [P₂O₇] dimers and [LiO₄] tetrahedra. These crystals demonstrate short UV absorption cutoff edges around 200 nm along with wide bandgaps. Notably, Li₂Rb₂P₂O₇ and LiK₂RbP₂O₇ exhibit a moderate powder SHG effect, approximately 0.2 times that of KDP (KH₂PO₄). First-principles calculations were performed to elucidate the origin of optical activities in these compounds. These findings highlight the potential of the A-site cation substitution strategy for the exploration of new nonlinear optical (NLO) crystals in the short-wave UV range.