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

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.