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.