AIn3Si4P9 (A = Ca, Sr): quaternary phosphides with double-helix structures exhibiting high laser-induced damage thresholds

Two quaternary phosphides AIn3Si4P9 (A = Ca, 1; Sr, 2) belonging to the non-centrosymmetric space group P41212 are discovered and synthesized through the solid-state method for the first time. They present extraordinary double-helix frameworks composed of corner-shared tetrahedral [InP4] and [SiP4] units. Although compounds 1 and 2 have undesirable band gaps (1.25 and 1.15 eV), their extremely low thermal expansion coefficients (∼10−6) result in high laser-induced damage thresholds approximately seven times that of benchmark AgGaS2 under a 2050 nm laser. Moreover, both compounds show weak second-harmonic generation intensities primarily caused by Kleinman forbidden nonlinear optical coefficients. Theoretical calculations further reveal that the electron transitions mainly originated from the tetrahedral [InP4] and [SiP4] units.