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

Abstract

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.