α-Ca₂CdP₂ and β-Ca₂CdP₂: Two Polymorphic Phosphide-Based Infrared Nonlinear Crystals with Distorted NLO-Active...

Two polymorphic phosphide-based infrared (IR) nonlinear optical (NLO) crystals, α-Ca₂CdP₂ and β-Ca₂CdP₂, were obtained by combining alkaline-earth metals and d¹⁰ transition metals using metal flux and metal salt flux methods, respectively. The crystal structure of α-Ca₂CdP₂ is orthorhombic in the space group Cmc2₁ (no. 36), while the structure of β-Ca₂CdP₂ is monoclinic in the space group Cm (no. 8). Both are two-dimensional layered structures that are composed of CdP₄ tetrahedra layers via sharing vertices, which stack along the b-axis and the c-axis, respectively. The second harmonic generation (SHG) measurements manifest that α-Ca₂CdP₂ and β-Ca₂CdP₂ exhibit strong SHG intensities (1.41 and 3.28× that of AgGaS₂ at a 2050 nm laser, respectively). Other optical measurements indicate that α-Ca₂CdP₂ and β-Ca₂CdP₂ have suitable band gaps (1.98 and 1.55 eV, respectively), high laser-induced damage thresholds (4.5 and 3.1× that of AgGaS₂ at 1064 nm laser, respectively) and appropriate birefringence (0.12 and 0.20 at 2050 nm, respectively) in addition to covering wide infrared transparent regions. The research on α-Ca₂CdP₂ and β-Ca₂CdP₂ demonstrates that they are potential IR NLO candidates. Theoretical calculations uncover that their SHG effects are from distorted CdP₄ tetrahedra, highlighting that tetrahedral motifs, including d¹⁰ transition metals, would be ideal NLO-active building blocks.