α-Ca2CdP2 and β-Ca2CdP2: Two Polymorphic Phosphide-Based Infrared Nonlinear Crystals with Distorted NLO-Active Tetrahedral Motifs Realizing Large Second Harmonic Generation Effects and Suitable Band Gaps.

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.