From H12C4N2CdI4 to H11C4N2CdI3: a highly polarizable CdNI3 tetrahedron induced a sharp enhancement of second harmonic generation response and birefringence

Abstract

In this study, we identify a novel class of second-order nonlinear optical (NLO) crystals, non-π-conjugated piperazine (H₁₀C₄N₂, PIP) metal halides, represented by two centimeter-sized, noncentrosymmetric organic–inorganic metal halides (OIMHs), namely H₁₂C₄N₂CdI₄ (P2₁2₁2₁) and H₁₁C₄N₂CdI₃ (Cc). H₁₂C₄N₂CdI₄ is the first to be prepared, and its structure contains a CdI₄ tetrahedron, which led to a poor NLO performance, including a weak and non-phase-matchable second harmonic generation (SHG) response of 0.5 × KH₂PO₄ (KDP), a small birefringence of 0.047 @1064 nm and a narrow bandgap of 3.86 eV. Moreover, H₁₂C₄N₂CdI₄ is regarded as the model compound, and we further obtain H₁₁C₄N₂CdI₃via the replacement of CdI₄ with a highly polarizable CdNI₃ tetrahedron, which results in a sharp enhancement of SHG response and birefringence. H₁₁C₄N₂CdI₃ exhibits a promising NLO performance including 6 × KDP, 4.10 eV, Δn = 0.074 @1064 nm and phase matchability, indicating that it is the first OIMH to simultaneously exhibit strong SHG response (>5 × KDP) and a wide bandgap (>4.0 eV). Our work presents a novel direction for designing high-performance NLO crystals based on organic–inorganic halides and provides important insights into the role of the hybridized tetrahedron in enhancing the SHG response and birefringence.