From H 12 C 4 N 2 CdI 4 to H 11 C 4 N 2 CdI 3 : a highly polarizable CdNI 3 tetrahedron induced a sharp enhancement of second harmonic generation response and birefringence.

In this study, we identify a novel class of second-order nonlinear optical (NLO) crystals, non-π-conjugated piperazine (H 10 C 4 N 2 , PIP) metal halides, represented by two centimeter-sized, noncentrosymmetric organic-inorganic metal halides (OIMHs), namely H 12 C 4 N 2 CdI 4 ( P 2 1 2 1 2 1 ) and H 11 C 4 N 2 CdI 3 ( Cc ). H 12 C 4 N 2 CdI 4 is the first to be prepared, and its structure contains a CdI 4 tetrahedron, which led to a poor NLO performance, including a weak and non-phase-matchable second harmonic generation (SHG) response of 0.5 × KH 2 PO 4 (KDP), a small birefringence of 0.047 @1064 nm and a narrow bandgap of 3.86 eV. Moreover, H 12 C 4 N 2 CdI 4 is regarded as the model compound, and we further obtain H 11 C 4 N 2 CdI 3 via the replacement of CdI 4 with a highly polarizable CdNI 3 tetrahedron, which results in a sharp enhancement of SHG response and birefringence. H 11 C 4 N 2 CdI 3 exhibits a promising NLO performance including 6 × KDP, 4.10 eV, Δ n = 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.