Abstract
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) (P2(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, 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.