Relativistic Effect Engineered Ag2Hg1–xCdxI4 Crystals: Achieving Optimal Balance between Bandgap and Nonlinear Optical Performance

The first successful synthesis of the infrared nonlinear optical crystals Ag₂Hg_1–xCd_xI₄ was reported in this work via a low-temperature solid-state method, demonstrating controlled optimization of nonlinear optical (NLO) properties through isovalent Cd-substitution. Structural analyses revealed a continuous lattice expansion and space group transition from I4̅ to I4̅2m induced by longer Cd–I bonds compared to Hg–I in the crystal structure. Bandgap engineering from 2.36 eV (x = 0) to 3.10 eV (x = 1) was achieved through conduction band modulation where relativistic effects lowered Hg-6s orbitals below Cd-5s levels, while second harmonic generation (SHG) efficiency decreased from 4.5 to 1.6 times that of AgGaS₂ (AGS). First-principles calculations confirmed the dual enhancement mechanisms for the superior nonlinear optical response in Hg-rich compositions: (1) superior polarizability of Hg-5d¹⁰ orbitals and (2) strengthened Hg–I coupling via relativistic 6s² contraction. This work established a “relativistic effect modulation” strategy for the tailored trade-off between bandgap and SHG efficiency, providing a practical strategy for optimizing NLO performance.