Hierarchical Contribution Of Anions To Second Harmonic Generation In A(2)Hgi(4) (A = Na, Ag, Cu) Regulated Dramatically By Metal Cations

Screening infrared (IR) nonlinear optical (NLO) materials remains an urgent concern, owing to the lack of efficient IR frequency-conversion materials with good NLO response. The deepened cognition of how cations regulate the contribution of anions to the SHG response is necessary to guide the exploration of IR NLO materials. In this work, the typical NLO materials A(2)HgI(4) (A = Ag, Cu) were studied systematically by the first-principles method. It is shown that they exhibit excellent SHG responses d(14) = 23.09 pm/V for Ag2HgI4 and d(14) = 29.50 pm/V for Cu2HgI4, which are about two times the IR benchmark AgGaS2. A(2)HgI(4) (A = Ag, Cu) also possess moderate band gaps of 2.38 and 2.04 eV. To analyze the regulation of A-site cation to the anions, two hypothetical compounds, alpha-Na2HgI4 and beta-Na2HgI4, were designed via atom substitution, which maintain strong NLO response about d(14) = 11.68 pm/V and d(14) = 14.43 pm/V and larger band gaps (3.36 eV for alpha-Na2HgI4 , 3.13 eV for beta-Na(2)FIgI(4)), respectively. Interestingly, these four compounds show hierarchical properties for differences in the A-site cation. The real-space atom-cutting (RSAC) method and SHG-density method reveal that the difference is not obtained directly from the change of A-site metals. Via the analysis of the electronic density method and SHG-density method, the iodine anions display hierarchical contributions to SHG response under the unexpected regulation by the A-site metals. Our findings offer a strategy to explore new NLO materials with desirable performances.