Zn_0.7Hg_0.3Ga₂Se₄: a defect diamond-like selenide with a balanced band gap and nonlinear optical response

Balancing wide band gaps with strong second-order nonlinear optical (NLO) responses remains a central challenge for infrared (IR) frequency-conversion materials. Here, we report a Zn/Hg co-occupied defect diamond-like (DDL) selenide, Zn_0.7Hg_0.3Ga₂Se₄, designed to address this trade-off through isovalent compositional regulation. The compound crystallizes in a noncentrosymmetric tetragonal framework composed of corner-sharing [GaSe₄] and [(Zn/Hg)Se₄] tetrahedra, in which ordered cation vacancies and mixed divalent-cation occupation jointly modulate the local bonding environment. The compound exhibits a wide optical band gap of 2.30 eV and a phase-matching second-harmonic generation response of 2.8 times that of AgGaS₂ under 2.09 μm laser irradiation. Its phase identity is preserved after melting treatment, further suggesting compatibility with bulk crystal growth. First-principles calculations reveal that the band edges are governed by Zn/Hg-Se and Ga-Se orbital hybridization, while the NLO response originates from the cooperative contributions of multiple tetrahedral units. In addition, another Zn/Hg composition, Zn_0.55Hg_0.45Ga₂Se₄, was obtained, further indicating the compositional flexibility of this DDL system. This work provides a useful reference for the compositional design of congruently melting DDL chalcogenides with balanced mid- to far-IR NLO performance.