High-performance
infrared (IR) nonlinear optical (NLO) materials
with large NLO response and suitable birefringence are urgently needed
for various applications. A Hg-based IR NLO material KHg4Ga5Se12 with such desirable properties has
been newly discovered. In the structure, obviously distorted HgSe4 and GaSe4 tetrahedra are connected to each other
by vertex-sharing to form a three-dimensional framework with the counterion
K+ residing in the cavities. Remarkably, all the NLO-active
building units in the title compound are arranged in a completely
parallel manner. Such a topological structure and the large susceptibility
of the Hg–Se bonds enable the material to achieve good phase-matchability
with a tremendous powder second harmonic generation (SHG) response
at 2.09 μm that is about 20-times that of the benchmark material
AgGaS2 (one of the largest responses among all the phase-matchable
IR NLO chalcogenides reported to date). The optical band gap of KHg4Ga5Se12 was determined as 1.61 eV. Moreover,
on the basis of the electronic band structure, the real-space atom-cutting
analysis, the SHG-weighted electronic densities, and the local dipole
moments calculations, the origin of the superior linear and nonlinear
optical properties of the title compound is ascribed to the (Hg/Ga)Se4 group. The calculated values for the maximum coefficient d
33 and birefringence (Δn) at 2.09 μm
are −65.257 pm/V and 0.072, respectively. Such values agree
well with experimental observations. Our study demonstrates that Hg-based
metal chalcogenides are a class of IR NLO material with competitive
features (good phase-matchability, very large SHG efficiency, wide
transparency) desirable for practical applications.