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LiGa 0.5 In 0.5 Se 2 is a new quaternary nonlinear optical crystal for the mid-IR application grown as a mixed crystal of the LiGaSe 2 −LiInSe 2 solid−solution system. It is transparent in the 0.47− 14 μm range and has an appropriate bandgap and a lower melting point than LiGaSe 2 and LiInSe 2 . It is more technological about the growth process since its homogeneity range is broader in the phase diagram. In this work, we have synthesized the LiGa 0.5 In 0.5 Se 2 polycrystal by the two-zone temperature method. LiGa 0.5 In 0.5 Se 2 single crystals (Φ26 mm × 50 mm) were grown through the modified Bridgman method with the c-axis seed crystal which has the smallest thermal expansion coefficient of the three main axes in 293−773 K. The crystal structure was studied by X-ray diffraction and the Rietveld refinement method. Due to the low transmittance of the as-grown crystals, a systematic thermal treatment experiment was carried out. In the annealing experiment, the crystal surface is seriously enriched with selenium due to the thermal diffusion of selenium, resulting in the crystal opacity and cracking, while after vacuum quenching at 873 K, the transmittance of the LiGa 0.5 In 0.5 Se 2 crystal wafer was greatly improved, the bandgap shows a large increase from 2.13 to 2.51 eV, and the quenched crystal shows strong SHG response (×1.91 LiGaSe 2 ). The chemical states and vibration modes of surface elements for both conditions were characterized by X-ray photoelectron and Raman spectra. Density functional theory calculations were carried out to simulate the phonon spectrum and phonon density of states, which can help to study the phonon vibration modes in the lattice.