Crystal Growth, Characterization,\nand Properties of\nNonlinear Optical Crystals of Li<sub>0.5</sub>Ag<sub>0.5</sub>GaTe<sub>2</sub> for Mid-Infrared Applications

LiGaTe<sub>2</sub> is a promising nonlinear optical crystal\nwith\na large figure of merit (<i>d</i><sub>36</sub><sup>2</sup>/<i>n</i><sup>3</sup>), but it is difficult to grow the\nLiGaTe<sub>2</sub> single crystal due to its extreme instability.\nIn this work, we used Ag to replace Li and successfully grew a Li<sub>0.5</sub>Ag<sub>0.5</sub>GaTe<sub>2</sub> crystal by the modified\nBridgman method for the first time. We found it has thermal stability\nbelow 200 °C in an atmospheric environment, and the thermal expansion\ncoefficient is positive. This is different from LiGaTe<sub>2</sub> and AgGaTe<sub>2</sub> which have a negative thermal expansion coefficient\nalong the <i>c</i>-axis. When the temperature exceeds 200\n°C, Li<sub>0.5</sub>Ag<sub>0.5</sub>GaTe<sub>2</sub> is easily\ndecomposed and oxidized. Under closed vacuum conditions, the melting\nand solidifying points of Li<sub>0.5</sub>Ag<sub>0.5</sub>GaTe<sub>2</sub> are measured to be 719 and 694 °C. XPS spectra show\nthat the binding energies of Li, Ga, and Te are higher than LiGaTe<sub>2</sub>, and the surface is easily oxidized. The A1 vibration modes\nare found at 117.72 and 135.44 cm<sup>–1</sup> by the Raman\nspectrum which is related to the Te and Ga–Te bond in [GaTe<sub>4</sub>]<sup>5–</sup> tetrahedra. Li<sub>0.5</sub>Ag<sub>0.5</sub>GaTe<sub>2</sub> has a wide transmittance range from 0.94 to 20\nμm, and there was two-photon absorption near 16 μm. The\nphonon spectrum and PDOS were simulated by the DFT calculation to\nstudy the phonon vibration modes in the lattice, which shows that\nthe density of the Raman vibration is higher than those of AgGaTe<sub>2</sub> and LiGaTe<sub>2</sub>. The SHG test results showed that\nthe SHG response intensity of Li<sub>0.5</sub>Ag<sub>0.5</sub>GaTe<sub>2</sub> is 1.5 times that of AgGaS<sub>2</sub>, which shows its excellent\nnonlinear optical properties for mid-IR applications.