Evolution of Structures and Optical Properties in a Series of Infrared Nonlinear Optical Crystals LixAg1-xInSe2 (0 ≤ x ≤ 1)

https://doi.org/10.1021/acs.inorgchem.3c01993

In this work, a number of new infrared nonlinear optical (NLO) crystals of Li_xAg_1–xInSe₂, in which the ratio x of Li/Ag varies in a wide range from 0 to 1, are investigated. Structural analysis reveals that the space group of Li_xAg_1–xInSe₂ evolved from I4̅2d in AgInSe₂ to Pna2₁ in LiInSe₂ as x increases from low values (0, 0.2, 0.37) to large values (0.55, 0.78, 0.81, 1). Compared to other Li/Ag coexisting chalcogenides such as Li_xAg_1–xGaS₂ and Li_xAg_1–xGaSe₂, the structural distortions in Li_xAg_1–xInSe₂ are much more prominent. This may explain the limited crystallization region in the phase graph of the tetragonal structure Li_xAg_1–xInSe₂. The fundamental optical absorption edges in these Li_xAg_1–xInSe₂ compounds are determined from the direct electronic transitions and the band gaps E_g gradually increase as the lithium content increases, consistent with the first-principles calculations. The composition x = 0.78 is calculated to have a good set of optical properties with a large NLO coefficient (d_powder = 28.8 pm/V) and moderate birefringence (Δn ∼ 0.04). Accordingly, the Li_0.78Ag_0.22InSe₂ crystal is grown by the modified Bridgman–Stockbarger method, and it exhibits a wide transparency range from 0.546 to 14.3 μm at the 2% transmittance level.