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

Isaenko, Lyudmila; Dong, Linfeng; Korzhneva, Ksenia; Yelisseyev, Alexander; Lobanov, Sergey; Gromilov, Sergey; Molokeev, Maxim S; Kurus, Alexey; Lin, Zheshuai// Inorganic chemistry//


In this work, a number of new infrared nonlinear optical (NLO) crystals of LixAg1–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 LixAg1–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 LixAg1–xGaS2 and LixAg1–xGaSe2, the structural distortions in LixAg1–xInSe2 are much more prominent. This may explain the limited crystallization region in the phase graph of the tetragonal structure LixAg1–xInSe2. The fundamental optical absorption edges in these LixAg1–xInSe2 compounds are determined from the direct electronic transitions and the band gaps Eg 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 (dpowder = 28.8 pm/V) and moderate birefringence (Δn ∼ 0.04). Accordingly, the Li0.78Ag0.22InSe2 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.