Evolution of Structures and Optical Properties in a Series of Infrared Nonlinear Optical Crystals Li x Ag 1- x InSe 2 (0 ≤ x ≤ 1).

In this work, a number of new infrared nonlinear optical (NLO) crystals of Li x Ag 1- x InSe 2 , 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 x Ag 1- x InSe 2 evolved from I 4̅2 d in AgInSe 2 to Pna2 1 in LiInSe 2 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 x Ag 1- x GaS 2 and Li x Ag 1- x GaSe 2 , the structural distortions in Li x Ag 1- x InSe 2 are much more prominent. This may explain the limited crystallization region in the phase graph of the tetragonal structure Li x Ag 1- x InSe 2 . The fundamental optical absorption edges in these Li x Ag 1- x InSe 2 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 (Δ n ∼ 0.04). Accordingly, the Li 0.78 Ag 0.22 InSe 2 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.