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 LiAgInSe, in which the ratio of Li/Ag varies in a wide range from 0 to 1, are investigated. Structural analysis reveals that the space group of LiAgInSe evolved from 4̅2 in AgInSe to Pna2 in LiInSe as 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 LiAgGaS and LiAgGaSe, the structural distortions in LiAgInSe are much more prominent. This may explain the limited crystallization region in the phase graph of the tetragonal structure LiAgInSe. The fundamental optical absorption edges in these LiAgInSe compounds are determined from the direct electronic transitions and the band gaps gradually increase as the lithium content increases, consistent with the first-principles calculations. The composition = 0.78 is calculated to have a good set of optical properties with a large NLO coefficient ( = 28.8 pm/V) and moderate birefringence (Δ ∼ 0.04). Accordingly, the LiAgInSe 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.