Evolution of Structures and Optical Properties in a Series of Infrared Nonlinear Optical Crystals Li_<i>x</i>Ag_1–<i>x</i>InSe₂ (0 ≤ <i>x</i> ≤ 1)

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

Synthesis of Ultra-incompressible sp³‑Hybridized Carbon Nitride with 1:1 Stoichiometry

The search of compounds with C_<i>x</i>N_<i>y</i> composition holds great promise for creating materials which would rival diamond in hardness due to the very strong covalent C–N bond. Early theoretical and experimental works on C_<i>x</i>N_<i>y</i> compounds were based on the hypothetical structural similarity of predicted C₃N₄ phases with known binary A₃B₄ structural types; however, the synthesis of C₃N₄ other than g-C₃N₄ remains elusive. Here, we explore an “elemental synthesis” at high pressures and temperatures in which the compositional limitations due to the use of precursors in the early works are substantially lifted. Using in situ synchrotron X-ray diffraction and Raman spectroscopy, we demonstrate the synthesis of a highly incompressible <i>Pnnm</i> CN compound (<i>x</i> = <i>y</i> = 1) with sp³-hybridized carbon above 55 GPa and 7000 K. This result is supported by first-principles evolutionary search, which finds that CN is the most stable compound above 14 GPa. On pressure release below 6 GPa, the synthesized CN compound amorphizes, maintaining its 1:1 stoichiometry as confirmed by energy-dispersive X-ray spectroscopy. This work underscores the importance of understanding the novel high-pressure chemistry laws that promote extended 3D C-N structures, never observed at ambient conditions. Moreover, it opens a new route for synthesis of superhard materials based on novel stoichiometrie