Structure, Second-, and Third-Harmonic Generation of Li4P2S6: A Wide Gap Material with a High Laser-Induced Damage Threshold

Li4P2S6 is a Li-rich, wide bandgap (3.74 eV) material that has been extensively studied for use in solid-state batteries. We offer a further examination and report here the first single-crystal structure determination with a special emphasis on its nonlinear optical (NLO) properties. Li4P2S6 crystallizes in the P321 space group in a zero-dimensional (0-D) structure, with molecular [P2S6](4-) units surrounded by a noncentrosymmetric Li+ net. Li4P2S6 does not melt up to 700 degrees C but instead thermally decomposes at 641 degrees C in air, off-gassing a combination of H2S, S (measured as S-2 and S-3), and H2P2S4. Upon cooling, there is a thermal event at 387 degrees C, likely due to the recondensation of volatile species. Li4P2S6 is air-stable in dry conditions despite a very high Li-content; however, it fully decomposes in sufficiently humid environments. Second-harmonic generation (SHG), third-harmonic generation (THG), and laser-induced damage threshold (LIDT) data were measured for Li4P2S6. It was found to have a moderate SHG intensity (similar to 0.1 x AgGaSe2), but an exceptional LIDT due to its wide bandgap, outperforming AgGaSe2 by a factor of almost 16. Li4P2S6 was found to have moderate THG intensity (similar to 0.2 x AgGaSe2) at energies below the bandgap of the reference, but exceptional THG intensity (similar to 36 x AgGaSe2) at higher energies where the reference self-absorbs THG output. Li4P2S6 is an excellent material for high-power NLO applications, especially for the efficient conversion of infrared (IR) to visible via THG and potentially other third-order NLO processes.