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

Li 4 P 2 S 6 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. Li 4 P 2 S 6 crystallizes in the P 321 space group in a zero-dimensional (0-D) structure, with molecular [P 2 S 6 ] 4– units surrounded by a noncentrosymmetric Li + net. Li 4 P 2 S 6 does not melt up to 700 °C but instead thermally decomposes at 641 °C in air, off-gassing a combination of H 2 S, S (measured as S 2 and S 3 ), and H 2 P 2 S 4 . Upon cooling, there is a thermal event at 387 °C, likely due to the recondensation of volatile species. Li 4 P 2 S 6 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 Li 4 P 2 S 6 . It was found to have a moderate SHG intensity (∼0.1 × AgGaSe 2 ), but an exceptional LIDT due to its wide bandgap, outperforming AgGaSe 2 by a factor of almost 16. Li 4 P 2 S 6 was found to have moderate THG intensity (∼0.2 × AgGaSe 2 ) at energies below the bandgap of the reference, but exceptional THG intensity (∼36 × AgGaSe 2 ) at higher energies where the reference self-absorbs THG output. Li 4 P 2 S 6 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.