Stabilization of the Polar 0D Cs₄Nb_2–<i>_<i>x</i></i>Ta<i>_<i>x</i></i>S₁₁ with Phase-Matched Second-Harmonic Generation and High Laser-Induced Damage Threshold

The polysulfide compounds A4M2S11 (A = K, Rb, Cs; M = Nb, Ta) are a class of 0D noncentrosymmetric compounds made up of molecular anionic clusters and crystallize in the polar space group Pca21. The clusters in these materials feature pentagonal bipyramidal dimeric units with the descriptive formula [MV2(S2)4S3]4–. Differential thermal analysis and variable-temperature PXRD (VT-PXRD) show that the Ta-based materials melt incongruently, transforming to ATaS3, while the Nb-based materials melt congruently. Additionally, the Nb-based materials have been shown to partially incorporate oxide ions in the presence of NbxOy impurities to form A4Nb2S10.7O0.3, which is made of molecular units with the formula [Nb2(S2)4S2.7O0.3]4–, while the Ta-based materials do not form an oxychalcogenide. Upon alloying the two metals in a 1.75:0.25 Nb to Ta ratio, Cs4Nb1.75Ta0.25S11, the compound now melts congruently and is resistant to partial oxygen substitution. Second-harmonic generation was studied on Cs4Nb2S11, Cs4Ta2S11, and Cs4Nb1.75Ta0.25S11, showing exceptional laser-induced damage threshold (LIDT) at 1064 nm for all materials with the LIDT of Cs4Nb1.75Ta0.25S11 being 0.475 GW/cm2, 3 times that of the reported industry standard AgGaSe2. Additionally, phase matchability was observed at the mid-infrared wavelength of 3000 nm for Cs4Nb2S11 and Cs4Nb1.75Ta0.25S11.