Stabilization of the Polar 0D Cs₄Nb_2–xTa_xS₁₁ with Phase-Matched Second-Harmonic Generation and High Laser-Induced Damage Threshold

The polysulfide compounds A 4 M 2 S 11 (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 Pca 2 1 . The clusters in these materials feature pentagonal bipyramidal dimeric units with the descriptive formula [M V 2 (S 2 ) 4 S 3 ] 4– . Differential thermal analysis and variable-temperature PXRD (VT-PXRD) show that the Ta-based materials melt incongruently, transforming to ATaS 3, while the Nb-based materials melt congruently. Additionally, the Nb-based materials have been shown to partially incorporate oxide ions in the presence of Nb x O y impurities to form A 4 Nb 2 S 10.7 O 0.3, which is made of molecular units with the formula [Nb 2 (S 2 ) 4 S 2.7 O 0.3 ] 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, Cs 4 Nb 1.75 Ta 0.25 S 11, the compound now melts congruently and is resistant to partial oxygen substitution. Second-harmonic generation was studied on Cs 4 Nb 2 S 11, Cs 4 Ta 2 S 11, and Cs 4 Nb 1.75 Ta 0.25 S 11, showing exceptional laser-induced damage threshold (LIDT) at 1064 nm for all materials with the LIDT of Cs 4 Nb 1.75 Ta 0.25 S 11 being 0.475 GW/cm 2, 3 times that of the reported industry standard AgGaSe 2 . Additionally, phase matchability was observed at the mid-infrared wavelength of 3000 nm for Cs 4 Nb 2 S 11 and Cs 4 Nb 1.75 Ta 0.25 S 11 .