Unprecedented mid-infrared nonlinear optical materials achieved by crystal structure engineering, a case study of (KX)P 2 S 6 (X = Sb, Bi, Ba).

Three acentric type-I phase-matchable infrared nonlinear optical materials KSbP 2 S 6 , KBiP 2 S 6 , and K 2 BaP 2 S 6 , showing excellent balance between the second harmonic generation coefficient, bandgap, and laser damage threshold, were synthesized via a high-temperature solid-state method. KSbP 2 S 6 is isostructural to KBiP 2 S 6 , which both crystallize in the β-KSbP 2 Se 6 structure type. K 2 BaP 2 S 6 was discovered for the first time, which crystallizes in a new structure type. KSbP 2 S 6 and KBiP 2 S 6 exhibit close structural similarity to the parent compound, centrosymmetric Ba 2 P 2 S 6 . The [P 2 S 6 ] motifs, isotypic to ethane, exist in Ba 2 P 2 S 6 , KSbP 2 S 6 , KBiP 2 S 6 , and K 2 BaP 2 S 6 . The mixed cations, K/Sb pair, K/Bi pair, and K/Ba pair, play a dual-role of aligning the [P 2 S 6 ] structure motifs, contributing to a high SHG coefficient, as well as enlarging the bandgap. KSbP 2 S 6 , KBiP 2 S 6 , and K 2 BaP 2 S 6 are direct bandgap semiconductors with a bandgap of 2.9(1) eV, 2.3(1) eV and 4.1(1) eV, respectively. KSbP 2 S 6 , KBiP 2 S 6 , and K 2 BaP 2 S 6 exhibit a high second harmonic response of 2.2× AgGaS 2 , 1.8× AgGaS 2 , and 2.1× AgGaS 2 , respectively, coupled with a high laser damage threshold of 3× AgGaS 2 , 3× AgGaS 2 , and 8× AgGaS 2 , respectively. The DFT calculations also confirm that the large SHG coefficient mainly originates from [P 2 S 6 ] anionic motifs.