Unprecedented mid-infrared nonlinear optical materials achieved by crystal structure engineering, a case study of (KX)P2S6 (X = Sb, Bi, Ba)

Abstract

Three acentric type-I phase-matchable infrared nonlinear optical materials KSbP₂S₆, KBiP₂S₆, and K₂BaP₂S₆, showing excellent balance between the second harmonic generation coefficient, bandgap, and laser damage threshold, were synthesized via a high-temperature solid-state method. KSbP₂S₆ is isostructural to KBiP₂S₆, which both crystallize in the β-KSbP₂Se₆ structure type. K₂BaP₂S₆ was discovered for the first time, which crystallizes in a new structure type. KSbP₂S₆ and KBiP₂S₆ exhibit close structural similarity to the parent compound, centrosymmetric Ba₂P₂S₆. The [P₂S₆] motifs, isotypic to ethane, exist in Ba₂P₂S₆, KSbP₂S₆, KBiP₂S₆, and K₂BaP₂S₆. The mixed cations, K/Sb pair, K/Bi pair, and K/Ba pair, play a dual-role of aligning the [P₂S₆] structure motifs, contributing to a high SHG coefficient, as well as enlarging the bandgap. KSbP₂S₆, KBiP₂S₆, and K₂BaP₂S₆ are direct bandgap semiconductors with a bandgap of 2.9(1) eV, 2.3(1) eV and 4.1(1) eV, respectively. KSbP₂S₆, KBiP₂S₆, and K₂BaP₂S₆ exhibit a high second harmonic response of 2.2× AgGaS₂, 1.8× AgGaS₂, and 2.1× AgGaS₂, respectively, coupled with a high laser damage threshold of 3× AgGaS₂, 3× AgGaS₂, and 8× AgGaS₂, respectively. The DFT calculations also confirm that the large SHG coefficient mainly originates from [P₂S₆] anionic motifs.