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.