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.