Synthesis, crystal and electronic structures, nonlinear optical properties, and magnetic properties of two thiophosphates: KInP\(_2\)S\(_7\) and KCrP\(_2\)S\(_7\)

Abstract

Two thiophosphates, KInP\(_2\)S\(_7\) and KCrP\(_2\)S\(_7\), were structurally characterized without investigating any optical properties. Herein in this work, KInP\(_2\)S\(_7\) and KCrP\(_2\)S\(_7\) were revisited to investigate their optical and magnetic properties, respectively. Pure polycrystalline samples and crystals of KInP\(_2\)S\(_7\) and KCr\(_2\)S\(_7\) were grown by high temperature solid state reactions, where mm-sized crystals of KCrP\(_2\)S\(_7\) were collected. KCrP\(_2\)S\(_7\) is isostructural to KInP\(_2\)S\(_7\), which features a layered structure. KInP\(_2\)S\(_7\) and KCrP\(_2\)S\(_7\) possess close relationship to the layered thiophosphate M\(_2\)P\(_2\)S\(_6\) (M = Fe, Co, Zn, etc.). The bonding pictures of KInP\(_2\)S\(_7\) were studied using the electron localization function (ELF) coupled with crystal orbital Hamilton population (COHP) calculations. The intrinsically distorted [PS\(_4\)] tetrahedra and [InS\(_6\)] octahedra are made by strong covalent P-S interactions and ionic In-S interactions, respectively. Electronic structure analysis confirmed that the optical properties of KInP\(_2\)S\(_7\) are mainly contributed to by [PS\(_4\)] tetrahedra together with small amounts of the contributions coming from [InS\(_6\)] octahedra. Magnetic measurement on mm-sized crystals of KCrP\(_2\)S\(_7\) verified that there is an antiferromagnetic transition around 21 K, and the Cr atoms are trivalent. KInP\(_2\)S\(_7\) is predicated to be an indirect bandgap semiconductor of 2.38 eV, which is confirmed by the UV-Vis measurement of 2.4(1) eV. KInP\(_2\)S\(_7\) is not a type-I phase-matching material and exhibits moderate second harmonic generation (SHG) response (0.51 × AgGaS\(_2\), sample of particle size of 100 µm). The laser damage threshold (LDT) of KInP\(_2\)S\(_7\) is very high of 5.2 × AgGaS\(_2\). Bandgap engineering were undergone to enhance the SHG response of KInP\(_2\)S\(_7\).