Centrosymmetric or Noncentrosymmetric? Transition Metals Talking in $K_2TGe_3S_8(T = Co, Fe)$

Abstract

Two new quaternary sulfides $K_2TGe_3S_8(T = Co, Fe)$ have been synthesized by a high-temperature solid-state routine and flux growth method. The crystal growth process of $K_2TGe_3S_8(T = Co, Fe)$ was elucidated by in situ powder X-ray diffraction and DSC thermal analysis. The millimeter-sized crystals of $K_2TGe_3S_8(T = Co, Fe)$ were grown. K2CoGe3S8 crystallizes in a new structure type in centrosymmetric space group P1 (no. 2) with unit cell parameters of a = 7.016(1) Å, b= 7.770(1) Å, c = 14.342(1) Å, α = 93.80(1)°, β = 92.65(1)°, γ = 114.04(1)°. $K_2TGe_3S_8$ crystallizes in the $K_2TGe_3S_8$ structure type and the noncentrosymmetric space group P21 (no. 4) with unit cell parameters of a = 7.1089(5)Å, b = 11.8823(8) Å, c = 16.7588(11) Å, β = 96.604(2)°. There is a high structural similarity between $K_2TGe_3S_8$ and $K_2TGe_3S_8$. The larger volume coupled with higher degrees of distortion of the [FeS4] tetrahedra compared to the [CoS4] tetrahedra accounts for the structure’s shift from centrosymmetric to noncentrosymmetric. The theory simulation confirms that [TS4]T= Co or Fe tetrahedra play a crucial role in controlling the structure and properties of $K_2$TG$e_3$$S_8$(T = Co, Fe). The measured optical bandgaps of $K_2TGe_3S_8$ and $K_2TGe_3S_8$ are 2.1(1) eV and 2.6(1) eV, respectively. $K_2TGe_3S_8$ shows antiferromagnetic ordering at 24 K while no magnetic ordering was detected in $K_2TGe_3S_8$. The magnetic measurements also demonstrate the divalent nature of transition metals in $K_2$TG$e_3$$S_8$(T = Co, Fe).