Centrosymmetric or Noncentrosymmetric? Transition Metals Talking in K2TGe3S8(T = Co, Fe)

Two new quaternary sulfides K2TGe3S8(T = Co, Fe) have been synthesized by a high-temperature solid-state routine and flux growth method. The crystal growth process of K2TGe3S8(T = Co, Fe) was elucidated by in situ powder X-ray diffraction and DSC thermal analysis. The millimeter-sized crystals of K2TGe3S8(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) A, b= 7.770(1) A, c = 14.342(1) A, α = 93.80(1)°, β = 92.65(1)°, γ = 114.04(1)°. K2FeGe3S8 crystallizes in the K2FeGe3Se8 structure type and the noncentrosymmetric space group P21 (no. 4) with unit cell parameters of a = 7.1089(5)A, b = 11.8823(8) A, c = 16.7588(11) A, β = 96.604(2)°. There is a high structural similarity between K2CoGe3S8 and K2FeGe3S8. 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 K2TGe3S8(T = Co, Fe). The measured optical bandgaps of K2CoGe3S8 and K2FeGe3S8 are 2.1(1) eV and 2.6(1) eV, respectively. K2FeGe3S8 shows antiferromagnetic ordering at 24 K while no magnetic ordering was detected in K2CoGe3S8. The magnetic measurements also demonstrate the divalent nature of transition metals in K2TGe3S8(T = Co, Fe).