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

Abstract

Two new quaternary sulfides K 2 TGe 3 S 8 (T = Co, Fe) have been synthesized by a high-temperature solid-state routine and flux growth method. The crystal growth process of K 2 TGe 3 S 8 (T = Co, Fe) was elucidated by in situ powder X-ray diffraction and DSC thermal analysis. The millimeter-sized crystals of K 2 TGe 3 S 8 (T = Co, Fe) were grown. K 2 CoGe 3 S 8 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 2 FeGe 3 S 8 crystallizes in the K 2 FeGe 3 Se 8 structure type and the noncentrosymmetric space group P2 1 (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 2 CoGe 3 S 8 and K 2 FeGe 3 S 8 . The larger volume coupled with higher degrees of distortion of the [FeS 4 ] tetrahedra compared to the [CoS 4 ] tetrahedra accounts for the structure's shift from centrosymmetric to noncentrosymmetric. The theory simulation confirms that [TS 4 ] T= Co or Fe tetrahedra play a crucial role in controlling the structure and properties of K 2 TGe 3 S 8 (T = Co, Fe). The measured optical bandgaps of K 2 CoGe 3 S 8 and K 2 FeGe 3 S 8 are 2.1(1) eV and 2.6(1) eV, respectively. K 2 FeGe 3 S 8 shows antiferromagnetic ordering at 24 K while no magnetic ordering was detected in K 2 CoGe 3 S 8 . The magnetic measurements also demonstrate the divalent nature of transition metals in K 2 TGe 3 S 8 (T = Co, Fe).

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Noncentrosymmetric Ba6In2Q10 (Q = S, Se): Structural Chemistry and Nonlinear-Optical Activity

“…The available MFIR crystals are far from the market’s requirement, and there is an increasing demand for better ones. Nowadays, the most successful commercial MFIR crystals, AgGaQ 2 (Q = S, Se) , and ZnGeP 2 , encounter several intrinsic defects, restricting their applications; therefore, it is imminent to explore new ones. − …”

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