Crystal structure refinement of MnTe₂, MnSe₂, and MnS₂: Cation-anion and anion-anion bonding distances in pyrite-type structures

The stability of hauerite (MnS₂) as compared to that of pyrite (FeS₂) can be explained by the long Mn-S distance and departure from the typical pyrite-type structures. The structural differences of MnX₂ compounds (X=S, Se, and Te) are the result of spin configurations that are different than those of other MX₂ compounds; however, the arrangement of d-electrons and the size of the ions in MnX₂ compounds do not clearly explain why Mn²⁺ in MnX₂ does not exist as a low spin state. To investigate the structural differences of MnX₂ compounds, we synthesized single-crystal MnTe₂ and MnSe₂ and performed single-crsytal X-ray diffraction experiments. The single-crystal X-ray diffraction experiments were conducted on MnTe₂ [a=6.9513(1) Å, u-parameter=0.38554(2), space group Pa3̄ , Z=4], MnSe₂ [a=6.4275(2) Å, u-parameter=0.39358(2)], MnS₂ [hauerite; a=6.1013(1) Å, u-parameter=0.40105(4), obtained from Osorezan, Aomori, Japan], and FeS₂ [pyrite; a=5.4190(1) Å, u-parameter 0.38484(5), obtained from Kawarakoba, Nagasaki, Japan]. The X-ray intensity datasets of these compounds do not show any evidence of symmetry reduction. In MnS₂, the S-S distance is 2.0915(8) Å, which is significantly shorter than that of FeS₂ (2.1618(9) Å), and the mean square displacement of S (U₁₁=0.00915(9) Ų) is smaller than that of Mn (U₁₁=0.01137(9) Ų). The thermal vibration characteristics of MnX₂ compounds are significantly different than those of FeS₂. Based on structural refinement data, we discuss the low spin state of MnX₂ compounds and the structural stability of pyrite-type structures.