新穎硒化物TMxSn2-xBi4Se8（TM = Mn, x = 1~2; Fe, x =0.5~0.75）的合成與物性分析

Abstract

本論文成功以固態燒結法在823K 的溫度下，合成出兩系列皆具有礦物Grumiplucite 結構之新穎硒化合物系統MnxSn2-xBi4Se8 以及FexSn2-xBi4Se8。它們的晶系都屬於單斜體，空間群同為 C2/m。晶體結構由兩層NaCl（311）結構單元平行c 軸延伸但不同厚度的平板交替堆疊而成。 從導電度的測量得知，MnxSn2-xBi4Se8 （ x = 1, 1.5, 2 ） 及 FexSn2-xBi4Se8（x = 0.5, 0.75）的電阻率隨溫度上升而下降，屬於半導體行為。MnxSn2-xBi4Se8 （x = 1, 1.5, 2）的電阻率隨著Mn 的含量下降，導電度增加，FexSn2-xBi4Se8 （x = 0.5, 0.75）則是Fe 含量上升時導電性增加。從UV-VIS 漫反射式吸收光譜得知，半導體能距皆小於0.69eV。MnSnBi4Se8 及FexSn2-xBi4Se8 （x =0.5, 0.75）皆為n-type 的半導體，各個化合物室溫的Seebeck量測值約-100μV/K (FexSn2-xBi4Se8)與-200μV/K(MnSnBi4Se8)。在磁性性質方面，除Fe0.75Sn1.25Bi4Se8 在275K以下為鐵磁性外，其餘化合物皆為反鐵磁性且過渡金屬在高自旋（ high spin ） 的狀態。電子結構計算顯示在電子自旋極化（spin-polarized）的模型計算下有較穩定的總能量。

New selenides TMxSn2-xBi4Se8 (TM = Mn, Fe) were prepared bydirectly reacting the elements in stoichiometric ratios at 823K.These compounds crystallized in the monoclinic system with space group C2/m(No.12). All phases are isostructural to the Grumiplucite type that features three-dimensional framework with building units of rod type units running parallel to the c-axis. The resistivity decrease with increasing temperature for all compounds, indicative of semiconducting behaviors. The resistivity of Mn-phases decreases with decreasing contents of Mn but the resistivity of Fe-phases exhibit the reverse trend. Diffuse-reflectance spectra show that the band gaps of all phases are lower than 0.69 eV. According to the Seebeck coefficient measurements, MnSnBi4Se8 and FexSn2-xBi4Se8（x = 0.5, 0.75）phases are n-type semiconductors. Temperature dependence susceptibilities measurements indicate paramagnetic property for MnxSn2-xBi4Se8（x = 1, 1.5, 2）and Fe0.5Sn1.5Bi4Se8, but Fe0.75Sn1.25Bi4Se8 exhibits ferromagnetic property below 275K. Band structure calculations using LMTO method show that the calculations with spin-polarized models are more stable than those with the nonmagnetic models.