Layered manganese bismuth tellurides with GeBi4Te7- and GeBi6Te10-type structures: towards multifunctional materials

Abstract

The crystal structures of new layered manganese bismuth tellurides with the compositions Mn_0.85(3)Bi_4.10(2)Te₇ and Mn_0.73(4)Bi_6.18(2)Te₁₀ were determined by single-crystal X-ray diffraction, including the use of microfocused synchrotron radiation. These analyses reveal that the layered structures deviate from the idealized stoichiometry of the 12P-GeBi₄Te₇ (space group Pm1) and 51R-GeBi₆Te₁₀ (space group Rm) structure types they adopt. Modified compositions Mn_1−xBi_4+2x/3Te₇ (x = 0.15–0.2) and Mn_1−xBi_6+2x/3Te₁₀ (x = 0.19–0.26) assume cation vacancies and lead to homogenous bulk samples as confirmed by Rietveld refinements. Electron diffraction patterns exhibit no diffuse streaks that would indicate stacking disorder. The alternating quintuple-layer [M₂Te₃] and septuple-layer [M₃Te₄] slabs (M = mixed occupied by Bi and Mn) with 1 : 1 sequence (12P stacking) in Mn_0.85Bi_4.10Te₇ and 2 : 1 sequence (51R stacking) in Mn_0.81Bi_6.13Te₁₀ were also observed in HRTEM images. Temperature-dependent powder diffraction and differential scanning calorimetry show that the compounds are high-temperature phases, which are metastable at ambient temperature. Magnetization measurements are in accordance with a Mn^II oxidation state and point at predominantly ferromagnetic coupling in both compounds. The thermoelectric figures of merit of n-type conducting Mn_0.85Bi_4.10Te₇ and Mn_0.81Bi_6.13Te₁₀ reach zT = 0.25 at 375 °C and zT = 0.28 at 325 °C, respectively. Although the compounds are metastable, compact ingots exhibit still up to 80% of the main phases after thermoelectric measurements up to 400 °C.