Thermoelectric properties of a Mn substituted synthetic tetrahedrite

Abstract

Tetrahedrite compounds Cu_12−xMn_xSb₄S₁₃ (0 ≤ x ≤ 1.8) were prepared by solid state synthesis. A detailed crystal structure analysis of Cu_10.6Mn_1.4Sb₄S₁₃ was performed by single crystal X-ray diffraction (XRD) at 100, 200 and 300 K confirming the noncentrosymmetric structure (space group I3m) of a tetrahedrite. The large atomic displacement parameter of the Cu2 atoms was described by splitting the 12e site into a partially and randomly occupied 24g site (Cu22) in addition to the regular 12e site (Cu21), suggesting a mix of dynamic and static off-plane Cu2 atom disorder. Rietveld powder XRD pattern and electron probe microanalysis revealed that all the Mn substituted samples showed a single tetrahedrite phase. The electrical resistivity increased with increasing Mn due to substitution of Mn²⁺ at the Cu¹⁺ site. The positive Seebeck coefficient for all samples indicates that the dominant carriers are holes. Even though the thermal conductivity decreased as a function of increasing Mn, the thermoelectric figure of merit ZT decreased, because the decrease of the power factor is stronger than the decrease of the thermal conductivity. The maximum ZT = 0.76 at 623 K is obtained for Cu₁₂Sb₄S₁₃. The coefficient of thermal expansion 13.5 ± 0.1 × 10⁻⁶ K⁻¹ is obtained in the temperature range from 460 K to 670 K for Cu_10.2Mn_1.8Sb₄S₁₃. The Debye temperature, Θ_D = 244 K for Cu_10.2Mn_1.8Sb₄S₁₃, was estimated from an evaluation of the elastic properties. The effective paramagnetic moment 7.45 μ_B/f.u. for Cu_10.2Mn_1.8Sb₄S₁₃ is fairly consistent with a high spin 3d⁵ ground state of Mn.