Thermoelectric properties of a Mn substituted synthetic tetrahedrite

Abstract

Tetrahedrite compounds Cu(12-x)Mn(x)Sb4S13 (0 ≤x≤ 1.8) were prepared by solid state synthesis. A detailed crystal structure analysis of Cu10.6Mn1.4Sb4S13 was performed by single crystal X-ray diffraction (XRD) at 100, 200 and 300 K confirming the noncentrosymmetric structure (space group I4[combining macron]3m) 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 Mn(2+) at the Cu(1+) 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 Cu10.2Mn1.8Sb4S13. The Debye temperature, Θ(D) = 244 K for Cu10.2Mn1.8Sb4S13, was estimated from an evaluation of the elastic properties. The effective paramagnetic moment 7.45 μB/f.u. for Cu10.2Mn1.8Sb4S13 is fairly consistent with a high spin 3d(5) ground state of Mn.

Research Update: Cu–S based synthetic minerals as efficient thermoelectric materials at medium temperatures

“…For Tr = Ni, high ZT value at ∼700 K and good thermal stability up to 783 K were confirmed by other research groups, [49][50][51] whereas for Tr = Ni and Zn (co-doped), ZT = 1 has not been followed up so far. Good figures of merit have been demonstrated also for Tr = Mn, 37,52,53 and Tr = Co, 54,55 as well as samples made of mixed natural and synthetic tetrahedrites. 13,[56][57][58] It should be noted that the natural minerals showed rather low ZT's due to excessive ρ.…”

“…14,[36][37][38] The large ADP is interpreted as the dynamical occupation of Cu(2) at off-plane split sites 0.3-0.4 Å away from the 12e site. [37][38][39][40] The S atoms in Cu 12 Sb 4 S 13 occupy tetrahedral 24k and octahedral 2a sites, which are denoted as S(1) and S(2), respectively. The Sb atom at the 8c position is bonded in a trigonal pyramidal manner to three S atoms.…”

Thermoelectric properties of Co substituted synthetic tetrahedrite

“…A new approach of doping by Te on the Sb site achieved the maximum zT of 0.92 at 723 K for synthetic Cu 12 Sb 3 Te 1 S 13 [10]. Chetty et al reported the adverse effect of Mn doping on thermoelectric properties and showed the maximum zT of 0.76 at 623 K for the parent compound [11]. Barbier et al [12] studied phase purity, structural and thermal stability of undoped and Ni doped tetrahedrite compounds and a maximum zT = 0.8 at 700 K for Cu 10.4 Ni 1.6 Sb 4 S 13 was obtained.…”

Structural phase transition and phonon instability inCu12Sb4S13

“…17,18,53 Thus, the suppression of the transition does not appear to be directly related to the disorder induced by the substitutions since the Cu(2) position appears to dominate the transition. Also, the change in unit cell volume does not appear critical.…”