Copper-Ion Dynamics and Phase Segregation in Cu-Rich Tetrahedrite: an NMR Study

Abstract

63 Cu NMR measurements are reported for the Cu-rich phase of Cu 12+x Sb 4 S 13 (x 2) and compared to Cu 12 Sb 4 S 13 . We identify the NMR signatures of the phase segregation into Cu-poor (x ≈ 0) and Cu-rich (x 2) phases, with the metal-insulator transition observed in Cu 12 Sb 4 S 13 suppressed in the Cu-rich phase. Based on NMR T1 and T2 measurements, the results demonstrate Cu-ion hopping below room temperature with an activation energy of ∼150 meV for the Cu-rich phase, consistent with superionic behavior. The NMR results also demonstrate the effects of Cu-ion mobility in the Cu 12 Sb 4 S 13 phase, but with a larger activation barrier. We identify a small difference in NMR Knight shift for the metallic phase of Cu 12 Sb 4 S 13 , compared to the Cu-rich phase, and when compared to DFT calculations the results indicate a mix of hyperfine contributions to the metallic shift.I.

Rattling and Band-Filling Effects in Substituted Tetrahedrites: An NMR Study

“…This is slightly larger than expected, for example, substituting all 1.4 Zn/Ni atoms per formula unit on Cu-I sites would yield a ratio 6/4.6 = 1.3. The NMR results are thus in general agreement with DFT results that show that Ni and Zn atoms prefer Cu-I sites, but the ratio is enhanced for reasons not yet known, for example, substitution might also cause promotion of Cu-I atoms to interstitial sites as discussed for Cu 14 Sb 4 S 13 …”

“…The T -dependent Cu-I shift for all Zn- and Ni-substituted samples is displayed in Figure . The plot also includes Mn and Te substitution results described below, as well as unsubstituted results from earlier reports. , Since paramagnetic broadening prevented fitting quadrupole parameters in some cases, these shifts were determined by finding the center of gravity of the Cu-I peaks. For most of the doped samples, a common shift position near 950 ppm is seen.…”

“…The dashed line shows the common shift position schematically. Cu 14 Sb 4 S 13 and Cu 12 Sb 4 S 13 results are from refs and , respectively. Connected Cu 12 Sb 4 S 13 and Zn 0.5 Ni 0.9 data points are shown to emphasize the metal–semiconductor transition (MST).…”

“…In addition, the metal–insulator transition and associated structural transition ,− observed in unsubstituted Cu 12 Sb 4 S 13 at 88 K has attracted great interest. While this transition was originally speculated to be antiferromagnetic in nature, it is now understood that the tetrahedrite Cu ions are nonmagnetic, although magnetic behavior can be induced by transition metal substitution. − Another important feature of these materials is the superionic behavior observed at high temperatures, especially in Cu-rich compositions. − The associated Cu-ion mobility, connected to an additional high-temperature phase transition, can be a concern in TE device applications, which require the chemical stability of the material. Here, we have used NMR methods as a highly sensitive probe of the anharmonic phonon behavior as well as local electronic and magnetic properties of these substituted materials.…”

Phase Analysis and Thermoelectric Properties of Cu-Rich Tetrahedrite Prepared by Solvothermal Synthesis

“…Such a strongly disturbed carrier transport could be related to the mobile Cu ions in tetrahedrites, as is proposed in refs. [48,59]. Because the Cu(1) site neighbors the partially filled Cu(3) sites, the migration of copper ions should be mainly located between these positions.…”