Suppression of Nonmagnetic Insulating State by Application of Pressure in Mineral Tetrahedrite Cu₁₂Sb₄S₁₃

Abstract

The mineral tetrahedrite Cu12Sb4S13 exhibits a first-order metal-insulator transition (MIT) at TMI = 85 K and ambient pressure. We measured the 63 Cu-NMR at ambient pressure and the resistivity and magnetic susceptibility at high pressures. 63 Cu-NMR results indicate a nonmagnetic insulating ground state in this compound. The MIT is monotonically suppressed by pressure and disappears at ∼ 1.0 GPa. Two other anomalies are observed in the resistivity measurements, and the pressure -temperature phase diagram of Cu12Sb4S13 is constructed.Mixed-valent transition-metal (TM) compounds with a fractional number of electrons per TM site often exhibit a metal-insulator transition (MIT) associated with charge ordering. The insulating ground state is either nonmagnetic or magnetically ordered depending on the compound. Typical compounds with nonmagnetic ground states include Magneli phase Ti 4 O 7 with Ti 3.5+ (3d 0.5 ), 1) oxide spinel AlV 2 O 4 with V 2.5+ (3d 2.5 ), 2) and thiospinel CuIr 2 S 4 with Ir 3.5+ (5d 5.5 ). 3) For instance, CuIr 2 S 4 experiences an MIT at 230 K associated with the charge ordering of the mixed-valent Ir ion. The formation of isomorphic octamers of Ir 3+ 8 S 24 and Ir 4+ 8 S 24 with the dimerization of Ir 4+ (5d 5 , S = 1/2) pairs results in a spin-singlet ground state. 3, 4) On the other hand, vanadium bronze β-Na 0.33 V 2 O 5 with (1/6)V 4+ + (5/6)V 5+ (3d 1/6 ), 5) chromium hollandite K 2 Cr 8 O 16 with Cr 3.75+ (3d 2.25 ), 6) and magnetite Fe 3 O 4 with Fe 2.5+ (3d 5.5 ) for a B sublattice 7, 8) exhibit magnetically ordered ground states. Interestingly, superconductivity appears in β-Na 0.33 V 2 O 5 5) and CuIr 2 S 4 9, 10) when charge ordering is suppressed by applying hydrostatic pressure and chemical doping, respectively.The mineral tetrahedrite Cu 12 Sb 4 S 13 is another example of a mixed-valent TM compound and has been attracting considerable interest because of its enhanced thermoelectric properties. 11-15) Apart from its thermoelectricity, Cu 12 Sb 4 S 13 exhibits intriguing physical properties such as an MIT and a simultaneous sudden decrease in the magnetic susceptibility χ at 85 K, 11,16) which is reminiscent of thiospinel CuIr 2 S 4 . 9, 10) However, understanding this transition is not intuitive be- *

Zn-Substitution Effect on Metal-Semiconductor Transition in Tetrahedrite Cu12Sb4S13

“…The NMR spectrum consists of the sharp line and the broad one. A similar spectrum was observed in the metallic state of Zn0 [3,6]. Therefore the sharp line and the broad one correspond to the signals from Cu(1) and Cu(2) site, respectively.…”

Precursor of Metal–Semiconductor Transition in Tetrahedrite Probed by Cu-NMR

“…Figure 1 (b) displays the frequency-sweep 63 Cu-NMR spectra at around 78.5 MHz measured at the constant field of H 0 = 6.97 T and at 120 K, the NMR line consists of two kinds of signals, i.e., a sharp line at 78.68 MHz and a broad line with two peaks distributed from 77.3 to 79.6 MHz. The sharp line corresponds to the signals from Cu(1) atoms, whereas the broad line corresponds to the signals from Cu(2) atoms [7,8]. The broadened line profile for Cu(2) site is due to the second order nuclear quadrupole effect [7], where nuclear quadrupole frequency and asymmetry parameter are obtained to be ν Q = 18.6 MHz and η = 0.03, respectively [7,8].…”

“…The sharp line corresponds to the signals from Cu(1) atoms, whereas the broad line corresponds to the signals from Cu(2) atoms [7,8]. The broadened line profile for Cu(2) site is due to the second order nuclear quadrupole effect [7], where nuclear quadrupole frequency and asymmetry parameter are obtained to be ν Q = 18.6 MHz and η = 0.03, respectively [7,8]. To investigate the change of the local structure near T MST , we measured temperature dependences of NMR parameters for Cu(2) site.…”

High temperature neutron powder diffraction study of the Cu 12 Sb 4 S 13 and Cu 4 Sn 7 S 16 phases

“…Such structural description decreases the magnitude of the displacement parameters, and consequently improves the crystal structure refinement when isotropic (instead of anisotropic) displacement parameters are considered. Moreover, synthetic Cu 12 Sb 4 S 13 exhibits at low temperature a metal-semiconductor (also reported as a paramagneticnonmagnetic) transition around 85 K [11,20,[25][26][27], which was recently associated to a concomitant structural transition from the well-known cubic structure to tetragonal structure [28,29]. Finally, synthetic Cu 12 Sb 4 S 13 sample evidences exsolution at low-temperature leading to Cu-poor and Cu-rich compounds.…”