Copper NQR studies of Ag-doped ternary sulfide Cu12As4S13 (tennantite)

The investigations of Ag-contaning tennantite, belonging to the group of tetrahedrite-based sulfides, were performed by means of copper nuclear quadrupole resonance (NQR) in a temperature range from 4.2 K up to 60 K. These experiments showed that copper nuclear spin-lattice relaxation rate T-1(-1) and nuclear spin-echo decay rate T-2(-1) depend on temperature in the same way as earlier studied samples with other impurity compositions. The obtained data are interpreted from the viewpoint of internal motions in the structure of tennantite. The analysis of copper NQR data at low temperatures points to magnetic phase transition near 60 K from paramagnetic phase to the phase, which resembles spin-glass-like constitution.