Evidence for low-temperature internal dynamics in Cu12As4S13 according to copper NQR and nuclear relaxation

63,65Cu nuclear quadrupole resonance (NQR) was applied to study the natural mineral Cu12As4S13 (tennantite) in the temperature range 4.2–210 K. The obtained results point to the presence of field fluctuations caused by internal motions in tennantite. Consistently with the crystal structure, the experimental data can be described by an occurrence of a magnetic phase transition, which takes place near 65 K. The low-temperature phase is characterized by Cu(II) electron magnetic moments freezing in the form of a spin-glass-like constitution.