Nuclear-magnetic-resonance study of crystalline tellurium and selenium.

Abstract

Nuclear-magnetic-resonance measurements of $^{125}\mathrm{Te}$ and $^{77}\mathrm{Se}$ have been carried out in single crystals of tellurium and selenium, respectively, between room temperature and the melting point ${T}_{m}$. In particular, both the time evolution of the nuclear magnetization in the laboratory frame as well as in the rotating frame, following both broadband and site-selective excitations, have been investigated in detail. In tellurium, the Zeeman spin-lattice relaxation is determined by three different mechanisms: (1) Below 300 K, spin-lattice relaxation is governed by a two-phonon (Raman) process; (2) in the temperature range between about 300 and 450 K, the influence of conduction electrons overcoming the gap energy of 0.30 eV becomes important; (3) above 450 K, the spin-lattice relaxation is due to mobile charged vacancies. From the high-temperature data, the formation energy of a charged vacancy was found to be 0.66 eV. In selenium, on the other hand, the Zeeman spin-lattice relaxation time is essentially caused by a two-phonon (Raman) process over the entire temperature range. In either system, the phonon-induced spin-lattice relaxation shows the same dependence on the crystal orientation relative to the direction of the external field. For T\ensuremath{\ge}0.74${T}_{m}$ in tellurium as well as in selenium, a diffusional contribution to the rotating-frame relaxation rate is observed, arising from fluctuations in the nuclear dipole and chemical shift interaction due to atomic self-diffusion. However, from these data alone the correlation time of atomic motion could be determined only with a relative large error. In contrast, from site-selective excitation experiments the correlation time was extracted directly with a high degree of accuracy without any assumption regarding the nature of the nuclear spin interactions. The observed correlation times are discussed in terms of Mehrer's model for monovacancy diffusion via nearest- and next-nearest-neighbor jumps. Diffusivities in tellurium deduced from this model are in good agreement with the tracer data obtained recently by Mehrer et al. In contrast, the observed diffusion coefficients in selenium are about a factor of 20 smaller than the tracer diffusion coefficients measured some years ago by Br\"atter and Gobrecht.