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Nuclear-magnetic-resonance study of crystalline tellurium and selenium

NMR-Untersuchung von kristallinem Tellur und Selen
: Kanert, O.; Günther, B.


Physical Review. B 31 (1985), Nr.1, S.20-33 : Abb.,Tab.,Lit.
ISSN: 0163-1829
ISSN: 1098-0121
ISSN: 0556-2805
ISSN: 2469-9950
Fraunhofer IFAM ()
diffusion; Kernspinrelaxation; Kernspinresonanz; NMR; Phononen; Selbstdiffusion; Selen; Tellur

Nuclear-magnetic-resonance measuremnts of 125Te and 77Se have been carried out in single crystals of tellurium and selenium, respectively, between room temperature and the melting point Tm. 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 essential ly caused by a two-phonon (Raman) process over the entire temperature range. 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-neighbour jumps. Diffusivities in tellurium deduced from this model are in good agreement with the tracer data.