Glasses nuclear spin relaxation

Fig. 5. 59.8 MHz Si NMR spectra of two hexasilane isomers. The broad hump is the signal for silicate in the glass and serves for correct phasing of the signals. There is competition between dipole-dipole and spin-rotation interactions as dominant nuclear spin relaxation mechanisms for Si nuclei. Si nuclei in SiHs groups at a more peripheric position of the molecule relax predominantly by spin-rotation interactions as a result of the high mobility of these groups. The SiH2 or SiH groups in the chains are less mobile and therefore, dipole-dipole interactions become competitive. Adapted from ref. 25.

Fan J, Marzke R F, Sanchez E and Angell C A (1994) Conductivity and nuclear spin relaxation in superionic glasses, polymer electrolytes and the new polymer-in-salt electrolyte, J Non-Cryst Solids 172-174 1178-1189. 

NUCLEAR SPIN RELAXATION IN AEROGELS AND POROUS GLASSES... 

The temperature-dependance of the nuclear-spin relaxation in our very porous amorphous materials is qualitatively the same as in most of the explored classical glasses. . s. In the latters, the relaxation rates behave as T o+ ), with a lying between 0.1 and 0.5. As showed by previous studies, the faster ionic conductor the glass is, the faster the relaxation. When compared to non ionic-conducting amorphous materials, our oxydes present particularly short relaxation times. 

Methyl radicals formed on a silica gel surface are apparently less mobile and less stable than on porous glass (56, 57). The spectral intensity is noticeably reduced if the samples are heated to —130° for 5 min. The line shape is not symmetric, and the linewidth is a function of the nuclear spin quantum number. Hence, the amplitude of the derivative spectrum does not follow the binomial distribution 1 3 3 1 which would be expected for a rapidly tumbling molecule. A quantitative comparison of the spectrum with that predicted by relaxation theory has indicated a tumbling frequency of 2 X 107 and 1.3 X 107 sec-1 for CHr and CD3-, respectively (57). 

Fig. 101. Left Near-ZF (i.e., a small LF is applied to suppress relaxation effects from nuclear moments) pSR spectra in YjMojO, stated as polarization (A(0 divided by the instrumental asymmetry, A, ), at various temperatures. The inset shows the early time behavior tar below the glass transition temperature (Dunsiger et al. 1996a). Right Near-ZF muon spin relaxation rate 1/T, as a function of temperatuie in TbjTijO,. The inset shows representative pSR spectra, all of which exhibit exponential relaxation. From Gardner et al. (1999).

T.L. Reinecke, K.L. Ngai, Low-temperature nuclear spin-lattice relaxation in glasses, Phys. Rev. B 12... 

In the paramagnetic regime (T > Tm), the spectra in a weak LF (needed to suppress the depolarization by Cu nuclear dipoles) for x > 0.08 were most easily fitted to a power exponential (exp[—(At) ]) relaxation. Hence the summary label relaxation rate in fig. 113 (left) refers to the static width Aeff (see eq. 74) for T dynamic rate A for r > Tu- The variation of power p was studied in some detail for the 10% sample. A decrease fromp w 1 at high temperatures top w 0.6 close to Tm was found. This is another indication that a disordered spin-glass-like state is approached and 7m might best be considered a spin freezing temperature. This spin-glass-like state, however,... 

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