Nuclear spin lattice relaxation in the amorphous state: Towards an understanding

Full reports concerning the nuclear relaxation of ¹¹B, ²³Na, ²⁹Si and ⁷⁷Se in the following glasses: B₂O₃, Na₂B₄O₇, (Na₂O)_0.3(SiO₂)_0.7, Se are presented. Extended measurements confirm the existence of an efficient quadrupolar mechanism typical of glasses and show that the two-phonon process, responsible for the quadropolar relaxation in insulating crystals, is always negligible in our whole range of investigation. The relaxation rate T₁^?1, field independent in almost all cases, varies like T^(1+γ) with 0 < γ < 1 between 1.2 K and 100 K but exhibits different behaviour at higher T depending upon the analysed material. Whereas T₁ goes through a minimum at 300 K in B₂O₃, its T-dependence for ²³Na in Na₂B₄O₇ and (Na₂O)_0.3(SiO₂)_0.7 becomes faster than in the low-T regime. Several attempts made to interpret the data within the framework of the tunneling model, commonly invoked to explain the anomalous thermal properties of glasses, are reviewed in detail. It is concluded that the T₁ results are best accounted for by a process consisting of the modulation of the electric field gradient (EFG) at the nuclear site by its nearest neighbour tunneling defect. This EFG fluctuation is shown to be dominated by interactions between defects and an average correlation time of about 10^?9 sec is assigned to it. A complete T₁ calculation requires the previous knowledge of the physical nature of the defects, so far not available. Each defect is then tentatively identified with a bridging oxygen atom tunneling between two potential wells, which gives the correct energy dependence for the EEG matrix element. A defect number of 10²⁷ m^?3 is shown to be consistent with the results.