75As NMR relaxation in glassy Ge-As-Se

We have used high magnetic fields to perform As-75 nuclear magnetic resonance (NMR) relaxation measurements in Ge As Se glasses. The spin lattice relaxation time T-1 has been studied as a function of average coordination number, < r > and temperature, T. We show that the relaxation time constant, T-1, in this system can be a useful tool to probe rigidity of the glassy matrix. In the two extreme cases, zero-field nuclear quadrupole resonance (NQR) and high-field NMR. T-1 exhibits a similar dependence on the average coordination number. Namely, there is a region in this dependence at low < r > values, where the spin lattice relaxation time does not change remarkably, followed by an increase in T-1 in glasses with < r > more than 2.54. This transition can be correlated with the onset of rigidity percolation in the glassy matrix. The temperature dependence of the spin lattice relaxation rate is found in the form T-1(-1) proportional to T-2. This behavior is typical for As and other nuclei in glasses at temperatures below 300 K and is indicative of Raman-like relaxation mechanism involving low-frequency modes. Spin-echo intensity for Ge2As2Se5 as a function of pulse separation exhibits exponential decay whose slope is the same as one of the average of the decay found from As-75 NQR measurements. This result indicates to the presence of indirect dipolar coupling of arsenic atoms in this composition. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim