Local Bonding Asymmetries in Ge-As-Se Glasses

High magnetic fields up to 22 T have been applied to determine the local bonding asymmetries in the Ge 2 As 2 Se 7 and Ge 2 As 2 Se 5 glasses by 75 As NMR. The results are analyzed using computer simulations of the line-shape. The asymmetry parameter η of the electric field gradient at arsenic sites in Ge 2 As 2 Se 7 is found to be about 0.2, indicating that the dominant arsenic structural units in the composition are nearly axially symmetric pyramids. In the Ge 2 As 2 Se 5 glass, however, the 75 As NMR spectrum exhibits no well-resolved structure, revealing the existence of highly asymmetric sites. The experimental data are fitted using the previously obtained distribution of quadrupole coupling constants from pulsed 75 As NQR experiments. These NMR simulations assume a broad distribution of the asymmetry parameter. The results are in agreement with the NQR studies in the same compositions.