Local Bonding Asymmetries in Ge-As-Se Glasses
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Local Bonding Asymmetries in Ge-As-Se Glasses. E. Mammadov1, P. C. Taylor2, D. Baker2, D. Bobela2, A. Reyes3, P. Kuhns3, S. Mehdiyeva1 1
Institute of Physics, National Academy of Sciences, Baku, AZ 1143, Azerbaijan Department of Physics, Colorado School of Mines, Golden, CO 80401, USA 3 National High Magnetic Field Laboratory, Tallahassee, FL 32000, USA 2
ABSTRACT High magnetic fields up to 22 T have been applied to determine the local bonding asymmetries in the Ge2As2Se7 and Ge2As2Se5 glasses by 75As 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 Ge2As2Se7 is found to be about 0.2, indicating that the dominant arsenic structural units in the composition are nearly axially symmetric pyramids. In the Ge2As2Se5 glass, however, the 75As 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 75As 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.
INTRODUCTION Magnetic resonance techniques provide very useful tools to probe the local bonding arrangements in amorphous solids. Nuclear Quadrupole Resonance (NQR) and Nuclear Magnetic Resonance (NMR) spectroscopies have been successfully applied for obtaining structural information in glassy materials [1-8]. Due to various important technological applications As-containing glasses are of special interest. 75As nucleus (100 % abundant) with spin I = 3 2 posses a quadrupole moment, which can interact with the electrical field gradient (EFG) created at the nuclear site. This field gradient is strongly influenced by the nearestneighbor atoms. The interaction between the nuclear quadrupole moment and the EFG can be probed by studying the response of the nuclear spin system to excitation by an rf pulse. For the arsenic nucleus a single transition between doubly degenerate nuclear states occurs at a frequency [9]: e 2 qQ 1 ν NQR = 1+ η 2 , (1) 2h 3 where e 2 qQ / h = vQ is the so-called quadrupole coupling constant and η is the asymmetry parameter of the EFG. In the case of glasses the NQR line is broadened due to a distribution of the transition frequencies, which occurs because the long-range order is different at different sites due mainly to variations in bond angles. Thus the two parameters, vQ and η , can uniquely describe the quadrupole interaction and, correspondingly, the local environment of a given atom.
However, since there is only one transition frequency and Eq. (1) does not depend strongly on η , this parameter is not easily determined just from NQR experiments. When a magnetic field is applied that is strong enough for the Zeeman interaction to prevail over the quadrupole one (perturbation limit), the NMR powder pattern exhibits two divergences at the central tra
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