NMR-STUDY OF PROTON MOTION IN THE DEFECT PYROCHLORES HTAWO6 AND HTAWO6.H2O

Both cw and pulsed proton NMR measurements have been made between 4 and 500 K to elucidate the nature of proton motion in the defect pyrochlores. The data show that at 4 K the water molecule is static and separate from the proton (no hydronium ions exist). About 200 K tumbling of the water molecule begins. At 300 K translational motion is evident. Sites for both the water molecule and the proton are suggested which are consistent with the data. Analysis of T1 and T2 measurements give an activation energy of ∼0.3 eV and an attempt frequency of 3 × 1010 sec-1. This abnormally" small attempt frequency suggests a tunneling mechanism for proton motion in these materials. © 1979 The American Physical Society."