NMR study of proton motion in the defect pyrochlores HTaW${\mathrm{O}}_6$ and HTaW${\mathrm{O}}_6$ • ${\mathrm{H}}_2$O

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 $T_1$ and $T_2$ measurements give an activation energy of ∼0.3 eV and an attempt frequency of 3 × ${10}^{10}$ ${\sec }^{-1\mathrm{}}$. This "abnormally" small attempt frequency suggests a tunneling mechanism for proton motion in these materials.