On the other hand, the T 1 values for the O V environments in both V-substituted materials have a weaker dependence on temperature. While the earlier linewidth analysis of the O V environment clearly probes increasing local VO 4 rotational or librational motion, the lack of any change to the T 1 relaxation rates would suggest that this motion is not on the order of the Larmor frequency (∼ 10 8 Hz). We note that the T 1 values for the O V site are surprisingly short; the previous This supports the inference that quadrupolar relaxation mechanism leads to the observed changes in T 1 .