The measurement of 1H nuclear magnetic resonance in proton-exchanged crystallites (prepared by using sulfuric acid as a proton donor) indicated the presence of 2 different motional regimes as a function of temperature. The low-temperature regime appeared to be especially significant since it suggested that there was a to-and-fro motion, between a normal and an interstitial site, which was very rapid at room temperature. This was suggested to be evidence for a substantial population of interstitial sites and might help explain the unusual optical properties of the material.

M.Engelsberg, G.C.Do Nascimento, L.H.Pacobahyba: Journal of Applied Physics, 1993, 74[10], 6427-9