Unusual doublet structure in proton magnetic-resonance spectra of yttrium and lutetium trihydrides

The proton magnetic resonance spectra of yttrium and lutetium trihydride, $\mathrm{Y}{\mathrm{H}}_{3}$ and $\mathrm{Lu}{\mathrm{H}}_{3}$, respectively, show an unusual doublet structure qualitatively similar to the Pake doublet that results from the mutual dipolar interaction of closely spaced, isolated proton pairs. However, both the magnitude of the splittings, roughly $70\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$, and the second moment of the spectra, roughly $200\phantom{\rule{0.3em}{0ex}}{(\mathrm{Oe})}^{2}$, greatly exceed classical dipolar theoretical expectations based on known nearest-neighbor spacings. On the other hand, the $\mathrm{Y}{\mathrm{H}}_{3}$ doublet narrows to a single line above $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, consistent with the temperature dependence of the spin-lattice relaxation rate, whereas the $\mathrm{Lu}{\mathrm{H}}_{3}$ doublet shows no narrowing up to $454\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In addition, the peaks themselves are close to Lorentzian in shape, indicative of strong indirect exchange narrowing. These features of the spectra indicate that substantial contributions from electron-mediated pseudodipolar and indirect exchange interactions exceed the classical dipolar interaction.