Deuterium site occupancy and phase boundaries inZrNiDx(0.87≤x≤3.0)

Abstract

${\mathrm{ZrNiD}}_{x}$ samples with compositions between $x=0.87$ and $x=3.0$ were investigated by $^{2}\mathrm{H}$ magic-angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR), powder x-ray diffraction (XRD), neutron vibrational spectroscopy (NVS), and neutron powder diffraction (NPD). The rigid-lattice MAS-NMR spectrum for a ${\mathrm{ZrNiD}}_{0.88}$ sample in the triclinic $\ensuremath{\beta}$ phase shows a single phase with two well-resolved resonances at $+11.5$ and $\ensuremath{-}1.7\phantom{\rule{0.3em}{0ex}}\mathrm{ppm}$, indicating that two inequivalent $\mathrm{D}$ sites are occupied, as was observed previously in ${\mathrm{ZrNiD}}_{1.0}$. For ${\mathrm{ZrNiD}}_{0.88}$, the ratio of spectral intensities of the two lines is 1:0.76, indicating that the $\mathrm{D}$ site corresponding to the $+11.5\phantom{\rule{0.3em}{0ex}}\mathrm{ppm}$ line has the lower site energy and is fully occupied. Similarly, the neutron vibrational spectra for ${\mathrm{ZrNiD}}_{0.88}$ clearly confirm that at least two sites are occupied. For ${\mathrm{ZrNiD}}_{1.0}$, XRD indicates that $\ensuremath{\sim}5%$ of the metal atoms are in the $\ensuremath{\gamma}$ phase, corresponding to an upper composition for the $\ensuremath{\beta}$ phase of $x=0.90\ifmmode\pm\else\textpm\fi{}0.04$, consistent with the MAS-NMR and neutron vibrational spectra indicating that $x=0.88$ is single phase. The MAS-NMR and NVS of ${\mathrm{ZrNiD}}_{1.87}$ indicate a mixed-phase sample $(\ensuremath{\beta}+\ensuremath{\gamma})$ and clearly show that the two inequivalent sites observed at $x=0.88$ cannot be attributed to the sites normally occupied in the $\ensuremath{\gamma}$ phase. For ${\mathrm{ZrNiD}}_{2.75}$, NPD results indicate a $\ensuremath{\gamma}$-phase boundary of $x=2.86\ifmmode\pm\else\textpm\fi{}0.03$ at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, increasing to $2.93\ifmmode\pm\else\textpm\fi{}0.02$ at $180\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and below, in general agreement with the phase boundary estimated from the NVS and MAS-NMR spectra of ${\mathrm{ZrNiD}}_{1.87}$. Rigid-lattice $^{2}\mathrm{H}$ MAS-NMR spectra of ${\mathrm{ZrNiD}}_{2.75}$ and ${\mathrm{ZrNiD}}_{2.99}$ show a ratio of spectral intensities of $1.8\ifmmode\pm\else\textpm\fi{}0.1:1$ and $2.1\ifmmode\pm\else\textpm\fi{}0.1:1$ $({\mathrm{Zr}}_{3}\mathrm{Ni}:{\mathrm{Zr}}_{3}{\mathrm{Ni}}_{2})$, respectively, indicating complete occupancy of the lower-energy ${\mathrm{Zr}}_{3}{\mathrm{Ni}}_{2}$ site, consistent with the NPD results. For each composition, the correlation time for deuterium hopping was determined at the temperature where resolved peaks in the MAS-NMR spectrum coalesce due to motion between inequivalent $\mathrm{D}$ sites. The measured correlation times are consistent with previously determined motional parameters for ${\mathrm{ZrNiH}}_{x}$.