Structure and interstitial deuterium sites of $\beta$-phase ZrNi deuteride

$\beta \text{−}\mathrm{Zr}\mathrm{Ni}{\mathrm{D}}_{1-x}$ (for $x\approx 0.1$, near the $\beta \text{−}\gamma$ phase boundary) was found to possess a triclinic $P\overline{1}$ structure as determined by high-resolution neutron powder diffraction. This is very different from the widely accepted orthorhombic and distorted orthorhombic $Cmcm$ structures previously proposed. In contrast to the single type of $D$ site associated with these latter structures, the true $\beta \text{−}\mathrm{Zr}\mathrm{Ni}{\mathrm{D}}_{1-x}$ structure contains two crystallographically distinct interstitial $D$ sites: “${\mathrm{Zr}}_4{\mathrm{Ni}}_2$” octahedral sites and “${\mathrm{Zr}}_4$” tetrahedral sites, alternately ordered along the $a$ direction. From first-principles calculations, the total energy of the $P\overline{1}$ structure was found to be $\approx 0.24\mathrm{eV}$ per unit cell lower than $Cmcm$-symmetry ZrNiD and could be rationalized in terms of different $D$ local-bonding configurations and metal-deuterium interactions. Resultant phonon calculations based on this structure were also consistent with the measured neutron vibrational spectrum.

Figure 1

(Color online) Experimental (dots), calculated (line), and difference NPD profiles for $\mathrm{Zr}\mathrm{Ni}{\mathrm{D}}_{0.88}$ at $300\mathrm{K}$. Vertical bars indicate the calculated positions of Bragg peaks for $\beta \text{−}\mathrm{Zr}\mathrm{Ni}{\mathrm{D}}_{1-x}$, ${\mathrm{Zr}}_9{\mathrm{Ni}}_{11}$, and $\alpha \text{−}\mathrm{Zr}\mathrm{N}\mathrm{i}$ (from the top).

Figure 2

(Color online) (a) Crystal structure of $\beta \text{−}\mathrm{Zr}\mathrm{Ni}{\mathrm{D}}_{1-x}$ viewed in a near-projection along the $\mathbf{c}$ axis. Large dark (blue) and small light (green) spheres denote Zr and Ni atoms, respectively. Small white spheres denote D atoms in the shaded polyhedra. (b) Local configuration of ${\mathrm{Zr}}_4$-tetrahedral and ${\mathrm{Zr}}_4{\mathrm{Ni}}_2$-octahedral interstitial D sites viewed in a near-projection along the $\mathbf{b}$ axis. The metal-deuterium bond distances are listed in Table .

Figure 3

(Color online) Structure comparison between (a) $Cmcm$ ZrNi, (b) $Cmcm$ ZrNiD and (c) $P\overline{1}$ ZrNiD. Large dark (blue) and small light (green) spheres denote Zr and Ni atoms, respectively. Small white spheres denote D atoms in the shaded polyhedra. The axes of the orthorhombic structure are indicated with subscript “$o$.”

Figure 4

(Color online) Total electronic DOS of $P\overline{1}$ ZrNiD and projection around the different atomic sites. The $l$-projected DOS of Zr and Ni are also shown. D1 atoms occupy ${\mathrm{Zr}}_4{\mathrm{Ni}}_2$ sites and D2 occupy ${\mathrm{Zr}}_4$ sites. $E_{\mathrm{F}}$ is taken as the zero of energy and shown as the dashed line.

Figure 5

(Color online) NV spectrum of $\mathrm{Zr}\mathrm{Ni}{\mathrm{D}}_{0.88}$ at $10\mathrm{K}$. FWHM instrumental resolutions are depicted by the horizontal bars beneath the spectra. Calculated spectra for $P\overline{1}$ and $Cmcm$ $\beta \text{−}\mathrm{Zr}\mathrm{Ni}\mathrm{D}$ delineating both 1 phonon (dotted line) and $1+2$ phonon (solid line) contributions are shown with the experimental data.

Figure 6

Calculated phonon dispersion curves along high-symmetry directions in the Brillouin zone for $P\overline{1}$ $\beta \text{−}\mathrm{Zr}\mathrm{Ni}\mathrm{D}$. The phonon density of states is also shown.