Neutron scattering study of tantalum dihydride

A single-phase sample of $\mathrm{Ta}{\mathrm{H}}_{2.2\left(1\right)}$ with a hexagonal close-packed metal lattice (hcp; space group $P{6}_3/mmc$) was synthesized under a hydrogen pressure of 9 GPa and a temperature of 100 °C; quenched to the liquid nitrogen temperature; recovered to ambient pressure and studied by neutron diffraction (ND) and inelastic neutron scattering (INS). The ND study showed that hydrogen atoms occupied one half of the tetrahedral (T) and all octahedral (O) interstitial sites in the hcp lattice of Ta atoms. The arrangement of the H atoms over the T sites was proven to be ordered, which lowered the symmetry of the full crystal structure of the dihydride to $P{6}_{3}mc$. Due to the resulting asymmetry in the local environment of the O sites, the H atoms were considerably displaced from the centers of these sites along the $z$ axis, away from the H atoms occupying the neighboring T sites. The INS study demonstrated that the potential wells for H atoms at both the T and O sites are highly anharmonic and anisotropic. The potential wells at the O sites are softer along the $z$ axis than in the $x, y$ plane, while the T sites show opposite anisotropy.

Figure 1

Neutron powder diffraction pattern of the hcp-$\mathrm{Ta}{\mathrm{H}}_{2.2}$ sample (black curve) measured at $T=100\mathrm{K}$ with the D20 instrument at ILL, Grenoble, and the results of its profile analysis using three structural models with fully occupied O sites and different arrangements of H atoms on the half-occupied T sites. The refined structural parameters are listed in Table 1.

Figure 2

Three models of the arrangement of hydrogen atoms in the octahedral (O) and tetrahedral (T) interstitial sites of the hcp metal lattice of $\mathrm{Ta}{\mathrm{H}}_2$. The Ta and H(octa) atoms are shown with large cyan and small red spheres, respectively; the tetrahedra show the T sites occupied by the H(tetra) atoms. In each model, shown in two projections, H atoms fill all the O sites and one half of the T sites. (a) The ordered $P{6}_{3}mc$ model, in which H(tetra) atoms fill only the T sites with the lower $z$ coordinate. The H(octa) and H(tetra) atoms are displaced from the centers of the corresponding interstices by $-0.12c$ and $+0.006c$, respectively, along the $z$ axis. (b) The disordered $P{6}_3/mmc$ model, in which H(tetra) atoms are randomly distributed among all available T sites with equal probability. (c) The ordered Pnma model also known as a cotunnite-type structure. The H atoms are all displaced from the centers of interstices in the hcp tantalum lattice and form an orthorhombic sublattice. The refined lattice parameters and atomic coordinates for the three models are listed in Table 1.

Figure 3

The dynamical structure factor $S\left(E\right)$ of the powder $\mathrm{Ta}{\mathrm{H}}_{2.2}$ sample as a function of the neutron energy loss $E$ measured at $T=10\mathrm{K}$ with the IN1-Lagrange spectrometer at ILL, Grenoble. The spurious intensity below 50 meV resulted from the contamination of the incoming neutron beam with $\lambda /2$ neutrons. The bottom row of black tics shows the positions of the experimental peaks indicated in Table 4. The upper row of red ticks with red labels shows the energies and polarizations of vibrational modes at the Г point of the Brillouin zone of $P{6}_{3}mc-\mathrm{Ta}{\mathrm{H}}_2$ resulted from ab initio calculations and presented in Table 3. All calculated modes with the x,y polarization are twice degenerate, while the $z$-polarized modes are nondegenerate.

Figure 4

Corrected dynamical structure factor $S\left(E\right)$ of the powder $\mathrm{Ta}{\mathrm{H}}_{2.2}$ sample (black curve), and estimated contributions from multiphonon neutron scattering on the H(octa) atoms (cyan curve), H(tetra) atoms (magenta curve) and the sum of these contributions (red curve). The fundamental vibrational bands are cross-hatched according to the type of the interstitial site, in which the hydrogen atom vibrates. The positions and polarizations of the calculated multiphonon peaks are listed in Table 4.