Synthesis and stability of tantalum hydride at high pressures

Although many metal hydrides were predicted by theory, very few of those were so far realized in experiment. Here, we systematically investigated the Ta-H system below 85 GPa, and found that the hexagonal close-packed $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ can be gradually transformed into $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ above 60 GPa at room temperature. With the help of density-functional theory calculations, the space group of $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ can be determined as a $I\overline{4}3d$ phase, which is isostructural to the Domeykite mineral. Such structure is rather rare and was not predicted in the earlier theoretical works due to the large unit cell. We also tried laser heating at high pressures and found that temperature plays a key role in tuning the $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ phase to $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ phase; however, no other new tantalum hydrides were synthesized. During decompression, the $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ was completely decomposed to the $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ below 30 GPa. Further experiments are still required for the synthesis of other tantalum polyhydrides at a higher pressure range and for the comparison with the theoretical calculations.

Figure 1

representation of structures for (a) bcc-Ta, (b) $\mathrm{hcp}-\mathrm{Ta}{\mathrm{H}}_2$, and (c) cubic $\mathrm{Ta}{\mathrm{H}}_3$.

Figure 2

X-ray diffraction patterns of the Ta-H system under high pressure. (a) During compression, $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ phase gradually transforms to the $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ phase. (b). The $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ phase decomposes to $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ phase at low pressure.

Figure 3

X-ray diffraction spectrum of Ta-H system at high temperature with the pressure of 85 GPa. The $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ phase decomposes to $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ phase at high temperature.

Figure 4

XRD pattern and Rietveld refinement for $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ phase (a) and $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ phase (b) with wavelength $\lambda =0.3344\AA$. The insets show the cake-type image of the two dimensional x-ray-diffraction patterns.

Figure 5

Volume per Ta atom (V/Z) as a function of pressure for Ta, $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$, and $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$. The solid lines represent the BM fit results as given in Table 1. The experimental results for $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ match well the DFT calculation results. The dashed red line is the equation of state for $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ which is determined below 41 GPa in previous work [18].

Figure 6

Calculated convex-hull of Ta-H system at 100 GPa considering the ZPE.

Figure 7

(a) Calculated band structure and density of states for the predicted cubic $\mathrm{Ta}{\mathrm{H}}_3$ phase at 100 GPa. (b) Calculated phonon dispersion for $\mathrm{Ta}{\mathrm{H}}_3$ at 100 GPa. The absence of any imaginary frequency modes in the whole Brillouin zone suggests the dynamical stability of $\mathrm{Ta}{\mathrm{H}}_3$ at high pressures.