High-pressure synthesis of tantalum dihydride

The reaction of tantalum with molecular hydrogen was studied by x-ray diffraction in a diamond-anvil cell at room temperature and pressures from 1 to 41 GPa. At pressures up to 5.5 GPa, a substoichiometric tantalum monohydride with a distorted bcc structure was shown to be stable. Its hydrogen content gradually increased with the pressure increase, reaching $\mathrm{H}/\mathrm{Ta}=0.92\left(5\right)$ at 5 GPa. At higher pressures, a new dihydride phase of tantalum was formed. This phase had an hcp metal lattice, and its hydrogen content was virtually independent of pressure. When the pressure was decreased, the tantalum dihydride thus obtained transformed back to the monohydride at $P=2.2\mathrm{GPa}$. Single-phase samples of tantalum dihydride also were synthesized at a hydrogen pressure of 9 GPa in a toroid-type high-pressure apparatus, quenched to the liquid-${\mathrm{N}}_2$ temperature, and studied at ambient pressure. X-ray diffraction showed them to have an hcp metal lattice with $a=3.224\left(3\right)$ and $c=5.140\left(5\right)\AA$ at $T=85\mathrm{K}$. The hydrogen content determined by thermal desorption was $\mathrm{H}/\mathrm{Ta}=2.2\left(1\right)$.

Figure 1

Energy-dispersive x-ray diffraction patterns of tantalum (shifted vertically for clarity) in a hydrogen atmosphere measured in the course of a stepwise pressure increase in a DAC. The peaks at E < 12 keV are the L fluorescence emission lines from tantalum; the other peaks are of a diffraction origin. The ticks at the bottom of the figure indicate calculated peak positions for Ta at ambient pressure (bcc lattice with $a=3.30\AA$) and positions of the strongest diffraction peaks from the hydrogen solution in rhenium at $P=5\mathrm{GPa}$ (hcp metal lattice with $a=2.79$ and $c=4.41\AA$ [15]). The ticks at the top show the positions of the strongest peaks from the hcp tantalum dihydride at $P=41.1\mathrm{GPa}$ ($a=3.08$ and $c=4.88\AA$).

Figure 2

Energy-dispersive x-ray diffraction patterns of tantalum (shifted vertically for clarity) in a hydrogen atmosphere collected in the course of a stepwise pressure decrease in the second series of measurements with the maximal pressure of 9 GPa. The top ticks show the positions of the strongest peaks for the hcp tantalum dihydride at $P=9\mathrm{GPa}$ ($a=3.19$ and $c=5.07\AA$). The black curve labeled “gasket at 0 GPa” shows a diffraction pattern from the strongly textured rhenium gasket measured after decompressing the cell to ambient pressure. Other commentaries are the same as in the caption for Fig. 1.

Figure 3

The top panel: The pressure-volume dependencies for Ta under high hydrogen pressure. The vertical black lines with arrows indicate the pressures of phase transitions. The solid black curve and its dashed prolongation to ambient pressure show the equation of state (EoS) fit for $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ with the fitting parameters listed in Table 1. The solid olive line at the bottom of the panel shows the EoS for Ta in an argon medium [16]. The bottom panel: The $c/a$ ratios for the hcp $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$. The black squares show $V_0$ and $c/a$ for the recovered $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ and $\mathrm{Ta}{\mathrm{H}}_{1-x}$ samples and initial Ta measured at 85 K and ambient pressure, see Sec. 3b.

Figure 4

XRD patterns of the initial Ta foil (top) and Ta samples hydrogenated at 5 GPa (middle) and 9 GPa (bottom) at 150 °C. Ambient pressure $T=85\mathrm{K}$, Cu Kα radiation. The black points stand for the experimental data, the red curves show the Rietveld fits, and the blue curves are the fit residuals. The rather large residual R factors are mostly due to the large grain size and strong texture of the samples.

Figure 5

Hydrogen release from the recovered $\mathrm{Ta}{\mathrm{H}}_{2.2}$ and $\mathrm{Ta}{\mathrm{H}}_{0.92}$ samples heated in vacuum at a rate of 10 °C/min.

Figure 6

Lattice expansion of Ta upon hydrogenation. The black symbols stand for the present data; the other colors are for the literature data [2, 23, 24, 25, 26, 27]. The uncertainty in V is smaller than the symbol size.