Synthesis of superconducting hcp-$\mathrm{Zr}{\mathrm{H}}_3$ under high hydrogen pressure

Zirconium trihydride and, for comparison, trideuteride with hexagonal close-packed (hcp) metal lattices were synthesized at a hydrogen/deuterium pressure of 9 GPa and a temperature of 873 K using toroid-type high-pressure chambers. After a rapid cooling (quenching) to 100 K and lowering the pressure to atmospheric, the quenched sample was removed from the chamber and studied by hot extraction analysis, powder x-ray diffraction, and ac magnetic susceptometry. The hot extraction showed that thermal decomposition of both hcp-${\mathrm{ZrH}}_3$ and hcp-${\mathrm{ZrD}}_3$ in vacuum begins after heating at a rate of 10 K/min to ∼200 K and almost ceases at 270 K, leaving tetragonal $\varepsilon \text{−}{\mathrm{ZrH}}_2/\varepsilon \text{−}{\mathrm{ZrD}}_2$ as a result. The x-ray diffraction study at 85 K gave lattice parameters $a=3.443\left(2\right)\AA$ and $c=5.964\left(4\right)\AA$ for hcp-${\mathrm{ZrH}}_3$ and somewhat smaller values of $a=3.430\left(5\right)\AA$ and $c=5.947\left(5\right)\AA$ for hcp-${\mathrm{ZrD}}_3$. The measurements of magnetic susceptibility revealed superconductivity with an onset at 11.6(1) K for hcp-${\mathrm{ZrH}}_3$ and 9.5(2) K for hcp-${\mathrm{ZrD}}_3$.

Figure 1

X-ray powder diffraction patterns and Rietveld refinements of samples of (a) $\varepsilon \text{−}{\mathrm{ZrH}}_2$ and (b) hcp-${\mathrm{ZrH}}_3$ hydrides synthesized at high hydrogen pressures and studied at ambient pressure and $T=85\mathrm{K}$ using Cu $K\alpha$ radiation. The black curves show the experimental diffraction patterns. The red, orange, and green curves are the calculated contributions from, respectively, $\varepsilon \text{−}{\mathrm{ZrH}}_2$, hcp-${\mathrm{ZrH}}_3$, and hexagonal ice Ih condensed onto the surface of the sample holder while loading it into the x-ray cryostat. The blue curves are the Rietveld fit residuals.

Figure 2

Temperature dependences of the amount of hydrogen (deuterium) released from quenched two-phase hcp-${\mathrm{ZrH}}_3+\varepsilon \text{−}{\mathrm{ZrH}}_2$ (hcp-$\mathrm{Zr}{\mathrm{D}}_3+\varepsilon \text{−}\mathrm{Zr}{\mathrm{D}}_2$) samples heated at $10{}^{\circ }\mathrm{C}/\min$ in a closed-volume, pre-evacuated measuring system.

Figure 3

Atomic volumes $V$ of zirconium hydrides at atmospheric pressure as a function of their hydrogen content $x=\mathrm{H}/\mathrm{Zr}$. The point for hcp-${\mathrm{ZrH}}_3$ was obtained at $T=85\mathrm{K}$; the other points refer to room temperature. The points for ${\mathrm{Zr}}_4{\mathrm{H}}_{15}$ [23], ${\mathrm{Zr}}_4{\mathrm{H}}_{15}$, and bcc-${\mathrm{ZrH}}_3$ [24], and the ω phase of Zr metal [39] were determined by extrapolating the experimental dependences $V\left(P\right)$ from high pressures to $P=1\mathrm{atm}$. The segment of the dashed brown curve shows a smooth $V\left(x\right)$ dependence for the fcc solid solutions $\delta \text{−}\mathrm{Zr}{\mathrm{H}}_x$ at $1.5\le x\le 1.7$ and bct solutions $\varepsilon \text{−}\mathrm{Zr}{\mathrm{H}}_x$ at $1.7\le x\le 1.9$ [40]. The crystal structure of the $\gamma \text{−}\mathrm{Zr}{\mathrm{D}}_{0.98\left(3\right)}$ phase is orthorhombic, space group Cccm [41].

Figure 4

Temperature dependences of the ac magnetic susceptibility $\chi ={\chi }^{\prime }-i{\chi }^{\prime \prime }$ in the vicinity of superconducting transition in (a) powdered Zr-H and Zr-D samples containing the hcp-${\mathrm{ZrH}}_3$ and hcp-${\mathrm{ZrD}}_3$ phases, respectively, and (b) two Zr-H samples with different contents of the hcp-${\mathrm{ZrH}}_3$ phase. The real part ${\chi }^{\prime }$ of the susceptibility is shown in the upper panels, the imaginary part ${\chi }^{\prime \prime }$ in the bottom panels. The measurements were carried out in a drive magnetic field with an amplitude of 3.5 mOe and a frequency of 1.5 kHz.