Synthesis of technetium hydride $\mathrm{Tc}{\mathrm{H}}_{1.3}$ at 27 GPa

In this work, we synthesize and investigate lower technetium hydrides at pressures up to 45 GPa using synchrotron x-ray diffraction, reflectance spectroscopy, and ab initio calculations. In the Tc-H system, the hydrogen content in $\mathrm{Tc}{\mathrm{H}}_x$ phases increases when the pressure rises, and at 27 GPa we found a hexagonal (hcp) nonstoichiometric hydride $\mathrm{Tc}{\mathrm{H}}_{1.3}$. The formation of technetium hydrides is also confirmed by the emergence of a reflective band at 450–600 nm in the reflectance spectra of $\mathrm{Tc}{\mathrm{H}}_{1+x}$ samples synthesized at 45 GPa. On the basis of theoretical analysis, we propose crystal structures for the phases $\mathrm{Tc}{\mathrm{H}}_{0.45\pm 0.05} \left({\mathrm{Tc}}_{16}{\mathrm{H}}_7\right)$ and $\mathrm{Tc}{\mathrm{H}}_{0.75\pm 0.05} \left({\mathrm{Tc}}_4{\mathrm{H}}_3\right)$ previously obtained at 1–2 GPa. Calculations of the electron-phonon interaction show that technetium hydrides $\mathrm{Tc}{\mathrm{H}}_{1+x} \left(x=0–0.3\right)$ do not possess superconducting properties due to low electron-phonon interaction parameter $\left(\lambda \sim 0.23\right)$.

Figure 1

Calculated convex hulls for the Tc-H system at (a) 50 GPa, (b) 5 GPa, and (c), (d) the most stable crystal structures of pseudohexagonal $\mathrm{Tc}{\mathrm{H}}_y \left(y=0.75\pm 0.12\right)$. Inset: (pseudo)hexagonal structures of TcH and ${\mathrm{Tc}}_{12}{\mathrm{H}}_7$, where hydrogen (H) occupies octahedral voids. Figures were drawn in vesta [47]. The predicted XRD patterns of these structures and their CIF files are given in the Supplemental Material [12].

Figure 2

X-ray powder analysis of technetium hydrides in DAC T1. Microphotographs of the loaded Tc sample (a) before and (b) after the laser heating, where the pressure significantly decreased. (c) Experimental XRD pattern measured at 300 K and the Le Bail refinement of the unit cell parameters of $P{6}_3/mmc-\mathrm{TcH}$ at 12 GPa. The experimental data, fit, and residue are shown in red, blue, and green, respectively. The unidentified reflections are indicated by asterisks. Inset: Pressure dependence of the $c/a$ ratio. (d) Experimental diffraction pattern of TcH.

Figure 3

Physical properties of technetium hydrides. (a) Experimental and theoretical (vasp, quantum espresso) equations of state of various technetium hydrides and metallic technetium (see also Supplemental Material [12]). (b) X-ray diffraction patterns of the synthesized technetium hydrides during decompression from 27 to $\sim 1$ GPa. (c) Eliashberg function and the electron-phonon interaction parameter of technetium monohydride TcH at 30 GPa. The calculations were performed using the PBE-SP-HGH pseudopotentials in the quantum espresso code [70, 71]. Inset: Crystal structure of hcp-TcH. (d) Schematic representation of different variants of the pressure dependence of the critical temperature of conventional Bardeen-Cooper-Schrieffer superconductivity [72] for metals, where $M$ is the mass of an atom and $P$ is the pressure. The maximum critical temperature of Tc can lie both at zero pressure and in the region of a small “negative” pressure.

Figure 4

Reflectance spectroscopy of technetium (Tc) and technetium hydride (TcH) at 45 GPa in DAC T2. Microphotographs of the sample (a) before and (b) after the laser heating at 45 GPa; the pressure was practically unchanged due to the small sample volume (Re is the gasket material, AB is ammonia borane). (c) Schematic of the study of the relative reflectivity $I_{\mathrm{sample}}/I_{\mathrm{ref}}$. (d) Changes in the electronic density of states in the reaction $2\mathrm{Tc}+{\mathrm{H}}_2\to 2\mathrm{TcH}$ at 20 GPa. (e) Smoothed measurement results of the relative reflectivity of Tc (green) and $\mathrm{Tc}{\mathrm{H}}_{1+x}$ (red), calculated data for hcp-TcH (purple) at 45 GPa, and literature data for technetium at 0 GPa (dashed black line). (f) Experimental reflectivity of Tc at 45 GPa (blue) compared with the literature data at 0 GPa (dashed black line) and vasp [77, 78, 79] calculation results at 0 (red) and 45 GPa (green).