High-pressure stabilization of Mg2IrH7: Structural proximity to high-Tc superconductivity

Abstract

Mg$_2$IrH$_6$ is a metastable complex metal hydride with a predicted superconducting transition temperature as high as 170 K at ambient pressure. Following the synthesis of isomorphic, insulating Mg$_2$IrH$_5$ at low pressure, higher-pressure studies were conducted to investigate the phase behavior and compound formation in this system. X-ray diffraction and Raman spectroscopic measurements indicate that cubic Mg$_2$IrH$_7$ is stabilized above ca. 40 GPa and coexists with a related hexagonal hydride with likely composition near Mg$_2$IrH$_5$. Electrical transport measurements show that the cubic Mg$_2$IrH$_7$ is insulating, in agreement with ab initio predictions, and persists during room-temperature decompression until $\sim$20 GPa before reverting back to the cubic Mg$_2$IrH$_5$. The experimental results confirm ground-state structure predictions in the Mg-Ir-H system, and the formation of two nearly identical phases with surrounding compositions opens new opportunities to access superconducting Mg$_2$IrH$_6$ through non-equilibrium processing pathways.

Figures (5)

• Figure 1: The crystal structures of (a) FCC Mg$_2$IrH$_6$ and (b) FCC Mg$_2$IrH$_7$. The structures are distinguished by an additional interstitial H per formula unit, shown in black.
• Figure 2: (a) Experimental Raman spectra of Mg2IrH5 before and after heating at 38.7GPa in ammonia borane. The calculated spectrum of Mg2IrH7 is shown for comparison using Lorentzian peak shapes with arbitrary widths. Orange arrows highlight new peaks attributed to [IrH6]$_{3-}$ observed after heating.(b) Raman spectra of a heated sample obtained at room temperature while decompressing from 38.7 GPa. The inset compares experimental peak positions for Mg2IrH7 (points) with DFT-calculated (QE-PBE) frequencies (lines).
• Figure 3: (a) Representative XRD pattern showing the formation of an expanded FCC phase after heating Mg2IrH5 in hydrogen. The sample pressure dropped from 40GPa to 32GPa after heating. The red points correspond to the experimental data, while the black and gray curves represent the refinement and residual, respectively. Orange, black, and blue ticks indicate the calculated peak positions for Mg2IrH7, Mg2IrH5 and Ir. (b) Volume per formula unit plotted as a function of pressure for Mg2IrH5, Hansen2024Mg2IrH7, and hexagonal Mg$_2$IrH$_x$. The orange and purple lines show the DFT-calculated equations of state. Ordered $Cmc2_1$Mg2IrH5 is used to approximate the disordered hexagonal variation.
• Figure 4: (a) Crystal structure of $P6_3mc$ Mg$_2$IrH$_x$. The calculated $P6_3mc$Mg2IrH7 structure possesses [IrH$_6$]$^{3-}$ units and intersitial H atoms (black spheres), like $Fm\bar{3}m$Mg2IrH7. In space group $P6_3mc$, Mg2IrH5 contains disordered [IrH$_5$]$^{4-}$ units (i.e., $\frac{5}{6}$ H occupancy represented by pie-chart spheres) with no interstitial hydrogen and can be described as an ordered orthorhombic $Cmc$2$_1$ structure in calculations. (b) Representative powder XRD pattern obtained after laser heating the samples to 1500K and corresponding Le Bail refinement. The red points correspond to the experimental data, while the black and gray curves represent the refinement and residual, respectively. Orange, black, green, and purple ticks indicate the calculated peak positions for $Fm\bar{3}m$Mg2IrH7, $Fm\bar{3}m$Mg2IrH5, $Pm\bar{3}m$ MgIrH$_x$, and $P6_3mc$ Mg$_2$IrH$_x$. Inset: comparison of experimental data and calculated powder intensity profile for the $P6_3mc$ structure. Dashed lines mark select predicted Bragg positions for the $P6_3mc$ structure that do not overlap with coexisting phases.
• Figure 5: Temperature-dependent electrical transport measurements performed after the synthesis of FCC Mg2IrH7 at 43 GPa and at subsequent decompression steps. Inset: Mg2IrH5 sample pellet with electrical contacts embedded in ammonia borane. The culet diameter is 300µ.