# Feasible route to high-temperature ambient-pressure hydride
superconductivity

Kapildeb Dolui  Department of Materials Science and Metallurgy, University of
Cambridge, 27 Charles Babbage Road, Cambridge CB30FS, UK  Lewis J. Conway
Department of Materials Science and Metallurgy, University of Cambridge, 27
Charles Babbage Road, Cambridge CB30FS, UK  Advanced Institute for Materials
Research, Tohoku University, Sendai, 980-8577, Japan  Christoph Heil
Institute of Theoretical and Computational Physics, Graz University of
Technology, NAWI Graz, 8010 Graz, Austria  Timothy A. Strobel  Earth and
Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road,
NW, Washington, DC 20015, USA  Rohit Prasankumar  Intellectual Ventures,
Bellevue, Washington, United States  Chris J. Pickard<EMAIL_ADDRESS>Department of Materials Science and Metallurgy, University of Cambridge, 27
Charles Babbage Road, Cambridge CB30FS, UK  Advanced Institute for Materials
Research, Tohoku University, Sendai, 980-8577, Japan

###### Abstract

A key challenge in materials discovery is to find a viable high temperature
superconductor. Hydrogen and hydride materials have long been considered
promising materials displaying conventional phonon-mediated superconductivity.
However, the high pressures required to stabilise these materials have
restricted their application. Here, we present results from high-throughput
computation, considering a wide range of high-symmetry ternary hydrides from
across the periodic table at ambient-pressure. This large composition space is
then reduced by considerations of thermodynamic, dynamic and magnetic
stability, before direct estimations of the superconducting critical
temperature. This approach, combined with first-principles structure
prediction has revealed a metastable ambient-pressure hydride superconductor,
Mg2IrH6, with a predicted critical temperature of 160 K, comparable to the
highest temperature superconducting cuprates. We propose a synthesis route via
a structurally related insulator, Mg2IrH7, which is thermodynamically stable
above 15 GPa and discuss the potential challenges in doing so. This supports a
long-held hypothesis that, hydrogen, in the right chemical environment, can
promote high-temperature superconductivity at ambient pressure.

A superconducting material exhibits zero resistance and the Meissner effect
below a critical temperature, $T_{\rm c}$. This phenomenon has many
applications where powerful electromagnets or loss-free power transfer is
desired. Most practical superconductors require cooling with liquid helium
($\sim$ 4 K) or nitrogen ($\sim$ 77 K) to reach such sufficiently low
temperatures. Historically, the highest $T_{\rm c}$ ambient-pressure materials
were cuprates where a $T_{\rm c}$ of 130 K is seen in an Hg–Ba–Ca–Cu–O
compound Schilling  _et al._ (1993). The $T_{\rm c}$ of some cuprates were
then shown to increase with pressure Chu  _et al._ (1993). However, no
cuprates with significantly higher $T_{\rm c}$s have been realized since.

A proposed route to high-$T_{\rm c}$ superconductivity is through hydrogen.
Under BCS theory, metallic hydrogen formed at high pressure (above 500 GPa) is
expected to be a superconductor Ashcroft (1968). Ashcroft and Carlsson
suggested in 1983 that the pressure required to transform hydrogen into a
metallic superconducting phase could be reduced by the introduction of
impurities Carlsson and Ashcroft (1983) — an effect which could be
rationalised in terms of ‘chemical precompression’ Ashcroft (2004); Hilleke
and Zurek (2022).

Hydride superconductivity remains at the forefront of the quest for room-
temperature superconductivity. High-pressure hydrides of sulfur Drozdov  _et
al._ (2015), lanthanum Drozdov  _et al._ (2019), yttrium Troyan  _et al._
(2021); Kong  _et al._ (2021), cerium Chen  _et al._ (2021a) and calcium Ma
_et al._ (2022), have been synthesized and shown to have critical temperatures
between 161 and 224 K at pressures well above 100 GPa.

Computational efforts to predict high-$T_{\rm c}$ hydrides have been ongoing
Zurek (2017); Pickard  _et al._ (2020); Boeri  _et al._ (2022). These works
comprise of several approaches: first-principles structure prediction Pickard
and Needs (2006, 2007); Duan  _et al._ (2014); Zurek (2017); Shipley  _et al._
(2021); Chen  _et al._ (2021b), high-throughput screening Saha  _et al._
(2023), and machine-learned property prediction Hutcheon  _et al._ (2020);
Cerqueira  _et al._ (2023); Xie  _et al._ (2022a).

The aim of computational approaches is to identify crystal structures
possessing a high $T_{\rm c}$s with thermodynamic and dynamic stability.
Thermodynamic stability indicates a resilience to transform into competing
phases or decomposition into constituent species. This is calculated by
Maxwell construction; how far a structure is from the convex hull formed of
free energy and composition coordinates. A structure on the convex hull is
considered thermodynamically stable. A structure close to the convex hull can
be described as metastable. Dynamic stability indicates a resilience to small
fluctuations in atomic positions and is determined through phonon dispersion
calculations. A structure with no ‘imaginary’ (or zero-frequency) phonon modes
is considered dynamically stable.

Most of the predicted high-$T_{\rm c}$ hydride structures are stabilized at
extremely high pressures (typically over 100 GPa) Pickard  _et al._ (2020);
Shipley  _et al._ (2021). A few of these predictions — including H3S Drozdov
_et al._ (2015), LaH10 Drozdov  _et al._ (2019), CaH6Ma  _et al._ (2022) —
have since been successfully synthesized in diamond anvil cells. However, most
theoretically predicted hydrides have not yet been synthesized. This may in
part be because many of these predicted structures are metastable. Since there
are significantly more crystal structures off the convex hull than on it, it
is to be expected that there are a multitude of energetically plausible
metastable high-$T_{\rm c}$ hydrides.

Many functional materials are in fact ‘metastable’ states obtained through a
well-designed synthesis pathway. Indeed, a recent survey of the Inorganic
Crystal Structure Database found that half of all registered structures are
metastable Sun  _et al._ (2016). Given the large number of predicted
high-$T_{\rm c}$ hydrides, it is difficult to decide which of these are worth
pursuing experimentally. This situation could improve if increased emphasis
was placed on the feasibility of synthetic pathways when proposing new
metastable structures.

Although most of the synthesized superconducting hydrides are only stable at
impractically high pressures, synthesis routes exploiting high pressures may
be viable to obtain metastable, ambient-pressure superconducting hydrides. A
structure may form at high pressure where it is thermodynamically stable and
remain in a metastable state on recovery to lower pressures due to its dynamic
stability and resilience to kinetic barriers. Several ternary hydrides
structures such as those with clathrate hydrogen structures, have recently
been predicted to have $T_{\rm c}$s around 70 K, to be thermodynamically
stable above 130 GPa, and to remain dynamically stable upon decompression as
low as 5 GPa. Di Cataldo  _et al._ (2021); Lucrezi  _et al._ (2022a); Belli
and Errea (2022)

For practical use, a superconducting material must be at least dynamically
stable at ambient pressure. Here we propose a viable high-$T_{\rm c}$
superconducting hydride predicted through a combination of first-principles
structure searches and high-throughput screening. Through this approach, we
identify a cubic phase of Mg2IrH6 — a metastable ternary hydride with $T_{\rm
c}=160$ K at ambient pressure with a viable high-pressure synthesis route. The
structure consists of octahedral IrH6 hydrido complexes and Mg ions on a FCC
lattice (see Figure 1(c)) and is less than 20 meV/atom from the convex hull at
15 GPa. We propose that this phase may be recovered via the synthesis of a
second, structurally related, insulating compound, Mg2IrH7, which contains
additional interstitial non-bonded hydrogen atoms (maroon spheres in Figure
1(b)) and is thermodynamically stable above 15 GPa. At 0 GPa, both
compositions are metastable, but the superconducting Mg2IrH6 configuration may
be obtained by removal of weakly bound interstitial H atoms from Mg2IrH7. The
viability of such a mechanism is reinforced by the high diffusivity of
hydrogen observed in hydrides such as LaH10 Wang  _et al._ (2023); Caussé  _et
al._ (2023). In this Letter, we propose and discuss the synthesis route
$\frac{7}{2}$ H2 \+ 2 Mg + Ir $\xrightarrow[]{+15{\rm GPa}}$ Mg2IrH7
$\xrightarrow[]{-15{\rm GPa}}$ Mg2IrH6 $+\frac{1}{2}$ H2.

We performed an ab initio random structure search (AIRSS) Pickard and Needs
(2006, 2011) at 1 GPa for $X_{a}Y_{b}$Hc compositions where $a=0-3$, $b=0-3$
and $c=1-18$ with $X$ and $Y$ randomly selected from a feasible subset of
elements on the periodic table 111 The subset of the periodic table used was;
Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn,
Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd,
Ag, Cd, In, Sn, Sb, Te, I, Cs, Ba, La, Ce, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au,
Hg, Tl, Pb, Bi and Po.. Structures were generated with 24 or 48 symmetry
operations (restricting the search to 35 cubic and hexagonal space groups
222The following space groups have 24 or 48 symmetry operations:
$F\overline{4}3c$, $F\overline{4}3m$, $F4_{1}32$, $F432$, $Fd\overline{3}$,
$Fm\overline{3}$, $I\overline{4}3d$, $I\overline{4}3m$, $I4_{1}32$, $I432$,
$Ia\overline{3}$, $Im\overline{3}$, $P\overline{4}3m$, $P\overline{4}3n$,
$P4_{1}32$, $P4_{2}32$, $P432$, $P4_{3}32$, $P6/mcc$, $P6/mmm$, $P6_{3}/mcm$,
$P6_{3}/mmc$, $Pa\overline{3}$, $Pm\overline{3}$, $Pn\overline{3}$,
$Fd\overline{3}c$, $Fd\overline{3}m$, $Fm\overline{3}c$, $Fm\overline{3}m$,
$Ia\overline{3}d$, $Im\overline{3}m$, $Pm\overline{3}m$, $Pm\overline{3}n$,
$Pn\overline{3}m$ and $Pn\overline{3}n$ ). Geometry optimizations were
performed using castep Clark  _et al._ (2005) with Perdew-Burke-Ernzerhof
generalized gradient approximation Perdew  _et al._ (1996), castep QC5
pseudopotentials, a 340 eV plane-wave cutoff, and a $k$-point spacing of
$2\pi\times~{}0.07$ Å-1.

The search resulted in 226,348 structures. To filter these structures for
synthesizable superconductors, we perform a multi-stage, high-throughput
screening process to test for thermodynamic, dynamic and magnetic
instabilities. Firstly, for thermodynamic stability, we calculated ternary
convex hulls for all permutations of $X$+$Y$+H at 1 and at 10 GPa (by linear
extrapolation of the $PV$ term — see section 6.4 in Pickard and Needs (2011))
and retained structures which are close to the convex hull. This reduced the
dataset to 1,586 structures. Second, for dynamic stability, we performed two
iterations of castep phonon calculations, first on a $1\times 1\times 1$
$q$-grid and then $2\times 2\times 2$ $q$-grid, removing structures with
imaginary phonon modes, reducing the dataset to 772 and then to 233
structures. Third, we calculated the electronic densities of states and
eliminated all non-metallic structures. Finally, we performed a spin-polarized
DFT calculation and removed any structures with non-zero spin. This procedure
resulted in 122 structures for which we estimated $T_{\rm c}$ by calculating
the electron–phonon interactions within density functional perturbation theory
(DFPT) using Quantum Espresso Giannozzi  _et al._ (2020), with coarse
parameters detailed in the supplementary material (SM), and solving the
isotropic Migdal-Eliashberg (ME) equations for the Eliashberg functions,
$\alpha^{2}F(\omega)$.

Figure 1: (a) Pressure dependence of the formation enthalpies of Mg–Ir–H
phases. Pressure synthesis route is indicated by arrows, starting from Mg2IrH7
at 15 GPa (1) and trackin decompression to 6 GPa below which cubic Mg2IrH6 \+
H should form (2). (b) Crystal structures of Mg2IrH7 and Mg2IrH6: green,
orange, pink and maroon spheres denote the Mg, Ir, hydrido H and interstitial
H atoms, respectively.

Of the remaining 122 structures, the face-centered cubic Mg2IrH6 structure had
a notably high estimated $T_{\rm c}$ and is the focus of this Letter. Mg2IrH6
is isostructural with potassium chloroplatinate, K2PtCl6 Ewing and Pauling
(1928), with $Fm\bar{3}m$ (space group 225) symmetry as shown in Fig. 1(b).
The Mg and Ir atoms occupy the 8$c$ (1/4, 1/4, 1/4) and 4$a$ (0, 0, 0) Wyckoff
sites, respectively, and H is located on the 24$e$ (0.26, 0, 0) Wyckoff site.
The PBE-relaxed lattice parameter is $a=6.655$ Åat 0 GPa. Simulated X-ray
diffraction patterns are shown in Figure S1 in the SM.

Mg2IrH6 consists of octahedral [IrH6]3- hydrido complexes and Mg2+ cations.
Notably Mg2IrH6 is also isostructural with a family of insulating ternary
hydrides, including Mg2(Fe,Ru,Ni,Os)H6 and K2SiH6, which have emerged as
appealing energy storage or electrode materials Huang  _et al._ (1991); Sai
Raman  _et al._ (2002); Zaïdi  _et al._ (2013); Xie  _et al._ (2022b).

To investigate the thermodynamic stability of Mg2IrH6, we carried out
extensive searches on the H–Mg–Ir ternary system using an Ephemeral Data
Derived Potential (EDDP) Pickard (2022), trained by a similar method as used
for the Lu–N–H system Ferreira  _et al._ (2023). The most stable structures
predicted by the EDDP, along with all Mg–Ir–H structures obtained from
Materials Project Jain  _et al._ (2013), were re-optimized at a range of
pressures up to 40 GPa by castep using PBE and PBEsol exchange–correlation
functionals. For these calculations, we used a 600 eV plane-wave cutoff, a
$k$-point spacing of $2\pi\times~{}0.03$ Å-1, and castep C19 pseudopotentials.

Figure 2: Convex hull at 20 GPa, calculated with PBEsol. Thermodynamically
stable points are shown by filled symbols. The cubic Mg2IrH7 and Mg2IrH6
structures are indicated by star symbols. Empty symbols represent structures
within 200 meV of the convex hull and are smaller (and more blue) with
increasing distance from the convex hull. At 20 GPa pressure, without
considering quantum nuclear effects, the $Amm2$ phase of Mg2IrH6 is more
stable than $Fm\bar{3}m$.

Figure 2 shows the resulting convex hull at 20 GPa, where Mg2IrH6 is present
but around 10 meV/atom above the convex hull. Also present are insulating
phases of Mg2IrH5, Mg2IrH9, as well as cubic and rhombohedral phases of
Mg2IrH7. The rhombohedral phase of Mg2IrH7 is on the convex hull at 0 GPa.
Above 15 GPa, the cubic phase of Mg2IrH7 is on the hull.

Figure 1(a) shows the pressure evolution of cubic and rhombohedral Mg2IrH7 and
decomposition routes, Mg2IrH6 \+ H and Mg2IrH5 \+ Mg2IrH9. The thick lines and
arrows indicate a proposed synthesis route from rhombohedral Mg2IrH7 to
Mg2IrH5 \+ Mg2IrH9 and then cubic Mg2IrH7 above 15 GPa. Then, under
decompression, we follow cubic Mg2IrH7 down to 6 GPa after which Mg2IrH6 \+ H
is favorable down to ambient pressure.

Given the high diffusivity of hydrogen in other known hydrides, it is possible
a low energy barrier exists for the non-bonded interstitial hydrogen atoms to
diffuse out of the system and a higher barrier exists for the bonded H atoms.
As the local hydrogen content reduces, the structure may become metallic and
superconducting as it forms cubic Mg2IrH6.

We also performed a comprehensive analysis of alternative decomposition routes
from cubic Mg2IrH7 at 0 GPa by enumerating all combinations of Mg2IrHx for
$5\leq x\leq 7$ formed by removing hydrogen atoms starting from the
conventional unit cell shown in figure 1(b). This enumeration process yielded
in 35,178 symmetrically inequivalent structures. The single-point energy of
each structure was calculating using castep, after which full geometry
optimisations were performed for the lowest-energy structures. Most
permutations of hydrogen vacancies result in a lowering of structural
symmetry, corresponding to small lattice distortions away from cubic. The
binary convex hull of these permutations comparing energies of cubic and
nearly-cubic structures is shown in Figure S2 in the SM. Although the
$Fm\bar{3}m$ phase of MgIr2H6 is one of the lower-energy configurations, we
note the presence of even lower-energy configurations in which some of the
IrH6 ions are broken into IrH5 units and some remaining interstitial H atoms.
The lowest-energy structure of this type is an insulating orthorhombic
($Amm2$) phase, shown in figure 1(d) and S4 in the SM, with a $c/a$ ratio of
1.03.

This presents a challenge in the synthesis route from cubic MgIr2H7 to cubic
MgIr2H6 such that in order to obtain the superconducting phase, the hydrogen
content must be controlled while preserving the IrH6 complexes. Free energy
calculations suggest that at higher pressures and temperatures, the cubic
structure is favourable (see figure S5 in the SM). This indicates a
potentially complex synthesis route involving heating, cooling, compression
and decompression may be required to obtain the superconducting phase.

Of the many experimental observables associated with a metal–insulator
transition, we specifically note that, compared to the cubic structure,
orthorhombic Mg2IrH6 exhibits a hardened Raman-active Ir–H stretching mode as
shown in Figure S6 in the SM. Compared with cubic Mg2IrH7, cubic Mg2IrH6
exhibits a softened $T_{2g}$ mode associated with Ir–H bending.

Figure 3: Energy-dependent distribution of anisotropic superconducting gap
$\Delta_{\mathbf{k}}$ on the Fermi surface of Mg2IrH6, evaluated for each
temperature solving the ME equations using Coulomb pseudopotential parameter
$\mu^{*}=$ 0.1 (blue), 0.125 (red), and 0.16 (green). The histograms for each
temperature indicate the number of $k$-points associated with the specific
superconducting gap values. The solid dashed lines are guides to the eye for
energy-averaged $\Delta_{\mathbf{k}}$. The inset shows the 3D Fermi surface
(left) and 00$\overline{1}$ projection (right) colored according to the
$k$-dependent gap values $\Delta_{\mathbf{k}}$ obtained from solving the
anisotropic ME equation at a temperature of 15 K using $\mu^{*}$= 0.125.

Having identified a promising and accessible metastable high-$T_{\rm c}$
structure, we performed robust electron–phonon coupling calculations on
denser, well-converged, Wannier interpolated, $k$\- and $q$-meshes using
Quantum Espresso Giannozzi  _et al._ (2020) and EPW Giustino  _et al._ (2007);
Margine and Giustino (2013); Poncé  _et al._ (2016). The superconducting gap
was then calculated by solving the anisotropic ME equations. The details of
these calculations are provided in the SM.

Figure 3 shows the superconducting gap distribution as a function of
temperature for a range of $\mu^{*}$ values. The calculated $T_{\rm c}$ is
between 160 K and 175 K at ambient pressure. This high $T_{\rm c}$ can be
rationalised by examining the density of states at the Fermi level in Figure
4, where there is a van-Hove-like singularity caused by flat bands along the
L–W high-symmetry path. Moreover, Figure 5(a) shows that the anisotropic
electron–phonon coupling strength, $\lambda_{\mathbf{q}\nu}$, exhibits a
maximum contribution around the Brillouin-zone center, i.e., $\mathbf{q}=0$,
which suggests stronger intraband coupling than interband coupling.

Figure 5(c) shows the Eliashberg spectral function,
$\alpha^{2}F\rm(\omega\rm)$, and the cumulative electron–phonon coupling
parameter, $\lambda(\omega)$, obtained by integrating
$\alpha^{2}F\rm(\omega\rm)/\omega$ over $\omega$, revealing a total $\lambda$
of about 2.52. The projected densities of states in figure 4(b) and (c) show a
significant contribution from H-1$s$ orbitals, enabling the high-frequency
phonon modes to couple with electrons near the Fermi surface. As such, we see
a significant contribution to $\lambda$ from hydrogen phonon bands at $\sim$
0.1 and 0.2 eV. Notably, $\lambda$ in Mg2IrH6 at 0 GPa is very close to the
calculated values of $\lambda$ for known high-pressure hydrides such as H3S at
200 GPa ($\lambda$=2.19) Duan  _et al._ (2014) and LaH10 at 250 GPa
($\lambda$=2.20) Liu  _et al._ (2017).

Figure 4: (a) Electronic band structure of $Fm\overline{3}m$ Mg2IrH6 at
ambient pressure. (b) The total electron density of states (eDOSs) projected
into Mg (red), Ir (green), and H (blue) atoms. (c) The eDOS is projected into
Mg-3$s$ (red), Ir-$e_{g}$ (light green), Ir-$t_{2g}$ (dark green) and H-1$s$
(blue). Dark green dashed line indicates the Fermi level. Figure 5: (a) Phonon
dispersion along high-symmetry $\Gamma-X-W-K-\Gamma-U-L-W$ $k$-path. The
radius of the red circles in the panel (a) is proportional to the magnitude of
$\lambda_{{\rm q}\nu}$ for each phonon mode. (b) Total (black) and projected
phonon densities of states (phDOS) onto Mg (magenta), Ir (green), and H (blue)
atoms. (c) Eliashberg spectral function, $\alpha^{2}F(\omega)$, and cumulative
frequency-dependent electron-phonon coupling strength $\lambda(\omega)$.

Quantum anharmonic effects can significantly impact vibrational properties,
particularly in materials rich in low-mass elements, like hydrides. To assess
their importance for Mg2IrH6, we employed the stochastic self-consistent
harmonic approximation (SSCHA) Monacelli  _et al._ (2021). We observe that the
vibrational modes of Ir and Mg are almost unchanged when going from the
harmonic approximation to the anharmonically corrected dispersion (see Figure
S8 in the SM). On the other hand, the low-frequency H-$T_{1g}$ modes of
hydrogen between 30–80 meV harden, while the high-frequency H-$T_{1u}$,
-$E_{g}$, -$A_{1g}$ modes above 150 meV soften, almost equally in magnitude by
15 meV. The intermediate H-$T_{2g}$, -$T_{2u}$, -$T_{1u}$ modes remain almost
unchanged. We attribute the hardening of H modes to the effects of
anharmonicity, while the softening is mainly due to the inclusion of quantum
ionic motion, as has been observed in other low-pressure hydride materials
Lucrezi  _et al._ (2022b). In fact, the H-24$e$ Wyckoff parameter changes
slightly from 0.258 (static-DFT) to 0.260 within the SSCHA, and the internal
pressure increases from 0 GPa to about 2 GPa. These calculations show that the
inclusion of quantum anharmonic effects do not significantly alter the phonon
dispersion relations in Mg2IrH6. In particular, quantum anharmonic effects do
not alter the dynamic stability at ambient pressure.

To understand the chemical bonding of cubic MgIr2H6, we calculated the
electron localization function (ELF) Becke and Edgecombe (1990) which is
plotted in the SM (Figure S2). The ELF shows isolated proto-spherical
isosurfaces surrounding the hydrogen atoms illustrating the ionic character of
the Ir–H bonds, originating from the large difference in electronegativity
between Ir and H atoms. A large gap below the Fermi level in the electronic
density of states (Figure 4b) indicates that electron doping would result in
the formation of a wide-gap insulator. Indeed, Mg2IrH6 is four electrons per
unit cell short of a balanced count, which is achieved for the case of Mg2IrH7
with interstitial H occupancy. For Mg2IrH6 hybridization between H-1$s$
orbitals and Ir-$e_{1g}$ orbitals results in splitting the bonding–antibonding
states, which contributes to a significant crystal field splitting of $\sim$4
eV between weakly hybridized occupied Ir-$t_{2g}$ states and unoccupied
Ir-$e_{1g}$ states. Therefore, if the Ir site is instead occupied by an atom
with one fewer $d$ electron (i.e., $d^{6}$ valency elements such as Fe, Ru and
Os) the material would exhibit a band gap within the $d$ manifold, consistent
with these well-known insulators. Therefore, total or partial substitution of
Ir with other transition metals, or adjustment of the overall H stiochiometry,
may significantly impact the bonding nature and the electronic structure,
leading to a variety of possible outcomes including the formation of
insulators, ferromagnets, or perhaps even higher-$T_{\rm c}$ superconductors.

In this Letter, we have presented a promising result from a wide search for
hydrides exhibiting phonon-mediated BCS superconductivity. The tendency for
electrons to form Cooper pairs — the mechanism behind BCS superconductivity —
must compete with other instabilities resulting from electron–electron and
electron–phonon coupling; magnetism, charge-density waves, disorder and
structural distortions. By employing AIRSS across the periodic table, high-
throughput calculations and DFPT, we have filtered a wide compositional and
structural space (albeit a high-symmetry subspace) to extract candidates with
only the Cooper pair instability. In doing so, we have predicted the existence
of cubic Mg2IrH6, an ambient-pressure, phonon-mediated, superconducting
hydride with a $T_{\rm c}$ of about 160 K. We demonstrate that there may exist
a viable high-pressure synthesis route via an intermediate cubic Mg2IrH7
phase, thermodynamically stable above 15 GPa. The extraction of non-bonded,
interstitial hydrogen from Mg2IrH7 represents a plausible route to create
superconducting Mg2IrH6, although kinetic barriers between competing phases
warrant careful consideration.

By studying the electronic structure of cubic Mg2IrH6, we suggest there may be
a route to increase either the thermodynamic stability or $T_{\rm c}$ by
doping this prototype crystal structure. This work demonstrates the
feasibility of high-$T_{\rm c}$ hydrides at ambient-pressure and reflects a
positive outlook for hydride superconductivity.

Acknowledgments. We thank Warren Pickett and Eva Zurek for their insightful
discussion. This work was supported by the Deep Science Fund at Intellectual
Ventures.

Supplementary Data. Crystal structures, calculated Raman and XRD spectra and a
full convex hull for the Mg–Ir–H system are provided in the Supplementary
Material (SM).

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