In recent years, hydrides have emerged as potential materials in the search of room-temperature superconductivity, not only from theoretical predictions1–5 but also from experimental verifications.6–22 In 2015, the predicted cubic H₃S was synthesized for the first time and was experimentally confirmed to have a T_c of 203 K at 155 GPa.7,8 Four years later, the T_c record passed to the LaH₁₀ metal hydride, with an H-clathrate structure, which was observed to have T_c values of 250–260 K at 170–190 GPa.9,10 Metallic clathrate hydrides were predicted to be a large class,23–28 and some of the predictions of superconductivity in this family have been successively verified experimentally, such as for ThH_9/10 (146/161 K at 170–175 GPa),11 YH₆ (224 K at 166 GPa and 220 K at 183 GPa),12,13 YH₉ (243 K at 201 GPa and 262 K at 182 GPa),13,14 and CaH₆ (215 K at 172 GPa and 210 K at 160 GPa).15,16 These achievements have greatly encouraged the exploration of room-temperature superconductivity in hydrides.

However, the possible application of superconducting hydrides is not only influenced by the superconducting transition temperature, as in the case of other superconductors, but also by the stabilization pressure. Although the hydrides mentioned above have high T_c values, the pressures required to stabilize them are also extremely high (>150 GPa). Therefore, it is essential to obtain superconducting hydrides that are stable at low pressures. Our previous study proposed that the introduction of light B into the La–H system should lead to a stable Fm3̄m LaBH₈ phase with BH₈ units29–31 that remains dynamically stable at 55 GPa and exhibits a T_c of 155 K. Subsequently, some other B-, C-, and Si-based ternary hydrides were also estimated to show good superconductivity at moderate pressures.32–35 In these hydrides, the covalent units formed by including light elements with H contribute significantly to their low-pressure stability. Furthermore, an La–Y alloy tetrahydride was synthesized at 110 GPa, exhibiting a T_c of 92 K, and it can be recovered at 80 GPa, both of which are lower pressure thresholds than those required for the synthesis of YH₄.18 Bi et al.19 synthesized an (La,Ce)H₉ alloy at megabar pressures of 97–172 GPa, which exhibited a T_c of 148–178 K. More recently, the metastable compound P6₃/mmc-LaH₁₀ was stabilized at 146 GPa by introducing Al atoms to form P6₃/mmc-(La,Al)H₁₀, with a T_c of 178 K.20 An increase in the configurational entropy of a mixed alloy hydride will decrease its Gibbs free energy and enhance its stability. Therefore, alloying binary metal hydrides may be an alternative approach to optimize the stabilization pressure of superconducting hydrides.

Trihydrides are commonly found in binary metal hydrides, and their superconductivity under pressure has been extensively studied.36–45 On the basis of their structures, common metal trihydrides can be divided into two categories, one with the Pm3̄n structure and the other with the Fm3̄m structure. This provides a platform to tune the stability and superconductivity of hydrides by alloying. The Pm3̄n structure, also called the A15 structure, is well known for its excellent superconducting performance. To date, about 50 alloys with this structure have been found to be superconductors, and some of them remain the most promising materials for practical applications.46 Among binary hydrides, AlH₃, GaH₃, ZrH₃, and HfH₃ were predicted to become stable in the A15 structure at 73, 160, 8, and 27 GPa, respectively, with the metal atoms forming a body-centered cubic (bcc) lattice and the six H atoms occupying half of the tetrahedral interstices of this lattice.36–40 Interestingly, the A15 phases of AlH₃ and ZrH₃ were confirmed in experiments at 100 and 30 GPa, respectively, but they do not exhibit high T_c values.37,39,41 On the other hand, the Al5 phase of GaH₃ was estimated to have the highest T_c of 102 K at 120 GPa.38 However, A15 GaH₃ requires relatively high minimum pressures of ∼160 and ∼84 GPa for thermodynamic and dynamic stability respectively,38,47 making its synthesis difficult. Below 160 GPa, A15 GaH₃ becomes thermodynamically unstable and transforms into a structure containing H₂ units, with an insulating character.26 Compared with Al and Ga, group IIIB and IVB metal atoms are less electronegative and could transfer more electrons to H₂ units, allowing structures with atomic hydrogen to remain stable at lower pressures. In addition, group IIIB and IVB metals can also exist in the trivalent state and form trihydrides. Therefore, alloying AlH₃/GaH₃ with group IIIB/IVB metals could give stable alloy hydrides with an A15-type structure and the expected superconductivity at low pressures. For ternary hydrides, A15-type GaAsH₆ and YZrH₆ were predicted to have T_c values of 98 and 16 K at 180 GPa and ambient pressure, respectively.28,48 Recently, our group designed a series of A15-type ternary hydrides, with the CaSnH₆ alloy demonstrating the lowest thermodynamically stable pressure of 110 GPa and CaSnH₆ with the lowest dynamically stable pressure of 41 GPa,47 which also facilitated our exploration of ternary alloy hydrides. To some extent, alloying AlH₃/GaH₃ with group IIIB/IVB metals also provides a way to control the physical properties of ternary hydrides through the choice of their constituent elements.

In this work, we investigate theoretically the structure, stability, and superconductivity of alloy hydrides with the AMH₆ composition (A = Al or Ga; M = a group IIIB or IVB metals). We chose the AMH₆ composition because the substitution of an M atom for an A atom in the A₂H₆ unit cell is one of the most intuitive ways to form alloy hydrides. Furthermore, considering the promoting effect of configurational entropy in material formation, mixing metal elements in equal proportions may yield a highly disordered alloy hydride, making it more favorable for experimental synthesis. The results show that seven ternary hydrides, AlMH₆ (M = Ti, Zr, or Hf) and GaMH₆ (M = Sc, Ti, Zr, or Hf) are identified to be stable in the A15-type structure. A15-type AlZrH₆, AlHfH₆, and GaMH₆ for all M are thermodynamically stable at pressures much lower than those required to stabilize A15 AlH₃ and GaH₃. Except for GaScH₆, all of these hydrides also have significant advantages over AlH₃ and GaH₃ in terms of the lowest dynamically stable pressures, with AlZrH₆, AlHfH₆, GaZrH₆, and GaHfH₆ able to maintain their dynamical stability at ∼13, 6, 5, and 6 GPa, respectively. Electron–phonon coupling (EPC) calculations show that all of these ternary hydrides are superconducting. Among them, the estimated T_c values for AlZrH₆, AlHfH₆, GaScH₆, GaZrH₆, and GaHfH₆ are 75, 76, 88, 70, and 49 K at 20, 10, 100, 10, and 10 GPa, respectively. In addition, AlScH₆ and AlYH₆ are predicted to be stable in structures similar to Fm3̄m Sc(Y)H₃, and they are also potential superconductors under pressure. Our results indicate that they have great potential for the exploration of low-pressure stable high-T_c superconductors in alloy hydrides.

Structure searches of AMH₆ with simulation cells containing up to four formula units were performed at 50–300 GPa by using the particle swarm optimization technique implemented in the CALYPSO code.49,50 Structural relaxations and calculations of electronic properties were performed using the Vienna Ab Initio Simulation Package (VASP) code based on density functional theory (DFT) with the Perdew–Burke–Ernzerhof generalized gradient approximation.51,52 The ion–electron interaction was described by projector-augmented-wave potentials, with 1s¹, 3s²3p¹, 3d¹⁰4s²4p¹, 3s²3p⁶3d¹4s², 4s²4p⁶4d¹5s², 5s²5p⁶5d¹6s², 3s²3p⁶3d²4s², 4s²4p⁶4d²5s², and 5p⁶5d²6s² configurations being treated as valence electrons for H, Al, Ga, Sc, Y, La, Ti, Zr, and Hf atoms, respectively.53 The plane-wave kinetic energy cutoff was set to 700 eV, and the corresponding Monkhorst–Pack (MP) k-point meshes were adopted for different structures to ensure that enthalpy converged to 1 meV/atom. Phonon calculations were performed using the PHONOPY54 or Quantum-ESPRESSO codes.55 EPC calculations of Pm3̄ ternary hydrides were carried out with the Quantum-ESPRESSO code using ultrasoft pseudopotentials for all the atoms. A 9 × 9 × 9 q-point mesh in the first Brillouin zone (BZ) was used in the EPC calculation, and an MP grid of 36 × 36 × 36 was considered, to ensure k-point sampling convergence.

We performed structure searches for 12 ternary hydrides with the AMH₆ composition in the pressure range of 50–300 GPa. Seven hydrides (AlTiH₆, AlZrH₆, AlHfH₆, GaScH₆, GaTiH₆, GaZrH₆, and GaHfH₆) were identified as having an A15-type structure with the Pm3̄ symmetry, as shown in Fig. 1(a). In this structure, two metal atoms occupy the vertex and the center positions to form a bcc lattice, with H atoms occupying locations near their tetrahedral interstices. The A–H, M–H, and H–H bond distances for Pm3̄ AMH₆ at 100 GPa are shown in Table S1 (supplementary material). The A–H distance is shorter than the M–H distance in AlZrH₆ and AlHfH₆, while the reverse happens in AlTiH₆ and GaMH₆. Different metal atoms in the structure lead to two different lengths between adjacent H atoms. The calculated H–H distances of 1.45–1.75 Å are much longer than those of 0.74 and 1.2 Å in H₂ and LaH₁₀ at 100 GPa, respectively, indicating that H atoms are not bonded to each other. Except for GaTiH₆ and GaScH₆, the other five hydrides have no phase transition in the entire pressure range from 50 to 300 GPa. For GaTiH₆ and GaScH₆, the C2/m and Pmma structures, respectively, were predicted to be stable at 50 GPa, and they do not present H₂ molecules in their structures (Fig. S1, supplementary material). Furthermore, AlScH₆ and AlYH₆ were predicted to have structures similar to Fm3̄m Sc(Y)H₃, where metal atoms form a face-centered cubic (fcc) lattice and H atoms are located at the tetrahedral and octahedral interstices of this lattice. As shown in Fig. S2 (supplementary material), Pmmn AlScH₆ can be seen as a 2 × 2 × 1 supercell of Fm3̄m Sc(Y)H₃ with two metal atoms arranged alternately in two directions. P4/mmm AlScH₆ is formed by replacing the two Sc atoms in Fm3̄m ScH₃ with Al atoms. I4/mmm AlYH₆ is similar to P4/mmm AlScH₆, but with slightly shifted H positions. The predicted structures of AlLaH₆, GaYH₆ and GaLaH₆ are also shown in Fig. S3 (supplementary material).

Bader charge analyses56 were performed on A15-type ternary hydrides at 100 GPa, as shown in Table S2 (supplementary material). The results demonstrate the transfer of electronic charges from metal to H atoms, suggesting an ionic nature of the bonding between them. Each H atom in AlMH₆ and GaMH₆ accepts ∼0.60–0.64 and 0.39–0.43e, respectively. In AlMH₆, Al atoms transfer around 2.31–2.71e to H atoms, surpassing the ∼1.31 to 1.54e transferred from the M atoms. In GaMH₆, the M atom loses a greater amount of charge (∼1.34 to 1.61e) compared with the Ga atom (∼0.94 to 1.00e), indicating a greater ionic nature of the M–H bond. The electrons acquired by the H₂ molecule occupy its antibonding orbitals, leading to an elongation of the H–H bond length and potentially even dissociation of the H₂ unit. In AlMH₆, the significant electron transfer from Al to H atoms compensates for the relatively smaller electrons transfer from the M atom. In contrast to GaH₃, the doping M atoms enable H atoms to gain more charge in GaMH₆ at the same pressure, thereby allowing these ternary hydrides to exhibit structures with atomic H at lower pressures. Additionally, the electron localization functions (ELFs) were calculated for these A15 ternary hydrides at 100 GPa with an isosurface value of 0.5, as depicted in Fig. S4 (supplementary material). The ELFs indicate that electrons are predominantly localized around the H atoms, confirming their role as electron acceptors. The ELF values below 0.5 between adjacent H atoms indicate the absence of H–H covalent bonds. The ELF values between the metal and H atoms approach zero, confirming their ionic bonding character, which is consistent with the aforementioned Bader charge analysis.

The thermodynamic stability of these ternary hydrides has been assessed by calculating their formation enthalpies relative to elemental solids and binary compounds.36,38–40,42–45 The relative enthalpy curves for AMH₆ are presented in Fig. 2 and Fig. S5 (supplementary material). The calculations consider the most stable configuration for each component, and the total energy of AH₃ and MH₃ is used as the reference energy. Within certain pressure ranges, AMH₆ exhibits lower formation enthalpy compared with possible decomposition pathways, indicating that AMH₆ is thermodynamically stable. For AlMH₆ (M = Ti, Zr, or Hf), the predicted A15-type phases remain stable within the pressure ranges of 78–165, 69–123, and 43–157 GPa, respectively. On accounting for zero-point energy (ZPE), the stable pressure range for AlTiH₆ shrinks to 92–113 GPa and the relative formation enthalpy reduces to only −10 meV/f.u. Conversely, with ZPE corrections, the stable pressure ranges for AlZrH₆ and AlHfH₆ expand to 40–150 and 30–181 GPa, respectively. Furthermore, the stability pressure thresholds for both hydrides are lower than the 73 GPa threshold for AlH₃36 [Fig. 1(c)]. Regarding GaMH₆ (M = Sc, Ti, Zr, or Hf), the predicted A15-type phases are stable above 126, 96, 85, and 95 GPa, respectively, and remain stable with increasing pressure. With ZPE taken into account, the minimal stable pressures change to 116, 95, 80, and 86 GPa, respectively, which are well below the stability threshold of 160 GPa for GaH₃ [Fig. 1(c)]. Therefore, it is anticipated that the experimental synthesis of A15-type GaMH₆ might be easier compared with that of GaH₃. Inspired by the successful synthesis of the equal-atomic (La,Y)H₄ and (La,Ce)H₉ alloys, the high-temperature and high-pressure reaction of AM alloys with NH₃BH₃ might be a promising route for the synthesis of the alloys AMH₆.18,19 Figure S5 (supplementary material) demonstrates that the Fm3̄m-like AlScH₆ is stable above 150 GPa and undergoes a transformation from the Pmmn phase to the P4/mmm phase at 318 GPa. The C2/m phase of AlYH₆ is stable above 45 GPa and transforms to the Fm3̄m-like I4/mmm phase at 115 GPa. AlLaH₆ is predicted to exhibit the P6₃/mmc and Cmcm structures below ∼200 GPa. For GaYH₆ and GaLaH₆, the low-symmetry P2₁2₁2 and C2/m structures are stable near 100 GPa and below 150 GPa, respectively. Additionally, we also performed structure predictions and first-principles calculations for hydrides with higher H content in the Al–Zr–H, Ga–Sc–H, Ga–Zr–H, and Ga–Hf–H systems at 200 and 300 GPa. As shown in Fig. S6 (supplementary material), these H-rich hydrides have higher formation enthalpies relative to AH₃+ BH₃ + H₂ or AMH₆ + H₂, indicating that they are unstable at the corresponding pressures.

To better understand why the A15-type AMH₆ structure is stable, we conducted an analysis of the influence of the relative internal energy (ΔU) and the product of pressure and volume (ΔPV) on the relative enthalpy (ΔH) under pressure. The findings for AlZrH₆ and AlHfH₆ are illustrated in Fig. S7 (supplementary material). As pressure increases, the ΔPV term also rises and eventually exhibits positive values. Conversely, the ΔU term exhibits the opposite trend, indicating that the bonding in both ternary hydrides contributes significantly to their stabilization. For the A15-type GaMH₆ (Fig. S8, supplementary material), the ΔPV contribution in all four hydrides is negative in comparison with the benchmark, indicating that the ΔPV term plays a dominant role in their thermodynamic stability. Meanwhile, the magnitude of ΔU decreases with increasing pressure, leading to an overall enhancement of the stability of these ternary hydrides. Furthermore, in GaScH₆, the ΔPV and ΔU terms exhibit contrasting trends when compared with ScH₃ + GaH₃. The negative ΔPV contribution counteracts the negative effect of the ΔU term, ultimately resulting in the stabilization of GaScH₆. Both the ΔPV and ΔU contribution in GaTiH₆, GaZrH₆, and GaHfH₆ have favorable effects on thermodynamic stability compared with their respective binary hydrides.

We also investigated the dynamical stability of the predicted A15-type AMH₆ compounds by calculating the phonon spectra using the supercell approach implemented in the PHONOPY code54 (Figs. S14 and S15, supplementary material). Within their respective thermodynamically stable pressure ranges, no imaginary frequencies were observed in the phonon spectra of these compounds, indicating their dynamical stability. Furthermore, we systematically explored the minimum pressure required for dynamical stability of these A15-type AMH₆. As pressure decreases, phonon softening begins to occur, eventually leading to instability with the appearance of imaginary frequencies at certain q-wave vectors. We plotted the evolution of the frequency with pressure at the q-wave vector where the largest imaginary frequency occurs. As depicted in Figs. S9 and S10 (supplementary material), A15-type GaScH₆ exhibited the highest critical pressure for dynamical stability, estimated to be around 97 GPa. GaTiH₆ followed, with a critical pressure of ∼28 GPa. AlTiH₆ and AlZrH₆ were found to be dynamically stable at lower pressures of about 18 and 13 GPa, respectively. Interestingly, the critical pressures for dynamical stability of AlHfH₆, GaZrH₆, and GaHfH₆ were considerably lower, at around 6, 5, and 6 GPa, respectively. Moreover, the threshold pressures for dynamical stability of the five ternary hydrides (GaTiH₆, AlTiH₆, AlHfH₆, GaZrH₆, and GaHfH₆) were much lower compared with those of AH₃ and MH₃ [Figs. S11 and S13 (supplementary material) and Fig. 1(d)]. This suggests that these ternary hydrides could potentially be recovered at lower pressures, provided that the barriers preventing them from decomposing are sufficiently high. Of the Fm3̄m-like ternary hydrides, Pmmn AlScH₆, P4/mmm AlScH₆, and I4/mmm AlYH₆ could maintain their dynamical stability to 65, 139, and 43 GPa, respectively (Figs. S12 and S16, supplementary material).

The electronic properties of the predicted A15-type AMH₆ compounds were investigated at different pressures, and the results are displayed in Fig. 3, as well as Figs. S17 and S18 (supplementary material). The calculated density of states (DOS) reveals the presence of electronic states at the Fermi energy level (E_f), indicating that these hydrides exhibit metallic behavior within the studied pressure range. The contributions of the metal and hydrogen atoms to the total DOS are also depicted in the figures. Figure 3(d) highlights a distinct difference between the DOS of GaScH₆ and the other six hydrides composed of Al/Ga and group IVB metals. In GaScH₆, the hydrogen atoms contribute significantly to the DOS at E_f, suggesting the potential for excellent superconducting properties. In the remaining six hydrides, the group IVB metals have one additional valence electron compared with Sc atoms, causing a shift of E_f to higher energies relative to GaScH₆. These six hydrides exhibit higher or comparable total DOS at E_f compared with GaScH₆. However, the DOS contributions from Al and Ga atoms are lower owing to significant charge transfer to the H atoms. In the case of the group IVB metals, the DOS at E_f is mainly contributed by d orbitals. Furthermore, the electronic band structures of AlHfH₆, GaScH₆, and GaZrH₆ were examined as examples at pressures of 10, 100, and 10 GPa, respectively. The band projections onto different elements are also displayed in the band structures. In these three hydrides, a band associated with Al/Ga atoms is observed to cross E_f steeply along the M–R–Γ direction. Additionally, in AlHfH₆ and GaZrH₆ there is an electron pocket at the M point and a flat band along the Γ–M direction, dominated by Hf and Zr atoms, located near E_f. In the case of GaScH₆, electron and hole pockets are observed along the X–M direction, along with a flat band along the Γ–M direction, which are attributed to H atoms. These localized electronic states contribute to a high DOS at the Fermi energy and play a role in electron–phonon interactions. Moreover, a comparison was made between the DOS of A15-type AlH₃, AlTiH₆, AlZrH₆, and AlHfH₆ at 100 GPa. As shown in Fig. S17 (supplementary material), the DOS at E_f for AlTiH₆, AlZrH₆, and AlHfH₆ are 0.030, 0.018, and 0.018 states eV⁻¹ Å⁻³ respectively, which are significantly higher than that of AlH₃ (0.008 states eV⁻¹ Å⁻³). The doping of AlH₃ with Ti, Zr and Hf atoms elevated E_f, resulting in an increased DOS at E_f. Ternary hydrides exhibit a more pronounced metallic character compared with AlH₃, potentially enhancing their superconducting properties.

After determining the stability and metallicity of A15-type ternary hydrides, we conducted EPC calculations to investigate their superconductivity. The calculated phonon dispersion curves, projected phonon density of states (PDOS), Eliashberg phonon spectral function α²F(ω)/ω, and its integral λ(ω) for Pm3̄ AlHfH₆, GaScH₆ and GaZrH₆ at pressures of 10, 100, and 10 GPa are displayed in Fig. 4. From the PDOS, it is evident that the high-frequency and low-frequency phonon modes are associated with the vibrations of H and metal atoms, respectively. The right-hand panels of Fig. 4 illustrate that the peaks of α²F(ω)/ω for AlHfH₆, GaScH₆, and GaZrH₆ are predominantly distributed below 15, 20, and 15 THz, respectively, with the corresponding λ(ω) increasing rapidly. The EPC strength on the different phonon modes is also depicted on the phonon dispersions. As a result, the significant contribution to the EPC comes from the soft modes associated with H-atom vibrations and the modes dominated by metal atoms. This leads to a total EPC parameter λ of 2.18, 1.56, and 1.80 for AlHfH₆, GaScH₆, and GaZrH₆ at pressures of 10, 100, and 10 GPa, respectively.

The superconducting critical temperatures of the predicted A15-type ternary hydrides were calculated using the Allen–Dynes modified McMillan equation with a Coulomb pseudopotential parameter μ* ranging from 0.1 to 0.13.57 The calculated T_c values, along with the EPC parameter λ and the phonon frequency logarithmic average ω_log are presented in Fig. 5 and Table I. For AlTiH₆, the calculated EPC parameter λ is 1.15 and the phonon frequency logarithmic average ω_log is 423 K. This results in a T_c of 32–36 K at 20 GPa. AlZrH₆ and AlHfH₆ exhibit stronger EPC interactions, with λ of 1.72 and 2.18, and the T_c values are estimated to be 59–64 and 57–60 K at 20 and 10 GPa, respectively. Considering that the λ values are higher than 1.5, the calculated T_c values are further rectified with strong-coupling (f₁) and shape (f₂) corrections and improved to 68–75 and 70–76 K, respectively. The T_c values obtained for AlMH₆ are indeed higher than those of AlH₃ and the corresponding MH₃ compounds.37,39,40,45 Similarly, GaScH₆ exhibits the highest T_c among the studied hydrides, reaching a range of 79–88 K at a higher pressure of 100 GPa, However, its T_c is slightly lower than that of GaH₃, which has a T_c of 102 K at 120 GPa. GaTiH₆ has a lower T_c range of 34–39 K at 35 GPa compared with the T_c of 63–70 K for GaZrH₆ and 45–49 K for GaHfH₆ at 10 GPa.

The evolution of T_c with pressure was also investigated for Pm3̄ AMH₆. For AlZrH₆, the calculated EPC parameter λ decreased from 1.72 at 20 GPa to 1.18 at 30 GPa, while ω_log increased from 494 to 667 K, resulting in a slight decrease in T_c with μ* of 0.1 from 64 to 59 K (Allen–Dynes modified McMillan equation). Similarly, as the pressure increased from 10 to 20 GPa, the λ of AlHfH₆ decreased from 2.18 to 1.28 and the ω_log increased from 397 to 596 K. As a result, the calculated T_c with μ* of 0.1 decreased slightly from 60 to 58 K. The increase in pressure leads to a stiffening of the phonon modes, which results in a decrease in the EPC parameter and an increase in ω_log. This compensating effect prevents a significant reduction in T_c. In addition, the T_c values of GaMH₆ compounds decrease with increasing pressure, owing to the competition between the decreasing EPC parameter λ and the increasing phonon frequency logarithmic average ω_log. Of the Fm3̄m-like ternary hydrides, Pmmn AlScH₆, P4/mmm AlScH₆, and I4/mmm AlYH₆ are found to be metallic (Fig. S19, supplementary material), and subsequent EPC calculations suggest that they exhibit superconductivity. The T_c values of these hydrides are estimated to be 42, 32, and 52 K at pressures of 80, 200, and 60 GPa, respectively (Table S3, supplementary material).

We have carried out crystal structural predictions and first-principles calculations on alloy hydrides with the AMH₆ composition under pressure. Seven ternary hydrides, AlMH₆ (M = Ti, Zr, or Hf) and GaMH₆ (M = Sc, Ti, Zr, or Hf) were predicted to adopt the A15-type structure, and the calculated enthalpy curves indicated that the A15-type AlMH₆ are stable within specific pressure ranges of 92–113, 40–150 and 30–181, respectively. The pressures for onset of stability for AlZrH₆ and AlHfH₆ are lower than the stability threshold of 73 GPa for AlH₃. Phonon calculations demonstrated that A15-type AlTiH₆, AlZrH₆, and AlHfH₆ can be dynamically stable at relatively low pressures, such as 18, 13, and 6 GPa, respectively. Additionally, the addition of Ti, Zr, and Hf into AlH₃ influenced the position of the Fermi level, resulting in improved metallicity compared with AlH₃. Consequently, the ternary hydrides exhibited higher T_c of 36, 75, and 76 K at 20, 20, and 10 GPa, respectively. The A15-type GaMH₆ hydrides (GaScH₆, GaTiH₆, GaZrH₆, and GaHfH₆) were found to have minimum pressure thresholds for thermodynamic stability at 116, 95, 80, and 86 GPa, respectively, which are significantly lower than the stability threshold of 160 GPa for GaH₃. Additionally, dynamical stability calculations indicated their potential retention down to lower pressures of 97, 28, 5, and 6 GPa, respectively. EPC calculations revealed that A15-type GaMH₆ hydrides are superconducting, with T_c of 88, 39, 70, and 49 K at 100, 35, 10, and 10 GPa, respectively. AlScH₆ and AlYH₆ were predicted to be stable in Fm3̄m Sc(Y)H₃-like structures under pressure. The estimated T_c values for Pmmn, P4/mmm AlScH₆, and I4/mmm AlYH₆ were 42, 32, and 52 K at 80, 200, and 60 GPa, respectively. These findings suggest that alloying holds promise as a route to lower the stabilization pressure of hydrides, enabling the exploration of high-T_c hydride superconductors that can be stable at lower or ambient pressures.

Acknowledgment. This work was supported by the Natural Science Foundation of China (Grant Nos. 52022089, 52372261, 52288102, and 11964026), the National Key R&D Program of China (Grant No. 2022YFA1402300), the Natural Science Foundation of Hebei Province (Grant No. E2022203109), the Doctoral Fund of Henan University of Technology (Grant No. 31401579). P.L. thanks the Science and Technology Leading Talents and Innovation Team Building Projects of the Inner Mongolia Autonomous Region (Grant No. GXKY22060). A.B. acknowledges financial support from the Spanish Ministry of Science and Innovation (Grant No. FIS2019-105488GB-I00) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (Grant No. IT1707-22). E.Z. acknowledges the National Science Foundation (Grant No. DMR-2136038) for financial support.