Since the discovery of hydride superconductors, a significant challenge has been to reduce the pressure required for their stabilization. In this context, we propose that alloying could be an effective strategy to achieve this. We focus on a series of alloyed hydrides with the AMH₆ composition, which can be made via alloying A15 AH₃ (A = Al or Ga) with M (M = a group ⅢB or IVB metal), and study their behavior under pressure. Seven of them are predicted to maintain the A15-type structure, similar to AH₃ under pressure, providing a platform for studying the effects of alloying on the stability and superconductivity of AH₃. Among these, the A15-type phases of AlZrH₆ and AlHfH₆ are found to be thermodynamically stable in the pressure ranges of 40–150 and 30–181 GPa, respectively. Furthermore, they remain dynamically stable at even lower pressures, as low as 13 GPa for AlZrH₆ and 6 GPa for AlHfH₆. These pressures are significantly lower than that required for stabilizing A15 AlH₃. Additionally, the introduction of Zr or Hf increases the electronic density of states at the Fermi level compared with AlH₃. This enhancement leads to higher critical temperatures (T_c) of 75 and 76 K for AlZrH₆ and AlHfH₆ at 20 and 10 GPa, respectively. In the case of GaMH₆ alloys, where M represents Sc, Ti, Zr, or Hf, these metals reinforce the stability of the A15-type structure and reduce the lowest thermodynamically stable pressure for GaH₃ from 160 GPa to 116, 95, 80, and 85 GPa, respectively. Particularly noteworthy are the A15-type GaMH₆ alloys, which remain dynamically stable at low pressures of 97, 28, 5, and 6 GPa, simultaneously exhibiting high T_c of 88, 39, 70, and 49 K at 100, 35, 10, and 10 GPa, respectively. Overall, these findings enrich the family of A15-type superconductors and provide insights for the future exploration of high-temperature hydride superconductors that can be stabilized at lower pressures.

Superionic and electride behaviors in materials, which induce a variety of exotic physical properties of ions and electrons, are of great importance both in fundamental research and for practical applications. However, their coexistence in hot alkali-metal borides has not been observed. In this work, we apply first-principles structure search calculations to identify eight Na–B compounds with host–guest structures, which exhibit a wide range of building blocks and interesting properties linked to the Na/B composition. Among the known borides, Na-rich Na₉B stands out as the composition with the highest alkali-metal content, featuring vertex- and face-sharing BNa₁₆ polyhedra. Notably, it exhibits electride characteristics and transforms into a superionic electride at 200 GPa and 2000 K, displaying unusual Na atomic diffusion behavior attributed to the modulation of the interstitial anion electrons. It demonstrates semiconductor behavior in the solid state, and metallic properties associated with Na 3p/3s states in the superionic and liquid regions. On the other hand, B-rich NaB₇, consisting of a unique covalent B framework, is predicted to exhibit low-frequency phonon-mediated superconductivity with a T_c of 16.8 K at 55 GPa. Our work advances the understanding of the structures and properties of alkali-metal borides.