Stable multifunctional aluminum phosphides at high pressures

Phosphides have been used in a wide range of applications due to their excellent optical, mechanical, and catalytic properties. Using an effective unbiased structure searching method combined with first-principles calculations, the phase diagram and physical and chemical properties of aluminum phosphides have been determined at high pressures. The results reveal that the unconventional stoichiometries of Al₂P, AlP₂, and AlP₃ remain stable above 66, 91, and 116 GPa, respectively. Interestingly, the analysis of the phonon spectrum suggests that AlP₂ with P2₁ symmetry can be dynamically stable at atmospheric pressure. In addition, the band gap of 1.51 eV at the HSE06 level and the estimated Vickers hardness of ∼10.54 GPa make P2₁-AlP₂ a hard photoelectric material. Moreover, our electronic properties show that AlP₃ with Immm symmetry and AlP₂ with I4/mmm structure are metallic at high pressures and further electron-phonon coupling calculations indicate Immm-AlP₃ and I4/mmm-AlP₂ are superconductors with estimated T_c values of 3.9 K at 150 GPa and 10.2 K at 100 GPa, respectively. Our work provides significant inputs toward understanding novel chemical bonding in aluminum phosphides and gives a direction for the experimental synthesis of multifunctional materials at high pressures.