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 Al 2 P, AlP 2 , and AlP 3 remain stable above 66, 91, and 116 GPa, respectively. Interestingly, the analysis of the phonon spectrum suggests that AlP 2 with P 2 1 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 P 2 1 -AlP 2 a hard photoelectric material. Moreover, our electronic properties show that AlP 3 with Immm symmetry and AlP 2 with I 4/ mmm structure are metallic at high pressures and further electron-phonon coupling calculations indicate Immm -AlP 3 and I 4/ mmm -AlP 2 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.