First-Principles Study on Structural, Mechanical, Anisotropic, Electronic and Thermal Properties of III-Phosphides: X P (X = Al, Ga, or In) in the P 6₄22 Phase

The structural, mechanical, electronic, and thermal properties, as well as the stability and elastic anisotropy, of XP (X = Al, Ga, or In) in the P6₄22 phase were studied via density functional theory (DFT) in this work. P6₄22-XP (X = Al, Ga, or In) are dynamically and thermodynamically stable via phonon spectra and enthalpy. At 0 GPa, P6₄22-XP (X = Al, Ga, or In) are more rigid than F 4 ¯ 3 m-XP (X = Al, Ga, or In), of which P6₄22-XP (X = Al or Ga) are brittle and P6₄22-InP is ductile. In the same plane (except for (001)-plane), P6₄22-AlP and P6₄22-InP exhibit the smallest and the largest anisotropy, respectively, and P6₄22-XP (X = Al, Ga, or In) is isotropic in the (001)-plane. In addition, Al, Ga, In, and P bonds bring different electrical properties: P6₄22-InP exhibits a direct band gap (0.42 eV) with potential application for an infrared detector, whereas P6₄22-XP (X = Al or Ga) exhibit indirect band gap (1.55 eV and 0.86 eV). At high temperature (approaching the melting point), the theoretical minimum thermal conductivities of P6₄22-XP (X = Al, Ga, or In) are AlP (1.338 W∙m^-1∙K^-1) > GaP (1.058 W∙m^-1∙K^-1) > InP (0.669 W∙m^-1∙K^-1), and are larger than those of F 4 ¯ 3 m-XP (X = Al, Ga, or In). Thus, P6₄22-XP (X = Al, Ga, or In) have high potential application at high temperature.