A new transition metal diphosphide α-MoP ₂ synthesized by a high-temperature and high-pressure technique

Monoclinic α-MoP ₂, with the OsGe ₂-type structure (space group C2/ m, Z = 4) and lattice parameters a = 8.7248(11) Å, b = 3.2322(4) Å, c = 7.4724(9)Å, and β = 119.263°, was synthesized under a pressure of 4~GPa at a temperature between 1100 °C and 1200 °C. The structure of α-MoP ₂ and its relationship to other transition metal diphosphides are discussed. Surprisingly, the ambient pressure phase orthorhombic β-MoP ₂ (space group Cmc2 ₁) is denser in structure than α-MoP ₂. Room-temperature high-pressure x-ray diffraction studies exclude the possibility of phase transition from β-MoP ₂ to α-MoP ₂, suggesting that α-MoP ₂ is a stable phase at ambient conditions; this is also supported by the total energy and phonon calculations.