High-pressure synthesis and crystal structures of molybdenum nitride Mo 3 N 5 with anisotropic compressibility by a nitrogen dimer.

A novel high-pressure molybdenum nitride phase, Mo 3 N 5 , was synthesized at above 45 GPa via a nitridation reaction of molybdenum with nitrogen under high pressure using a laser-heated diamond anvil cell. Mo 3 N 5 , having an N-N dimer and 7-coordinated Mo sites, crystallizes in an orthorhombic structure with a space group of Cmcm (No. 63) without other prototype structures. The refined lattice parameters for Mo 3 N 5 were a = 2.86201(2) Å, b = 7.07401(6) Å, and c = 14.59687(13) Å. The DFT enthalpy calculation suggested that Mo 3 N 5 is a high-pressure stable phase, which is also consistent with an increasing coordination number compared to ambient- and low-pressure phases. The zero-pressure bulk modulus of Mo 3 N 5 was determined to be K 0 = 328(4) GPa with K ' 0 = 10.1(6) by the fitting for the compression curve, which is almost consistent with the theoretical E - V curve and elastic stiffness constants. The compressibility of Mo 3 N 5 has axial anisotropy corresponding to the N-N dimer direction in the crystal structure.