High-Pressure Synthesis of a High-Pressure Phase of MnN Having NiAs-Type Structure.

A novel high-pressure phase of manganese mononitride, NiAs-type MnN, was successfully synthesized through a pressure-induced phase transition from a tetragonal distorted NaCl-type MnN at pressures above approximately 55 GPa. High-pressure experiments, including starting material preparation, were conducted using a laser-heated diamond anvil cell. This result is the first example of a nitride with a structural phase transition from the distorted NaCl-type to the NiAs-type structure. Upon decompression after the phase transition to NiAs-type structure, the NiAs-type MnN underwent a structural change to the distorted NaCl-type phase, indicating the phase transition was reversible. NiAs-type MnN has a higher density and bulk modulus in comparison to the distorted NaCl-type one. The phase transition pressure of MnN is lower than that of oxides, such as FeO and MnO, which show a structural phase transition from a NaCl-type to a NiAs-type structure. It is suggested that this is due to the lattice distortion caused by antiferromagnetic ordering.

High-pressure synthesis and crystal structures of molybdenum nitride Mo3N5 with anisotropic compressibility by a nitrogen dimer.

A novel high-pressure molybdenum nitride phase, Mo3N5, was synthesized at above 45 GPa via a nitridation reaction of molybdenum with nitrogen under high pressure using a laser-heated diamond anvil cell. Mo3N5, 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 Mo3N5 were a = 2.86201(2) Å, b = 7.07401(6) Å, and c = 14.59687(13) Å. The DFT enthalpy calculation suggested that Mo3N5 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 Mo3N5 was determined to be K0 = 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 Mo3N5 has axial anisotropy corresponding to the N-N dimer direction in the crystal structure.

High pressure synthesis and the valence state of vanadium ions for the novel transition metal pernitride, CuAl2-type VN2.

A novel transition metal pernitride, CuAl2-type VN2, has been synthesized at a pressure above 73.3 GPa. The bulk modulus has been determined to be K0 = 347(12) GPa. By hard X-ray absorption spectrum measurements of VN2, the valence state of transition metal ions in pernitrides has been for the first time experimentally reported.

Thermal Expansion of Incompressible U2S3-Type Nb2N3 Synthesized in a Diamond Anvil Cell.

A novel niobium nitride, U2S3-type Nb2N3, has been successfully synthesized by nitridation of δ-NbN above approximately 30 GPa in a laser-heated diamond anvil cell. Nb2N3 crystallizes in the same orthorhombic structure (space group: Pnma) as η-Ta2N3. Nb2N3 consists of regular-shaped polyhedra, and the bulk modulus has been determined to K0 = 300(2) GPa. The low-temperature X-ray diffraction measurements have been successfully conducted for the tiny novel Nb2N3 between 297.7(5) and 106.3(3) K under ambient pressure. Nb2N3 shows no structural phase transition down to 106.3(3) K, and investigation of the linear thermal expansion coefficients yields αa = 3.36(9) × 10-6 K-1, αb = 5.39(10) × 10-6 K-1, αc = 6.77(15) × 10-6 K-1, respectively. Our study reveals that the incompressible novel nitride shows low thermal expansion behavior, which offers new insights for the development of functional nitrides and their crystal chemistry.