Phase relationships in the Si₃N₄-SiO₂-Lu₂O₃ system

Phase relationships in the Si₃N₄-SiO₂-Lu₂O₃ system were investigated at 1850°C in 1 MPa N₂. Only J-phase, Lu₄Si₂O₇N₂ (monoclinic, space group P2₁/c, a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β = 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si₃N₄-SiO₂-Lu₂O₃ system. The Si₃N₄/Lu₂O₃ ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu-J-phase, β-Si₃N₄, and a new phase of Lu₃Si₅ON₉, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.606222(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si₃N₄-LuN-Lu₂O₃ system. The phase diagram suggests that Lu₄Si₂O₇N₂ is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.