Phase Relationships in the Si 3 N 4 –SiO 2 –Lu 2 O 3 System

Phase relationships in the Si 3 N 4 –SiO 2 –Lu 2 O 3 system were investigated at 1850°C in 1 MPa N 2 . Only J‐phase, Lu 4 Si 2 O 7 N 2 (monoclinic, space group P 2 1 / 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 Si 3 N 4 –SiO 2 –Lu 2 O 3 system. The Si 3 N 4 /Lu 2 O 3 ratio is 1, corresponding to the M‐phase composition, resulted in a mixture of Lu–J‐phase, β‐Si 3 N 4 , and a new phase of Lu 3 Si 5 ON 9 , having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si 3 N 4 –LuN–Lu 2 O 3 system. The phase diagram suggests that Lu 4 Si 2 O 7 N 2 is an excellent grain‐boundary phase of silicon nitride ceramics for high‐temperature applications.