Oxynitride Perovskites: Synthesis and Structures of LaZrO₂N, NdTiO₂N, and LaTiO₂N and Comparison with Oxide Perovskites

A new oxynitride, LaZrO 2 N, has been synthesized by the reaction of a highly reactive X-ray amorphous La 2 Zr 2 O 7 precursor with flowing ammonia at between 900 and 1000 °C. The white insulating material crystallizes in the GdFeO 3 distorted perovskite structure type with orthorhombic space group Pnma and with a = 5.875 25(5) Å, b = 8.250 31(7) Å, c = 5.810 08(5) Å, and Z = 4, as determined by neutron powder diffraction. A detailed structural analysis of the isostructural NdTiO 2 N is also reported: Pnma, a = 5.5492(1) Å, b = 7.8017(1) Å, c = 5.529 01(9) Å, and Z = 4. LaTiO 2 N crystallizes in a different distorted perovskite structure, which is triclinic, and consequently has two distinct Ti(O 4 N 2 ) octahedra (LaTiO 2 N: I 1̄, a = 5.6097(1) Å, b = 7.8719(2) Å, c = 5.5752(1) Å, α = 90.199(2)°, β = 90.154(3)°, γ = 89.988(8)°, Z = 4). Oxide and nitride ions are completely disordered in all three phases. The M(O,N) 6 octahedra (M = Ti, Zr) in these phases and a number of perovskite titanates and zirconates become more regular as the tilting distortion of the octahedral framework becomes more pronounced, as a result of quenching of the second-order Jahn−Teller distortion of the octahedra.