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

A new oxynitride, LaZrO2N, has been synthesized by the reaction of a highly reactive X-ray amorphous La2Zr2O7 precursor with flowing ammonia at between 900 and 1000 °C. The white insulating material crystallizes in the GdFeO3 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 NdTiO2N is also reported: Pnma, a = 5.5492(1) Å, b = 7.8017(1) Å, c = 5.529 01(9) Å, and Z = 4. LaTiO2N crystallizes in a different distorted perovskite structure, which is triclinic, and consequently has two distinct Ti(O4N2) octahedra (LaTiO2N: I1̄, 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.