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 degreesC. The white insulating material crystallizes in the GdFeO3 distorted perovskite structure type with orthorhombic space group Pnma and with a = 5.875 25(5) Angstrom, b = 8.250 31(7) Angstrom, c = 5.810 08(5) Angstrom, and Z = 4, as determined by neutron powder diffraction. A detailed structural analysis of the isostructural NdTiO2N is also reported: Pnma, a = 5.5492(l) Angstrom, b = 7.8017(1) Angstrom, c = 5.529 01(9) Angstrom, and Z = 4. LaTiO2N crystallizes in a different distorted perovskite structure, which is triclinic and consequently has two distinct Ti(O4N2) octahedra (LaTiO2N: I (1) over bar alpha = 5.6097(1)Angstrom, b = 7.8719(2) Angstrom, c = 5.5752(l) Angstrom, alpha = 90.199(2)degrees, beta = 90.154-(3)degrees, gamma = 89.988(8)degrees, 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.