The structures of the Ce and La N-phases RE3Si8 – xAlxN11 – xO4 + x (x≈ 1.75 for RE = Ce, x ≈ 1.5 for RE = La), determined by single-crystal X-ray and time-of-flight neutron powder diffraction, respectively

Abstract

Single crystals of the sialon Ce N-phase Ce₃Si_8 − xAl_xN_11 − xO_4 + x (x ≈ 1.75) were obtained via a novel route, i.e. by first preparing a fully compacted body with phase constitutions far away from thermal dynamic equilibrium by means of a fast densification technique, spark plasma sintering, and then heat treating this compact in a sialon powder bed at 1600 °C in N₂ atmosphere for 24 h. The structure of the Ce N-phase was solved using MoKα single-crystal data and direct methods. It was refined in space group I2/a with a = 15.798(1), b = 4.8939(3), c = 17.990(1) Å, β = 114.816(4)°, V = 1262.4 ų, to a weighted R(F_obs ²) = 6.7% for 977 unique reflections. The isostructural La phase structure was refined with anisotropic thermal parameters for La and N/O atoms using time-of flight neutron powder diffraction data. The structure exhibits three structural kinds of N/O anions: 4/15 of these are N atoms which are triangularly coordinated by 3 Si/Al atoms, 2/15 are tetrahedron free apex O atoms which are in addition bonded to 3 Ce/La atoms while the remaining 9/15 are on sites that are statistically occupied by N and O atoms and which are bonded to 2 tetrahedral Si/Al atoms and 1–2 Ce/La atoms. Comparisons are made with a previous structural proposal deduced from solid state nuclear magnetic resonance spectra.