Synthesis, structure and magnetic properties of R-W-O-N (R=Nd and Eu) oxynitrides

Neodymium and europium tungsten oxynitrides have been synthesized by the nitridation of corresponding R2W2O9 precursor oxides, in ammonia flow at 1173K during 24h. The obtained polycrystalline neodymium oxynitride phase, with NdWO3.05N0.95 composition, crystallizes with the tetragonal symmetry of the scheelite-type structure, space group S.G. I41/a (no. 88). The analogous europium derivative, with formula EuWO1.58N1.42, presents the cubic perovskite-type structure, S.G. Pm3-bar m (no. 221). Unit-cell parameters, a=5.255(1)A, c=11.399(3)A, and a=3.976(3)A, have been established from Rietveld refinements of collected X-ray powder diffraction patterns for the Nd and Eu- oxynitrides, respectively. Magnetic susceptibility measurements show that NdWO3.05N0.95 behaves as paramagnetic in a wide range of temperature T ∼50-300K. The downwards deviation from the Curie-Weiss law below 40K reflects the splitting of the 4I9/2 ground state of Nd3+ experienced under the influence of a S4 crystal field CF potential, as the successful reproduction of the magnetic susceptibility χ-1m vs. T, using semi-empirical structure-derived CF parameters, indicates. EuWO1.58N1.42 is paramagnetic down to 20K, and the measured effective magnetic moment 8.01μB is indicative of the presence of Eu2+ in this oxynitride. The observed sudden jump in the magnetic susceptibility at 20K and the value of 6μB for the saturation moment is attributed to the onset of ferrimagnetic interactions in which the Eu2+ and W5+ sublattices appear to be involved