Anionic and Magnetic Ordering in Rare Earth Tantalum Oxynitrides with an n = 1 Ruddlesden-Popper Structure

The new compounds R₂TaO_4-N with R = La, Ce, Nd, and Eu and 1.20 ≤ x ≤ 2.81 have been obtained by a solid-state reaction between metal nitrides and oxides or oxynitrides under N₂ gas at temperatures between 1200 and 1700 °C. They are the first examples of rare earth transition metal oxynitrides with an n = 1 Ruddlesden-Popper structure and show different anion stoichiometries, crystal structures, and magnetic properties. Synchrotron X-ray powder diffraction and electron diffraction indicate that the lanthanum, cerium, and neodymium compounds crystallize in the orthorhombic space group Pccn, with cell parameters a = 5.72949(2), b = 5.73055(5), and c = 12.77917(6) Å for La₂TaO_1.31N_2.69, a = 5.70500(5), b = 5.71182(4), and c = 12.61280(7) Å for Ce₂TaO_1.19N_2.81, and a = 5.70466(3), b = 5.70476(5), and c = 12.32365(5) Å for Nd₂TaO_1.46N_2.54. In contrast, Eu₂TaO_2.80N_1.20 shows a tetragonal I4₁/acd superstructure doubling the c axis, with parameters a = 5.71867(2) and c = 25.00092(19) Å. Refinement of neutron powder diffraction data of Ce₂TaO_1.19N_2.81 indicated the nitrogen order in the two equatorial positions of the tantalum octahedron, with refined N/O occupancies of 0.930(7)/0.070 and 0.876(13)/0.124, and the axial position is occupied by 50% of each anion. This anion ordering agrees with the distribution predicted by Pauling's second crystal rule. Magnetization measurements show that the cerium and europium compounds are ordered magnetically at low temperatures, while the neodymium compound remains paramagnetic down to 2 K, as a consequence of suppression of the effective magnetic moment of the latter when reducing the temperature.