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

Abstract

The new compounds R 2 TaO 4−x N x with R = La, Ce, Nd, and Eu and 1.20 ≤ x ≤ 2.81 have been obtained by a solidstate reaction between metal nitrides and oxides or oxynitrides under N 2 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 La 2 TaO 1.31 N 2.69 , a = 5.70500(5), b = 5.71182(4), and c = 12.61280(7) Å for Ce 2 TaO 1.19 N 2.81 , and a = 5.70466(3), b = 5.70476(5), and c = 12.32365(5) Å for Nd 2 TaO 1.46 N 2.54 . In contrast, Eu 2 TaO 2.80 N 1.20 shows a tetragonal I4 1 /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 Ce 2 TaO 1.19 N 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.