New scheelite-type oxynitrides in systems RWO3N?AWO4 (R = rare-earth element; A = Ca, Sr) from precursors obtained by the citrate route

Abstract

RWO 3 N oxynitrides were isolated as single phases for R = Nd and Pr, after thermal ammonolysis of reactive precursors prepared using the citrate complexation/calcination route. Both stoichiometric compounds crystallize with a scheelite-type tetragonal unit cell, with a = 5.2821(3) Å, c = 11.5893(8) Å (NdWO 3 N) and a = 5.299(3) Å, c = 11.631(9) Å (PrWO 3 N). Neutron diffraction experiments performed on the Nd-phase did not evidence any oxygen/nitrogen order within the scheelite anionic subnetwork (S.G. I4 1 /a). (Nd,A)W(O,N) 4 (A = Ca, Sr) solid solution domains were identified between NdWO 3 N and the AWO 4 oxide composition.