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

RWO3N 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) إ, c=11.5893(8) إ (NdWO3N) and a=5.299(3) إ, c=11.631(9) إ (PrWO3N). Neutron diffraction experiments performed on the Nd-phase did not evidence any oxygen/nitrogen order within the scheelite anionic subnetwork (space group: I41/a). (Nd,A)W(O,N)4 (A=Ca, Sr) solid solution domains were identified between NdWO3N and the AWO4 oxide composition.