The synthesis of reactive nitridation precursors prepared using citrate complexation/calcination route has shown interest in the study of the system R2Ta-O-N. Novel fluorite-type phases have been evidenced in the oxide series R2TaO5.5 (R = Nd-Yb, Y) and oxynitride series R2.67Ta1.33(O,N,square)(8). Chemical and optical analyses were performed to characterize these phases, as well as structural analogies with R2WO6 and R2MoO5 oxide families. (c) 2007 Elsevier Ltd. All rights reserved.