Thermal Ammonolysis Study of the Rare-Earth Tantalates RTaO₄

Reaction between rare-earth tantalates RTaO 4 and ammonia flow at 900−950 °C forms oxynitrides belonging to different structure types, perovskites RTaON 2, pyrochlores R 2 Ta 2 O 5 N 2, and defect fluorites RTa(O,N,□) 4, depending on the size of the R element. The nature of the oxide precursor is a crucial parameter affecting the ammonolysis reaction. A comparative study has been carried out between oxide powders prepared by a ceramic route and a chimie douce process. The pyrochlore structure is restricted to large rare-earths (R = Nd→Gd). For smaller R elements, thermal ammonolysis of reactive precursors elaborated from the citrate combustion route results in a fluorite-type oxynitride solid solution. X-ray and neutron diffraction studies evidence a totally disordered cubic fluorite unit cell, which, however, appears to be not suitable for the structure refinement, thus revealing a more complex atom arrangement.