Crystal structure and optical properties of oxynitride rare-earth tantalates RTa–(O, N) (R = Nd, Gd, Y)

X-ray absorption and photoluminescence were used to investigate the structure of rare-earth tantalum oxynitrides RTa–(O, N) (R = Nd, Gd, Y). Owing to the size of the rare-earth element, the crystal structure tends towards the pyrochlore or defect fluorite-type structure. EXAFS suggested neodymium and yttrium atoms are coordinated either by 6 + 2 or 6 oxygen/nitrogen atoms in the Nd or Y respective oxynitrides although the coordination number of tantalum is six in both compounds. Photoluminescence for 5 at.% doped europium showed the spectra compatible with the point symmetry C3v lower than Oh in fluorite and D3d in pyrochlore structure type for both the Gd and Y tantalum oxynitrides. These measurements supported that their structure types were basically pyrochlore for Nd and Gd tantalum oxynitrides and defect fluorite for Y tantalum oxynitride but they are highly defective.