Synthesis, crystal structure, and magnetic properties of pyrochlore-type Eu₂Ta₂(O,N)_7+δ

Pyrochlore-type Eu₂Ta₂(O,N)_7+δ phases were prepared by reaction of ammonia with an amorphous europium tantalum oxide precursor. ¹⁵¹Eu Moessbauer and EPR spectroscopy as well as magnetic susceptibility measurements point to the presence of exclusively Eu³⁺. For phase-pure samples (X-ray powder diffraction), the nitrogen content varies between 1.0 and 1.8 wt %, leading to compositions in the range Eu₂Ta₂O_7.1N_0.6 - Eu₂Ta₂O_6.5N_1.0. Pyrochlore-type phases are structurally derived from the fluorite type with 1/8 of the anions missing, resulting in an ideal composition A₂B₂X₇. In Eu₂Ta₂(O,N)_7+δ the excess anions partly occupy these vacancies. The prepared phases are colorless with a direct optical bandgap of 4.3 eV and they show the typical Van Vleck paramagnetic behavior known for trivalent Eu atoms. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)