Synthesis, Crystal Structure, and Magnetic Properties of Pyrochlore‐Type Eu 2 Ta 2 (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 Mossbauer 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 Eu2Ta2O7.1N0.6 – Eu2Ta2O6.5N1.0. 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.