Refined Crystal Structure and Idealized Structure of Mixed-Valent Eu4F5(CN2)2: Transition Possible

The new europium fluoride carbodiimide Eu4F5(CN2)(2) was synthesized by solid state reaction from mixtures of EuF3 and Li-2(CN2) at 700 degrees C. The crystal structure as refined by single crystal X-ray diffraction (P (4) over bar2(1)c, no. 114, a = 16.053(1) angstrom, c = 6.5150(6) angstrom, Z = 8) reveals three crystallographically distinct [N=C=N](2-) ions in the structure of mixed-valent Eu4F5(CN2)(2). The presence of one Eu3+ and three Eu2+ per formula unit Eu4F5(CN2)(2) is confirmed by magnetic measurements and Eu-151-Mossbauer spectroscopy. The arrangement of Eu ions and gravity centers of [NCN](2-) ions in the structure of Eu4F5(CN2)(2) follow the motif formed by atoms in the CuAl2-type structure. A possible high-symmetry structure of Eu4F5-(CN2)(2) is discussed on the basis of a group-subgroup scheme.