Mixed Valence Europium Nitridosilicate Eu2SiN3

The mixed valence europium nitriclosilicate Eu2SiN3 has been synthesized at 900 degrees C in welded tantalum ampules starting from europium and silicon diimide Si(NH)(2) in a lithium flux. The structure of the black material has been determined by single-crystal X-ray diffraction analysis (Cmca (no. 64), a = 542.3(11) pm, b = 1061.0(2) pm, c = 1162.9(2) pm, Z = 8, 767 independent reflections, 37 parameters, R1 = 0.017, wR2 = 0.032). Eu2SiN3 is a chain-type silicate comprising one-dimensional infinite nonbranched zweier chains of corner-sharing SiN4 tetrahedra running parallel [100] with a maximum stretching factor f(s) = 1.0. The compound is isostructural with Ca2PN3 and Rb2TiO3, and it represents the first example of a nonbranched chain silicate in the class of nitridosilicates. There are two crystallographically distinct europium sites (at two different Wyckoff positions 8f) being occupied with Eu2+ and Eu3+, respectively. Eu-151 Mossbauer spectroscopy of Eu2SiN3 differentiates unequivocally these two europium atoms and confirms their equiatomic multiplicity, showing static mixed valence with a constant ratio of the Eu2+ and Eull signals over the whole temperature range. The Eu2+ site shows magnetic hyperfine field splitting at 4.2 K. Magnetic susceptibility measurements exhibit Curie-Weiss behavior above 24 K with an effective magnetic moment of 7.5 mu(B)/f.u. and a small contribution of Eu3+, in accordance with Eull and Eu3+ in equiatomic ratio. Ferromagnetic ordering at unusually high temperature is detected at T-C = 24 K. DFT calculations of Eu2SiN3 reveal a band gap of similar to 0.2 eV, which is in agreement with the black color of the compound. Both DFT calculations and lattice energetic calculations (MAPLE) corroborate the assignment of two crystallographically independent Eu sites to Eu2+ and Eu3+.