Mixed Valence Europium Nitridosilicate Eu_2SiN_3

摘要

The mixed valence europium nitridosilicate Eu_2SiN_3 has been synthesized at 900℃ 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). Eu_2SiN_3 is a chain-type silicate comprising one-dimensional infinite nonbranched zweier chains of corner-sharing SiN_4 tetrahedra running parallel [100] with a maximum stretching factor f_s = 1.0. The compound is isostructural with Ca_2PN_3 and Rb_2TiO_3, 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 Eu~(2+) and Eu~(3+), respectively. ~(151)Eu Moessbauer spectroscopy of Eu_2SiN_3 differentiates unequivocally these two europium atoms and confirms their equiatomic multiplicity, showing static mixed valence with a constant ratio of the Eu~(2+) and Eu~(3+) signals over the whole temperature range. The Eu~(2+) 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 μ_B/f.u. and a small contribution of Eu~(3+), in accordance with Eu~(2+) and Eu~(3+) in equiatomic ratio. Ferromagnetic ordering at unusually high temperature is detected at T_c = 24 K. DFT calculations of Eu_2SiN_3 reveal a band gap of ～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 Eu~(2+) and Eu~(3+).