Na1.515EuGeS4, A Three-Dimensional Crystalline Assembly of Empty Nanotubules Constructed with Europium(II/III) Mixed Valence Ions

Abstract

A new compound, Na(1.515)EuGeS(4), has been synthesized at 750 °C from a reaction of elemental europium, germanium, and sulfur and Na(2)S. The compound crystallizes in the trigonal system with Z = 18 and the R3c space group with a = 23.322(3) Å, c = 6.838(1) Å, and V = 3221.2(9) Å(3). Na(1.515)EuGeS(4), which is isostructural with Na(2)EuSiSe(4), contains quasi-infinite nanoscale (∞)[EuGeS(4)](2-) tubules that are held together by sodium cations through electrostatic interactions. The tubules consist of a complex network of monoface-capped EuS(7) trigonal prisms and GeS(4) tetrahedra. The most striking structural feature of Na(1.515)EuGeS(4) is the absence of sodium cations inside the tubules, an absence that is balanced by the presence of mixed valence europium(II/III) ions. This mixed valence is confirmed by europium-151 Mössbauer spectroscopy, which indicates discrete mixed-valence europium ions at least up to 295 K. The stoichiometry has been determined by a fit of χ(M)T measured between 20 and 300 K with a combination of europium(II) ions, with a Curie constant of 7.877 emu K/mol, and europium(III) ions whose contribution to χ(M)T has been fit by using the Van Vleck expression for its molar susceptibility. The best fit corresponds to 51.5% of europium(II), 48.5% of europium(III), a stoichiometry of Na(1.515(5))EuGeS(4), and a splitting, E, between the J = 0 and the first excited J = 1 state of europium(III) of 360(6) cm(-1). The field dependence of the 1.8 K magnetization is in perfect agreement with a S = 7/2 Brillouin function with g = 2.00 and yields a saturation magnetization of 7 Nβ at 5 T.