Na₁.₅₁₅EuGeS₄, a Three-Dimensional Crystalline Assembly of Empty Nanotubules Constructed with Europium(II/III) Mixed Valence Ions
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A new compound, Na 1.515EuGeS 4, has been synthesized at 750 °C from a reaction of elemental europium, germanium, and sulfur and Na 2S. 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 1.515EuGeS 4, which is isostructural with Na 2EuSiSe 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 1.515EuGeS 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 MT 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 MT 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.