Optical and magnetic properties of EuSi u2 O u2 N u2
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K.V. Ramanujachary Department of Chemistry and Biochemistry, Rowan University, Glassboro, New Jersey 08028-1701
S.E. Lofland Department of Physics and Astronomy, Rowan University, Glassboro, New Jersey 08028-1701
G. de With and H.T. Hintzena) Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands (Received 18 May 2005; accepted 12 September 2005)
A new europium-silicon-oxynitride compound EuSi2O2N2 was obtained by a reaction of Eu2O3, SiO2, and ␣–Si3N4 at 1300 °C under a nitrogen atmosphere. EuSi2O2N2 is indexed on a monoclinic unit cell with a ⳱ 13.151(5) Å, b ⳱17.311(5) Å, c ⳱ 7.956(2) Å,  ⳱ 104.12(4)°, and V ⳱ 1756.56 Å3. EuSi2O2N2 shows a highly pure yellow color associated with a very steep drop in the reflection spectrum with an optical absorption edge at about 512 nm (2.43 eV). On the other hand, EuSi2O2N2 can be efficiently excited in the visible range 370–485 nm and shows a broad band emission peaking at about 568 nm corresponding to the Eu2+ 4f 65d1 → 4f 7 transition. EuSi2O2N2 shows paramagnetic Curie behavior with an experimental magnetic moment of 7.89(3) B in accordance with 7 unpaired spins of Eu2+. Additionally, no magnetic ordering can be observed down to 5 K. The divalent nature of the Eu ions in EuSi2O2N2 is evident from both luminescence and magnetic properties.
I. INTRODUCTION
The valence state of europium has a great influence on both luminescence and magnetic properties.1–3 Europium can exist in trivalent and/or divalent state in different host lattices strongly depending on the anion type and coordination; the luminescence of europium displays sharpline emission and broad band emission for Eu3+ and Eu2+, respectively.1 On the other hand, for magnetic properties, Eu3+ usually shows a nonlinear −1(T) characteristic with an effective magnetic moment of eff ⳱ 3.4 to 3.51 while Eu2+ displays a linear −1(T) behavior with eff ≈ 7.94.2 Apart from using a reducing atmosphere, the divalent europium also can be obtained in reductive lattices, such as silicate, silicon-nitride, sulfide, silicon-sulfide, boron-oxide, boron-nitride, boride, and cynamide.4–12 Because Eu3+ is smaller than Eu2+, a large divalent lattice site also has a high capability to reduce
Address all correspondence to these authors. a) e-mail: [email protected] DOI: 10.1557/JMR.2006.0057 396
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 2, Feb 2006 Downloaded: 14 Mar 2015
the valence of europium from Eu3+ into Eu2+.13 In addition, Eu2+ ions can also exist in several glasses showing interesting luminescence properties.14–17 It is wellknown that Eu2+ and Sr2+ have about the same ionic radius and therefore exhibit very similar structural characteristics, as reflected in a number of isotypic Eu2+- and Sr2+-compounds, such as Eu2Si5N86 and Sr2Si5N8,7 EuReSi4N76,8 and SrReSi4N7 (Re ⳱ Yb, Y).8,9 To our knowledge, no europium-silicon-oxynitride compounds have been reported up to date. In contrast, a SrSi2O2N2 compound has already been reported in the
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