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Hydrogen Induced Structure and Property Changes in Eu3Si4
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
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Abstract [en]

Hydrides Eu3Si4H2+x were obtained by exposing the Zintl phase Eu3Si4 to a hydrogen atmosphere at a pressure of 30 bar and temperatures from 25 to 300 °C. Structural analysis using powder X-ray diffraction (PXRD) data suggested that hydrogenations in a temperature range 25 – 200 ºC afford a uniform hydride phase with an orthorhombic structure (Immm, a ≈ 4.40 Å, b ≈ 3.97 Å, c ≈ 19.8 Å), whereas at 300 ºC mixtures of two orthorhombic phases with c ≈ 19.86 and ≈19.58 Å were obtained. The assignment of a composition Eu3Si4H2+x is based on first principles DFT calculations, which indicated a distinct crystallographic site for H to be occupied in the Eu3Si4 structure. In this position, H atoms are coordinated in a tetrahedral fashion by Eu atoms. The resulting hydride Eu3Si4H2 is stable by -0.46 eV/H atom with respect to Eu3Si4 and gaseous H2. Deviations between the lattice parameters of the DFT optimized Eu3Si4H2 structure and the ones extracted from PXRD patterns point to the presence of additional H in interstitials also involving Si atoms. Subsequent DFT modeling of compositions Eu3Si4H3 and Eu3Si4H4 showed considerably better agreement to the experimental unit cell volumes. However, the ordered monoclinic model structures do not provide a good match to the experimental, orthorhombic, PXRD patterns. It was then concluded that the hydrides of Eu3Si4 have a composition Eu3Si4H2+x (x < 2) and are disordered with respect to H in Si2Eu3 interstitials. Hydrides Eu3Si4H2+x decompose at temperatures above 300 °C in a dynamic vacuum into unidentified products. Thus the hydrogenation of Eu3Si4H2+x is not reversible. From magnetic measurements the Curie-Weiss constant and effective magnetic moment of Eu3Si4H2+x were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of ~8 mB which is typical for compounds containing Eu2+ 4f7 ions.  

Keyword [en]
Zintl phases. metal hydrides
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-141585OAI: oai:DiVA.org:su-141585DiVA: diva2:1087585
Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2017-04-27Bibliographically approved
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Nedumkandathil, RejiHäussermann, Ulrich
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