Title: Structure-dependent vibrational dynamics of Mg(BH ₄ ) ₂ polymorphs probed with neutron vibrational spectroscopy and first-principles calculations

Abstract

We investigated the structure-dependent vibrational properties of different Mg(BH₄)₂ polymorphs (α, β, γ, and δ phases) with a combination of neutron vibrational spectroscopy (NVS) measurements and density functional theory (DFT) calculations, with emphasis placed on the effects of the local structure and orientation of the BH₄^- anions. DFT simulations closely match the neutron vibrational spectra. The main bands in the low-energy region (20–80 meV) are associated with the BH4^- librational modes. The features in the intermediate energy region (80–120 meV) are attributed to overtones and combination bands arising from the lower-energy modes. The features in the high-energy region (120–200 meV) correspond to the BH₄^- symmetric and asymmetric bending vibrations, of which four peaks located at 140, 142, 160, and 172 meV are especially intense. There are noticeable intensity distribution variations in the vibrational bands for different polymorphs. We can explain these differences using the spatial distribution of BH₄^- anions within various structures. An example of the possible identification of products after the hydrogenation of MgB₂, using NVS measurements, is presented. Our results provide fundamental insights of benefit to researchers currently studying these promising hydrogen-storage materials.

Authors:

Dimitrievska, Mirjana ^[1]; White, James L. ^[2]; Zhou, Wei ^[3]; Stavila, Vitalie ^[2]; Klebanoff, Leonard E. ^[2]; Udovic, Terrence J. ^[3]

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+ Show Author Affiliations

1. National Renewable Energy Lab. (NREL), Golden, CO (United States); National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States). NIST Center for Neutron Research
2. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
3. National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States). NIST Center for Neutron Research

Publication Date:

Fri Aug 19 00:00:00 EDT 2016

Research Org.:

Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

Sponsoring Org.:

USDOE Office of Energy Efficiency and Renewable Energy (EERE), Sustainable Transportation Office. Hydrogen Fuel Cell Technologies Office; USDOE National Nuclear Security Administration (NNSA)

OSTI Identifier:

1338381

Report Number(s):

SAND2016-12365J
Journal ID: ISSN 1463-9076; PPCPFQ; 649735; TRN: US1701710

Grant/Contract Number:  

AC04-94AL85000; AC36-08GO28308

Resource Type:

Accepted Manuscript

Journal Name:

Physical Chemistry Chemical Physics. PCCP

Additional Journal Information:

Journal Volume: 18; Journal Issue: 36; Journal ID: ISSN 1463-9076

Publisher:

Royal Society of Chemistry

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY