De-ice, ice baby: Under pressure

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  • Published: Sep 1, 2016
  • Author: David Bradley
  • Channels: Raman
thumbnail image: De-ice, ice baby: Under pressure

Defining de-icer

Atmospheric-pressure (left) and high-pressure (right) 2D-layered Lawrencite-type crystal structures of magnesium chloride. Blue and green spheres indicate magnesium cations (positively charged ions) and chlorine anions (negatively charged ions) respectively.

Raman spectroscopy and X-ray diffraction measurements have worked together with first-principle calculations to examine the high-pressure structural behaviour of magnesium chloride at a million atmospheres.

A team of scientists from the Lawrence Livermore National Laboratory, Lawrence Berkeley National Laboratory and DESY Photon Science, Germany, and the University of Saskatchewan, Canada, have investigated the properties of beta-magnesium chloride under pressure, a compound used widely in the aviation industry to de-icing agent. There is also potential for magnesium compounds under extreme conditions as effective biocidal agents that could be used to neutralize biological weapons. Thus, the high pressure properties of these materials are important for understanding and predicting their behaviour in complex, chemically reactive environments such as explosions and as such the US Defense Threat Reduction Agency (DTRA) has helped fund the research.

EOS

The team found that the compound is very stable under high pressure, a finding that is in conflict with well-established structural systematic studies on the compound. The team revealed details in August in the journal Scientific Reports. Their primary aim, technically speaking, being to find the equations of state (EOS) and structural phase diagrams that would help them improve the confidence of semi-empirical thermochemical calculations used to predict the properties and performance of detonated chemical formulations.

"In order to determine accurate EOS data, we first conducted high-pressure X-ray diffraction measurements up to a nominal detonation pressure of 40 gigapascals (GPa) or 400,000 times more than atmospheric pressure," explains physical chemist Zaug who led the project. "The EOS data enable the development of thermochemical prediction tools to guide the development of effective formulations to defeat bioagents," adds physicist Bastea who heads the computational work. "According to previous theoretical studies and the well-established phase diagram of high pressure compounds, magnesium chloride should have transformed to a higher coordination number (become more dense) and 3D connectivity structure well below 40 GPa, through a first order phase transition," points out lead author Elissaios Stavrou of LLNL. By contrast, MgCl2 remained in a low coordination layered structure. "We were surprised by our first results," Bastea says. "We decided to compress MgCl2 to higher pressures to further explore this remarkable discrepancy." Indeed, even above the 1 megabar (1 million atmospheres) pressure threshold, the team saw no structural phase transition in their data. The team has corroborated the experimental data with first principles calculations performed by Saskatchewan physicist Yansun Yao. The calculated EOS, within small error estimates, agree very well with the measured data. Yao explains that, "The surprising pressure stability of this material is inherent to the compound itself and not simply an artefact of the kinetic barrier."

Archetype

As Stavrou explains further, "AX2 compounds are archetypal ionic solids and after nearly 50 years of systematic study, theorists tend to suggest that these pressure-dependent structures and transitions are predictable. Yao adds that, "Our results highlight the need to re-examine currently established structural systematics and to be prepared for unexpected results."

"Ultimately we want to understand the diverse structural behavior of AX2 compounds under pressure," Yao told SpectroscopyNOW. "The surprising finding in MgCl2 urges us to examine current understanding and theoretical predictions of these compounds and to develop new theories. As a next step we want to study other relevant compounds that don't belong to the main structural families."

Related Links

Sci Rep 2016, 6, 30631: "High-pressure X-ray diffraction, Raman, and computational studies of MgCl2 up to 1 Mbar: Extensive pressure stability of the beta-MgCl2 layered structure"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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