To counter the threat of chemical or biological warfare, the Defense Threat Reduction Agency (DTRA) looked at the high pressure properties of materials to understand and predict their behavior in complex, chemically reactive environments such as detonations. Materials that can function at extreme conditions could be used as effective biocidal agents and work to neutralize biological weapons.
Magnesium compounds, including Magnesium chloride (MgCl2) is one such material. Apart from the potential use as a biological counter weapon, MgCl2 is known in the aviation industry to be an effective de-icing agent. In a study at Lawrence Livermore National Laboratory (LLNL), researchers combined vibrational spectroscopy and X-ray diffraction measurements with first-principle calculations to inspect the high-pressure structural behavior of magnesium chloride.
The objective of the study was to provide structural phase diagrams and equations of state (EOS) to increase the confidence of semi-empirical thermochemical calculations that are used to predict the products and performance of detonated chemical formulations.
Joe Zaug, an LLNL physical chemist and project leader, first conducted high-pressure X-ray diffraction measurements up to a detonation pressure of 40 Giga Pascals (GPa) This is 400,000 times more than normal atmospheric pressure. The results of these tests were then used to determine accurate EOS data. Sorin Bastea, the project’s lead LLNL computational physicist, used the EOS data to develop thermochemical prediction tools to guide the improvement of effective formulations to defeat bioagents.
In theory, MgCl2 should have transformed to a 3D connectivity structure and a higher coordination number (denser) well below 40 GPa. These predictions are based on the well-established phase diagram of high-pressure compounds and previous theoretical studies. Surprisingly, MgCl2 contradicted the structural systematics by remaining in a low coordination layered structure even after exceeding the 1 Mega Bar (1 million atmospheres) pressure limit. At no stage was any structural phase transition observed.
Assistant Professor Yansun Yao at the University of Saskatchewan, the team’s collaborator, confirmed the experimental results by using first principle calculations. Yao confirms that the surprising pressure stability is not due to a kinetic barrier, but is inherent.
Stavrou noted that high-pressure compounds are archetypal ionic solids. After nearly 50 years of systematic study, theorists believe that these pressure-dependent transitions and structures are predictable. The results achieved in this study emphasize the need to re-examine currently established structural systematics and to be ready for unexpected results.
The research is published in the Scientific Reports journal.