Abstract
Magnesium chloride (MgCl2) with the rhombohedral layered CdCl2-type structure (α-MgCl2) has been studied experimentally using synchrotron angle-dispersive powder x-ray diffraction and Raman spectroscopy using a diamond-anvil cell up to 100 GPa at room temperature and theoretically using first-principles density functional calculations. The results reveal a pressure-induced second-order structural phase transition to a hexagonal layered CdI2-type structure (β-MgCl2) at 0.7 GPa: the stacking sequence of the Cl anions are altered resulting in a reduction of the c-axis length. Theoretical calculations confirm this phase transition sequence and the calculated transition pressure is in excellent agreement with the experiment. Lattice dynamics calculations also reproduce the experimental Raman spectra measured for the ambient and high-pressure phase. According to our experimental results MgCl2 remains in a 2D layered phase up to 100 GPa and further, the 6-fold coordination of Mg cations is retained. Theoretical calculations of relative enthalpy suggest that this extensive pressure stability is due to a low enthalpy of the layered structure ruling out kinetic barrier effects. This observation is unusual, as it contradicts with the general structural behavior of highly compressed AB2 compounds.
Highlights
According to Pauling’s first rule, the ambient pressure crystal structure of divalent metal halides and oxides AX2 is mainly determined by the cation-anion radius ratio R =rc/ra[1,2]
The reaction of Mg with chlorine (Cl) leading to MgCl2 production may occur for example in Mg-loaded energetic formulations with significant Cl content, e.g. due to the use of an oxidizer such as ammonium perchlorate (AP) e.g. ref
The evolution of the x-ray diffraction (XRD) data shows discontinuous changes beginning at approximately 0.7 GPa revealing the occurrence of a phase transition
Summary
According to Pauling’s first rule, the ambient pressure crystal structure of divalent metal halides and oxides AX2 is mainly determined by the cation-anion radius ratio R =rc/ra[1,2]. Confident semi-empirical thermochemical calculations of chemistry under extreme pressure-temperature conditions are made using equations of state data and phase diagrams of formulated reactants and likely high concentration product materials. The use of MgCl2 as an additive in energetic formulations is a plausible route to achieving high biocidal activity In both scenarios, knowledge of the MgCl2 equation of state (EOS) is crucial toward understanding and computing the reactive shock behavior of energetic systems that involve a significant amount of MgCl2. In order to examine the high pressure structural behavior of MgCl2 and to expand knowledge of the high pressure structural behavior of layered-structured metal halide compounds we have carried out a detailed synchrotron angle-dispersive powder x-ray diffraction and Raman spectroscopy studies up to 100 GPa and compared these results with first-principles density functional calculations. The results are discussed within the context of the well established structural behavior of highly compressed AX2 compounds
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