Comprehensive study of the structure of aluminum trihalides from electron diffraction and computation

Tracing mechanistic pathways and reaction kinetics toward equilibrium in reactive molten salts

In the dynamic environment of multi-component reactive molten salts, speciation unfolds as a complex process, involving multiple competing reaction pathways that are likely to face free energy barriers before reaching the reaction equilibria. Herein, we unravel intricate speciation in the AlCl3-KCl melt compositions with rate theory and ab initio molecular dynamics simulations. We find that the compositions with 100 and 50 mol% AlCl3 exclusively comprise neutral Al2Cl6 dimers and charged AlCl4- monomers, respectively. In intermediate AlCl3-KCl compositions, the chemical speciation proves to be a very complex process, requiring over 0.5 nanosecond to reach an equilibrium distribution of multiple species. It is a consequence of the competitive formation and dissociation of additional species, including charged Al dimers, trimers, and tetramers. Here, the species formation occurs through ion exchange events, which we explain by computing free energy landscapes and employing a Marcus-like rate theory. We show that both interspecies and intraspecies ion exchanges are probable and are dictated by the local structural reorganization reflected in the change of local coulombic fields. The species distributions are validated by comparing computed Raman spectra and neutron structure factors with the available experimental data. We find an excellent simulation-experiment agreement in both cases. Nevertheless, Raman spectroscopy turns out to be particularly advantageous for distinguishing between unique species distributions because of the distinct vibrational signatures of different species. The mechanistic insight into reaction dynamics gained in this study will be essential for the advancement of molten salts as reactive media in high-temperature energy applications.

Combined gas electron diffraction and mass spectrometric experimental setup at Bielefeld University

We have designed and constructed a combined experimental setup for synchronous measurements of electron diffraction patterns and mass-spectra of gas samples. Test measurements have been performed for acetic acid at two temperatures, 296 K and 457 K. Electron diffraction data have been analyzed taking into account mass spectra measured in the same experiments. From the diffraction intensities, molecular structures and mole fractions of the acetic acid monomer and dimer have been refined. The obtained results demonstrate the importance of measuring mass spectra in gas electron diffraction experiments. In particular, it is possible to detect the sample decomposition, which can be used for the optimization of experimental conditions and for the data interpretation. The length of the hydrogen bond in the acetic acid dimer determined in this work, r_e(O⋯H) = 1.657(9) Å, is in good agreement with modern theoretical predictions. We recommend measuring the diffraction patterns of acetic acid for the calibration of the sample pressure in the diffraction volume.

High pressure structural changes in aluminium triiodide: A first principles study

First principles calculations identified a phase transition in aluminium triiodide (AlI3) and predicted its physical and spectroscopic properties under high pressure conditions. A high pressure monoclinic phase is predicted to exist above 1.3 GPa accompanied with a coordination change of aluminium resulting from a transformation from the ambient pressure 4-coordinated primitive monoclinic phase with space group P21/c to the monoclinic 6-coordinated structure with space group C2/m. Density functional phonon calculations predicted its dynamical and mechanical stability. Infrared effective charge intensities and Raman scattering tensors were obtained to characterize its spectroscopic properties. First-principles metadynamics simulations were employed to reconstruct this phase transition and provide the mechanism details for energetically favourable path from the ambient pressure P21/c structure to the predicted C2/m structure.