Vibrational energies and thermal expansion of layered compounds: MgCl2

Discovering Hidden Material Properties of MgCl2 at Atomic Resolution with Structured Temporal Electron Illumination of Picosecond Time Resolution

“…Their extrapolation across temperature to find a + = a − = 0 yields thermal energy equivalents of 13 meV (155 K) and 8 meV (98 K) for the temperature onset of the recorded structural transformations. Theoretically, it is challenging to accurately calculate the difference in Gibbs energy between α‐ and β‐MgCl 2 because it is expected to be unusually small (1–2 meV) . Incidentally, the measured alteration parameters at 183 K in Figure C ( a − = −2.2*10 −5 Å 2 e −1 , a + = 0.58*10 −5 Å 2 e −1 ) reasonably match the expectations from Figure C,D (a − = −1.7*10 −5 Å 2 e −1 , a + = 0.33*10 −5 Å 2 e −1 ) at 298 K for a common beam current of 0.5 nA.…”

Elucidating water dynamics in MgCl 2 hydrates from molecular dynamics simulation

“…So far, empirical interaction potentials (force fields) for molecular simulations of MgCl 2 .nH 2 O crystals have been tested. In the literature several force fields have been reported that reproduce the structure of anhydrous MgCl 2 crystals in molecular mechanics calculations [28][29][30][31], however these do not include the water-ion interactions that play a key role in hydrated structures. Most of the work on Mg 2+ -H 2 O and Cl --H 2 O interactions has been done in the field of ionic solutions [32][33][34][35][36][37][38], but these force fields are optimized on their ability to reproduce the hydration characteristics of the ions and not on their ability to reproduce ion-ion contacts .…”

“…The closest distance between two Cl-ions sitting at different planes is only 3.6 Å [46]. Studies on anhydrous MgCl 2 with classical force fields (basically energy minimizations) always had to account for the polarizability of the Cl-ions to stabilize the crystal structure [29][30][31][32]. The only force fields designed for aqueous MgCl 2 solutions, able to produce stable anhydrous MgCl 2 structures in MD runs, explicitly took the polarizability of the Chloride ion into account [39].…”

Negative thermal expansion

“…With more complex structure and bonding it becomes more difficult to predict or explain the thermal expansion (see, e.g., [86] for a prediction of the thermal expansion of MgCl 2 ). In the examples above, negative expansion occurs in the elastically harder directions, and is smaller in magnitude than the positive expansion in other directions; but the semi-metal indium bismuth, InBi, breaks these rules.…”