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Molecular dynamics and phase transitions in alkali azides and thiocyanates
Abstract
Calculations based on an ab initio model developed for the intermolecular and intramolecular potentials in complex ionic solids were performed on selected alkali azides $\rm(KN\sb3,\ RbN\sb3,\ CsN\sb3)$ and thiocyanates (KSCN, RbSCN, CsSCN). With these parameter-free potentials we performed static structural relaxations, supercell molecular dynamics and lattice dynamical studies and predict with reasonable accuracy the temperatures for the onset of the order-disorder phase transitions in these materials. Most significantly, we found that the transitions to the high-temperature disordered phases in these systems are driven by rotational disordering of the linear N$\sb3\sp-$ and SCN$\sp-$ anions. In case of the azides (N$\sb3\sp-$ anions) we observed free rotations of the anions about all three crystallographic axes, in all three systems. However, in the thiocyanates (SCN$\sp-$ anions) only one system, caesium thiocyanate (CsSCN), yielded free rotations about all the axes. In potassium thiocyanate (KSCN) and rubidium thiocyanate (RbSCN) the order-disorder transition appears rather to involve large amplitude librations of the SCN$\sp-$ ions, primarily about the c axis. This reflects the fact that in the azides the center of mass of the N$\sb3\sp-$ ions coincides with the geometric center of these anions, whereas in the thiocyanates this is no longer so since the center of mass lies $\sim$30% of the distance from the central carbon atom to the sulphur atom. Given this asymmetry in SCN$\sp-$ it is inevitable that the rotational motion about the center of mass will necessarily be coupled to the translational motion of the center of mass of this ion.
Subject Area
Condensation|Chemistry|Materials science
Recommended Citation
Ossowski, Maciej, "Molecular dynamics and phase transitions in alkali azides and thiocyanates" (1998). ETD collection for University of Nebraska-Lincoln. AAI9908479.
https://digitalcommons.unl.edu/dissertations/AAI9908479