R. J. C. Brown

Displacement and distortion of the ammonium ion in rotational transition states in ammonium fluoride and ammonium chloride

Ab initio density functional calculations have been carried out on ammonium fluoride to determine the equilibrium structure and the transition state for rotation of the ammonium ion. The calculated equilibrium geometry agrees satisfactorily with crystallographic data. Optimization of the crystal geometry in the transition state for rotation results in significant distortion and displacement of the ammonium ion within the unit cell. Upon reexamination of the rotational transition states in ammonium chloride, similar distortion and displacement of the ammonium ion are found. The rotation process is similar to a carousel motion, in which the ion moves along the rotation axis and changes shape as it rotates. These results show that rigid ion models for ammonium ion rotational dynamics in crystals are, at best, incomplete.

Refinement of ammonium perrhenate structure using a pseudo-spin model for the ammonium ion orientation

A new Rietveld refinement of previously reported powder neutron diffraction data on ND 4 ReO 4 taken over a wide range of temperatures [Powell, Brown, Harnden & Reid (1993). Acta Cryst. B49, 463-468] is reported. Rigid-body constraints are applied to the cation geometry and occupation of a second cation orientation at higher temperatures is permitted. The resulting structure at higher temperatures is to be preferred to the previous structure with a single cation orientation and supports the pseudo-spin theory of cooperative disordering without a phase transition.

The pathway to reorientation in ammonium fluoride

Abstract Two alternative transition states for ammonium ion rotation in crystalline NH 4 F have been characterised using ab initio calculations. In one all the cations rotate in phase, while in the other the rotating ions are isolated from each other by non-rotated ions. These two transition states are characterised by the cation displacement and changes in the bond lengths and bond angles within the ammonium ion. The pathway for passing over the barrier was investigated by low temperature molecular dynamics runs from each transition state. In both cases the order of events in passing from the ground state to the transition state is first cation displacement towards a neighbouring fluoride ion, secondly, lengthening of the NH bond pointing to this ion, and thirdly, rotation about this bond.

The heat capacity of KVO3 from 25 to 310 K

Abstract The heat capacity of potassium metavanadate has been measured from 25 to 310 K. There is no indication of any phase transition or thermal anomaly in this temperature range. In the analysis of the heat capacities maximum use is made of known spectroscopic results.