Molecular-dynamics simulation of ${\mathrm{NH}}_4$${\mathrm{}}^{+}$ reorientations in ${\mathrm{NH}}_4$Cl

Computer simulations of ${\mathrm{NH}}_4$Cl in its CsCl-type phase have been carried out and show disorder in the ammonium orientations between the two possible quasistable states above 200 K. A potential function obtained from lattice dynamics and consistent with the parallel ordering of ${\mathrm{NH}}_4$ ${\mathrm{}}^{+}$ at low temperature is used. Contrary to earlier belief that the jumps between the two quasistable states of ${\mathrm{NH}}_4$ ${\mathrm{}}^{+}$ occur through 90° rotations about 〈001〉 axes only, such jumps are also seen to occur through 180° rotations about 〈110〉 axes with a comparable probability.