Activation energies and tunneling frequencies of ammonium halides and ammonium hexachlorides

The activation energies for threefold and fourfold reorientations of the ammonium ion in cubic crystalline fields have been derived using the potential function v(ω) = βB[β3v3(ω)+β4v4(ω)+β6v6(ω)]. The improved zero-point energy correction of Svare et al. has been used. The activation energies computed for both the ordered and disordered bcc phases of the ammonium halides are somewhat lower than the observed values. The activation energies have also been computed for the ammonium hexachlorides. The parameters in the potential function for (NH4)2PbCl6, (NH4)2PtCl6, and (NH4)2PdCl6 have been determined from the librational frequencies. It is found that the β6v6 term in the potential function is very important for the analysis of the hindered rotation of the ammonium ion in these compounds.