Thermal expansion of cubic ammonium compounds

摘要

The contribution to the thermal expansion due to the hindered rotation of the ammonium ion has been computed for the disordered bcc phases of NH4Cl and NH4Br and for the ammonium pervoskites NH4ZnF3and NH4CoF3. The thermal expansion due to the motion of the ammonium ion is computed from the hindered rotational energy levels. It is assumed that the potential functionV(ohgr;)prop;rminus;n. The value of lsquo;lsquo;nrsquo;rsquo; gives an indication of the type of bonding, whether due to electrostatic or valence forces, that are present in the solid. The thermal expansions computed for the disordered bcc phases of NH4Cl and NH4Br, for NH4CoF3, and for NH4ZnF3are in good agreement with the observed results, when electrostatic forces are assumed to predominate in these solids. The contribution of the thermal expansion from the hindered rotation of the ammonium ion amounts to about 10percnt; of the observed thermal expansion for the ammonium pervoskites and about 25percnt; for the disordered bcc phases of the ammonium halides. These percentages are nearly independent of temperature.