The Lattice Energies and Transition Temperatures of Caesium Chloride and Ammonium Chloride

The lattice energies of CsCl and N${\mathrm{H}}_4$Cl are calculated for both the CsCl type and NaCl type lattices, and the temperatures of transition between these two crystal forms determined from the equality of the free energies. This requires the computation of the elastic constants and proper frequencies. In determining the lattice energies, use is made of the method of Born and Mayer, four constants in the lattice energy expression being evaluated from experimental data. To obtain results consistent with known lattice constants, compressibilities and heats of transition, it is found necessary to multiply the van der Waals constants derived by Mayer by about 3.5 and to make the constant coefficient in the repulsion term for nearest like ions about 0.6 that for nearest (unlike) ions. Some success is attained in computing transition temperatures comparable with those known from experiment, but the temperature dependence found for the proper frequencies appears to be faulty.