THE LATTICE CONTRIBUTION TO THE SPECIFIC-HEAT IN TETRAMETHYLAMMONIUM TETRACHLOROMETALLATES

The lattice specific heat in tetramethylammonium tetrachlorometallates was calculated from accurate Cp measurements as well as from the available spectroscopic data. The experimental information on various members of this family and some related compounds was also used to analyse the various contributions to the lattice specific heat. The anharmonic correction was performed using the Nernst-Lindemann law, together with the elastic constants and with the thermal expansion coefficients. The frequencies of the Raman and infrared inactive modes were estimated from the active associated motions in related organic molecules. Einstein and Debye functions were used to calculate the harmonic specific heat from the phonon spectrum. The final result shows an excellent fit to the experimental data and provides a useful method to obtain a baseline for the calculation of the phase transition thermodynamic functions. This method has permitted the authors to study the specific heat contribution of the tetramethylammonium groups, which is also compared with the values obtained by other procedures. Finally, the main sources of errors are discussed.