Heat Capacities and Phase Transitions of Protonated and Deuterated Methylammonium Tetrafluoroborates†

Heat capacities of CH₃NH₃BF₄ and CD₃ND₃BF₄ were measured in the temperature ranges 5?300 and 13?300 K, respectively. A large heat capacity peak due to a first-order transition appeared at (251.3 ± 0.1) K for CH₃NH₃BF₄ and (251.6 ± 0.1) K for CD₃ND₃BF₄. The transition entropies of CH₃NH₃BF₄ and CD₃ND₃BF₄ were 20.47 ± 0.02 and 20.47 ± 0.02 J K^-1 mol^-1, respectively. A broad hump of the heat capacity was found at around 40 K in both samples. The excess entropy due to this anomaly was 2.90 J K^-1 mol^-1 for CH₃NH₃BF₄ and 2.77 J K^-1 mol^-1 for CD₃ND₃BF₄. These data indicate that the effects of deuterium substitution on both anomalies are minor in this compound. It was concluded from the analysis of entropy that both anomalies of CH₃NH₃BF₄ and CD₃ND₃BF₄ are due to ordering of the highly disordered orientations of methylammonium and tetrafluoroboric ions suggested by the previous X-ray and neutron diffraction study. Raman and IR spectra of CH₃NH₃BF₄ and CD₃ND₃BF₄ were measured and used in the estimation of the normal (vibrational) parts of the heat capacity.