Heat Capacities and Phase Transitions of Protonated and Deuterated Methylammonium Tetrafluoroborates

Abstract

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 norm...