Structural phase transitions and adduct release in calcium borohydride

▶ A small amount of THF is retained also in thermally treated samples. ▶ Two, non reversible, phase transformations are detected above room temperature. ▶ The amount of THF and H2 released in vacuum are greater than at room pressure. Ca(BH 4) 2 compounds were investigated above room temperature by anelastic spectroscopy (AS) and concomitant measurements of thermogravimetry and mass spectrometry (TGA/MS). Both AS and TGA/MS indicate that even after a thermal treatment at 125 °C for 20 h, a non-negligible residual of THF adduct is still present in the sample, which can be removed on a subsequent thermal treatment at temperatures lower than 250 °C. Above 250 °C dehydrogenation takes place. Moreover, AS sensitively detects the occurrence of the α → α’ structural phase transition around 180 °C, and the α’ → β transformation, which is completed around 330 °C. We also show that both transitions are irreversible and are not accompanied by a latent heat.