The tetragonal-to-orthorhombic phase transformation in ammonia borane and in its deuterium substituted compounds

The tetragonal to orthorhombic phase transition occurring in ammonia borane has been characterized by means of anelastic spectroscopy and differential scanning calorimetry. The transformation is of first-order, as appreciable latent heat is developed during the phase conversion; however, the transition has also a second-order character, as demonstrated by a dramatic modulus softening starting well above the transformation temperature; a physical mechanism to account for the transformation is proposed. The direct step-by-step monitoring of the dynamic Young modulus allowed us to observe that the real temperature hysteresis between cooling and heating is rather small (∼0.4 K), and the complete transformation evolves in a fraction of Kelvin with presence of etero-phase fluctuations. A comparison between the present deuterium substituted compounds and previous results in hydrogenated samples provided information about the effect of partial and selective deuteration on both the time constants and the width of the phase transformation.

Research highlights
► The structural phase transition occurring in ammonia borane is studied.
► The real temperature hysteresis between cooling and heating is small (0.4 K).
► The transition is of first order, as heat exchange is detected.
► The time constants of the transformations are measured.
► Partial deuteration induces the slowing down of the kinetics of the transition.