Phase coexistence and hysteresis effects in the pressure-temperature phase diagram of NH3BH3

Abstract

The potential hydrogen storage compound NH${}_{3}$BH${}_{3}$ has three known structural phases in the temperature and pressure ranges 110--300 K and 0--1.5 GPa, respectively. We report here the boundaries between, and the ranges of stability of, these phases. The phase boundaries were located by in situ measurements of the thermal conductivity, while the actual structures in selected areas were identified by in situ Raman spectroscopy and x-ray diffraction. Below 0.6 GPa, reversible transitions involving only small hysteresis effects occur between the room-temperature tetragonal plastic crystal $I$4$\mathit{mm}$ phase and the low-temperature orthorhombic Pmn2${}_{1}$ phase. Transformations of the $I$4$\mathit{mm}$ phase into the high-pressure orthorhombic Cmc2${}_{1}$ phase, occurring above 0.8 GPa, are associated with very large hysteresis effects, such that the reverse transition may occur at up to 0.5 GPa lower pressures. Below 230 K, a fraction of the Cmc2${}_{1}$ phase is metastable to atmospheric pressure, suggesting the possibility that dense structural phases of NH${}_{3}$BH${}_{3}$, stable at room temperature, could possibly be created and stabilized by alloying or by other methods. Mixed orthorhombic Pmn2${}_{1}$/Cmc2${}_{1}$ phases were observed in an intermediate pressure-temperature range, but a fourth structural phase predicted by Filinchuk et al. [Phys. Rev. B 79, 214111 (2009)] was not observed in the pressure-temperature ranges of this experiment. The thermal conductivity of the plastic crystal $I$4$\mathit{mm}$ phase is about 0.6 W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}1}$ and only weakly dependent on temperature, while the ordered orthorhombic phases have higher thermal conductivities limited by phonon-phonon scattering.