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 I4mm phase and the low-temperature orthorhombic Pmn2 1 phase. Transformations of the I4mm 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 I4mm phase is about 0.6 W m −1 K −1 and only weakly dependent on temperature, while the ordered orthorhombic phases have higher thermal conductivities limited by phonon-phonon scattering.

Ammonia borane at high pressures

“…Chen et al [63] reported a negative Clapeyron slope of the phase boundary above room temperature in their earlier XRD study in a multi-anvil apparatus, but their later Raman spectroscopy study using a DAC indicates that the boundary has a positive slope [69] and their previous XRD data were likely contaminated by the surrounding sample chamber materials in the solid pressure medium cell assembly. The positive Clapeyron slope above room temperature is consistent with the extrapolation from the II-III phase boundary below room temperature determined by Andersson et al [59] using in situ thermal conductivity measurements.…”

“…Custelcean and Dreger [53] used mineral oil as pressure-transmitting medium in the sample chamber of the DAC, whereas no pressure medium was used in Trudel and Gilson's [54] experiment. Further studies [59,60] confirmed that this phase transition was indeed very sluggish and the two phases may coexist over a wide pressure range (from 0.5 to 2.5 GPa).…”

“…Earlier studies were limited to Raman spectroscopy with controversial observations [53,54] on the pressure where possible new phases appear. The study was soon expanded beyond Raman spectroscopy [55][56][57][58][59][60] to X-ray/neutron diffraction (XRD/ND) [59,[61][62][63][64][65], infrared (IR) spectroscopy [57], differential scanning calorimetry [66], thermal conductivity [59] and theoretical modeling [61,62,64,67,68]. A number of pressure-induced phase transitions have been observed in the spectroscopy, diffraction and thermal conductivity experiments.…”

High Pressure and Low Temperature Study of Ammonia Borane and Lithium Amidoborane

“…Chen et al [63] has determined the pressure-temperature phase boundary between the I4mm phase and Cmc2 1 phase as negative Clapeyron slope indicating the transformation is of endothermic, although it is not in agreement with the observation of Anderson et al [64]. Kumar et al has reported the I4mm to Cmc2 1 phase transition occurring at 1.22 GPa and Cmc2 1 to P1 (monoclinic) phase transition occurring at 8 GPa by combined X-ray, Neutron and theoretical investigation [65].…”

“…Although, Anderson et al [83] did not observe this phase transition as they did not continue their experiment at higher pressure and low temperature region where this new phase is stable. Najiba et al [78] also observed phase B which is evident from the change of Raman modes.…”