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Low Temperature Phase Diagram of NH3BH3

Bertil Sundqvist, Ove Andersson, Issam Quwar and Alexandr Talyzin Department of Physics, Umeå University, SE-90187 Umeå, Sweden

ABSTRACT The pressure-temperature (p-T) phase diagram of NH3BH3 has been investigated by thermal conductivity measurements up to 1.5 GPa at temperatures between 100 and 300 K, and the phase boundaries between the three known structural phases have been identified. The transformation between the room temperature tetragonal I4mm phase and the low temperature orthorhombic Pmn21 phase (Tc = 218 K at p = 0) shows only a small hysteresis. The transformation into the high pressure orthorhombic Cmc21 phase (at 1.0 GPa near 292 K) has a very strong hysteresis, up to Δp = 0.5 GPa, and below 230 K a fraction of this phase is metastable even at atmospheric pressure. INTRODUCTION Future hydrogen storage systems for mobile applications should be optimized to contain as much hydrogen as possible, both in terms of volume and in terms of weight [1]. Storage in the form of solid compounds is very efficient, since many such compounds contain more hydrogen per unit volume than pure solid or liquid hydrogen. Therefore, many simple or complex hydrides containing high volume fractions of hydrogen have been studied recently in the search for safe, reversible storage materials. In particular, the temperature-pressure phase diagrams of several complex hydrides have been studied [2-5] to evaluate the possibility to increase the hydrogen density still further by creating structural phases with denser molecular packing. Ammonium borohydride, NH3BH3, contains 19.6 percent hydrogen by weight, and the hydrogen can be released in three stages by heating to temperatures in the range 80 – 500oC [6], or by other means. The structural evolution of this material under pressure has recently been studied by several groups [7-9] and three structural phases have been identified in the lowpressure region below 1.5 GPa [8]. Under ambient conditions NH3BH3 has a tetragonal lattice with symmetry I4mm and with rotationally disordered molecules aligned with the lattice c axis. Cooling below about 220 K results in a transformation into an orthorhombic Pmn21 structure with the molecules tilted by a temperature dependent, small (18-26o) angle relative to the c axis. Increasing instead the pressure at room temperature causes a transformation near 1.1 GPa into a second orthorhombic structure with Cmc21 symmetry , with strongly inclined molecules (69-79o, increasing with pressure). The high pressure transition is clearly of first order with a 4.4% volume change, a significant range of phase coexistence (300 MPa) and a large hysteresis in pressure, while the low temperature transition has only a weak first-order character with a volume change of 0.27% and a 2 K range of coexistence. Using thermodynamic arguments,

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Filinchuk et al. [8] also predicted the existence of a fourth phase, probably monoclinic P21, at high pressure and low temperature. Other recent studies have observed further phase transf