In situ X-ray study of ammonia borane at high pressures

In situ synchrotron X-ray diffraction experiments of ammonia borane synthesized using sonication assisted direct reaction (sodium borohydride and ammonium formate) were performed under high pressures up to 23 GPa at ambient temperature and up to 4 GPa at elevated temperatures (300 K-505 K). At ambient temperature, one first order phase transition (I4mm to Cmc2₁) was observed at 1.3 GPa and one second order phase transition was observed at about 5 GPa. Fitting the measured volumetric compression data to the third order Birch-Murnaghan equation of state reveals a bulk modulus of K = 9.3 ± 0.4 GPa (K′ = 4.8) for the I4mm phase, K = 11.9 ± 0.5 GPa (K′ = 4.6) for the Cmc2₁ phase below 5 GPa, and K = 37 ± 4 GPa (K′ = 4.6) for pressure above 5 GPa. There is a 6% volume drop at the first order phase transition. Transition pressures from I4mm to Cmc2₁ phases were determined at elevated temperatures. The phase boundary has a negative Clapeyron slope of -1.67 MPa/K. The experiments also reveal a new structural transition when temperature increases to 450 K at 4 GPa.