THERMOCHEMICAL INVESTIGATIONS ON BORAZANE (BH3-NH3) IN THE TEMPERATURE-RANGE FROM 10 TO 289 K

The molar heat capacity of borazane BH3-NH3 was determined in the 10-2 89 K range. Two different adiabatic calorimeter systems, each using th e classical heat-step method, were applied for the measurements. The m olar standard entropy of borazane at T=298.15 K was determined to be S -0=96.34 J K-1 mol(-1). For the phase transition at T=224 K, a transit ion entropy of Delta S=6.87 J K-1 mol(-1) was calculated from the heat capacity data. The results of X-ray powder diffraction confirm an ord er-disorder transition with a change of the lattice structure from ort horhombic to tetragonal. From low-temperature differential scanning ca lorimetry, a transition enthalpy of Delta H=1.34 kJ mol(-1) was found, the transition enthalpy derived from the heat capacity is Delta H=1.4 8 kJ mol(-1) (C) 1998 Elsevier Science B.V.